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Applying BEAST v1.10.4, we constructed a Bayesian Inference tree comprising 322 taxa, primarily representing Paleoptera (Odonata and Ephemeroptera; Pterygota), Zygentoma and Archaeognatha (Apterygota; paraphyly), and Neoptera (Plecoptera; Pterygota), based on a 2685 bp sequence dataset. Our analyses revealed that robust dating required the incorporation of both Quaternary and pre‐Quaternary dates. To achieve this, our dating incorporated a 1.55 Ma (Quaternary) geological event (the formation of the Ryukyu Islands) and a set of chronologically well‐founded fossil dates, spanning from up to 400 Ma (Devonian) for the stem Archaeognatha, 320 Ma (Carboniferous) for the crown of Paleoptera, 300 Ma (Carboniferous) for the crown Ephemeroptera, and 280 Ma (Permian) for the crown Odonata, down to 1.76 Ma (Quaternary) for Calopteryx japonica, encompassing a total of 22 calibration points (events: 6, fossils: 16; Quaternary: 7, pre‐Quaternary: 15). The resulting dated tree aligns with previous research, albeit with some dates being overestimated. This overestimation was mainly due to the lack of Quaternary calibration and the exclusive dependence on pre‐Quaternary calibration, though the application of maximum age constraints also played a role. Our minimum age dating demonstrates that the molecular clock did not uniformly progress, rendering rate dating an inapplicable approach. We observed that the base substitution rate is time‐dependent, with an exponential increase evident from around 20 Ma (Miocene) to the present time, exceeding an order of magnitude. The extensive radiation and speciation of Insecta and Paleoptera, potentially resulting from the severe climatic changes associated with the Quaternary, including the commencement of glacial and interglacial cycles, may have significantly contributed to this increase in base substitution rates. Additionally, we identified a potential peak in base substitution rates during the Carboniferous period, around 320 million years ago, possibly corresponding to the Late Paleozoic Ice Age. [ABSTRACT FROM AUTHOR]

The ocean's organic carbon export is a key control on atmospheric pCO2 and stimulating this export could potentially mitigate climate change. We use a data‐constrained model to calculate the sensitivity of atmospheric pCO2 to local changes in export using an adjoint approach. A perpetual enhancement of the biological pump's export by 0.1 PgC/yr could achieve a roughly 1% reduction in pCO2 at average sensitivity. The sensitivity varies roughly 5‐fold across different ocean regions and is proportional to the difference between the mean sequestration time τseq of regenerated carbon and the response time τpre of performed carbon, which is the reduction in the preformed carbon inventory per unit increase in local export production. Air‐sea CO2 disequilibrium modulates the geographic pattern of τpre, causing particularly high sensitivities (2–3 times the global mean) in the Antarctic Divergence region of the Southern Ocean. Plain Language Summary: Atmospheric CO2 levels could be reduced by stimulating plankton in the ocean to produce more organic carbon that then sinks to depth (is exported) at a higher rate. The resulting carbon deficit in surface waters drives atmospheric carbon into the ocean. The efficacy of this process depends on how long the exported carbon stays isolated from the surface and how quickly the surface deficit can be filled. Here we investigate how sensitive atmospheric CO2 levels are to a given enhancement in carbon export and where such enhancements would be most effective. We show that the sensitivity is determined by the difference between the time for which exported carbon stays sequestered at depth and the time with which the rest of the carbon in the ocean responds to the additional export rate. High sequestration times are found where the organic carbon is exported into old deep waters, while fast response times are found where it is easy for gas exchange to inject carbon into the ocean and where the additionally exported carbon is less likely to resurface and escape into the atmosphere. These factors result in the Southern Ocean and the tropics having greatest sensitivity. Key Points: pCO2 sensitivity to carbon export perturbations is governed by sequestration time, preformed response time, and atmospheric turnover timeHigh sensitivity occurs in the tropics and Southern Ocean where sequestration time is long and preformed response time is shortThe preformed response time is modulated by air‐sea CO2 disequilibrium and shortest in the Atlantic and Southern Ocean [ABSTRACT FROM AUTHOR]

[Display omitted] The Indo-Pacific warm pool (IPWP) is crucial for regional and global climates. However, the development of the IPWP and its effect on the regional climate during the Cenozoic remain unclear. Here, using a compilation of sea surface temperature (SST) records (mainly since the middle Miocene) and multimodel paleoclimate simulations, our results indicated that the extent, intensity and warmest temperature position of the IPWP changed markedly during the Cenozoic. Specifically, its extent decreased, its intensity weakened, and its warmest temperature position shifted from the Indian to western Pacific Ocean over time. The atmospheric CO 2 dominated its extent and intensity, while paleogeography, by restricting the distribution of the Indian Ocean and the width of the tropical seaways, controlled the shift in its warmest temperature position. In particular, the eastward shift to the western Pacific Ocean from the middle to late Miocene inferred from compiled SST records likely resulted from the constriction of tropical seaways. Furthermore, by changing the atmospheric thermal structure and atmospheric circulation, the reduced extent and intensity of the IPWP decreased the annual precipitation in the western Indian Ocean, eastern Asia and Australia, while the shift in the warmest temperature position from the Indian to western Pacific Ocean promoted aridification in Australia. Qualitative model-data agreements are obtained for both the IPWP SST and regional climate. From the perspective of past warm climates with high concentrations of atmospheric CO 2 , the expansion and strengthening of the IPWP will occur in a warmer future and favor excessive precipitation in eastern Asia and Australia. [ABSTRACT FROM AUTHOR]

The atmospheric carbon dioxide (CO2) concentration has been increasing rapidly since the Industrial Revolution, which has led to unequivocal global warming and crucial environmental change. It is extremely important to investigate the interactions among atmospheric CO2, the physical climate system, and the carbon cycle of the underlying surface for a better understanding of the Earth system. Earth system models are widely used to investigate these interactions via coupled carbon-climate simulations. The Chinese Academy of Sciences Earth System Model version 2 (CAS-ESM2.0) has successfully fixed a two-way coupling of atmospheric CO2 with the climate and carbon cycle on land and in the ocean. Using CAS-ESM2.0, we conducted a coupled carbon-climate simulation by following the CMIP6 proposal of a historical emissions-driven experiment. This paper examines the modeled CO2 by comparison with observed CO2 at the sites of Mauna Loa and Barrow, and the Greenhouse Gases Observing Satellite (GOSAT) CO2 product. The results showed that CAS-ESM2.0 agrees very well with observations in reproducing the increasing trend of annual CO2 during the period 1850–2014, and in capturing the seasonal cycle of CO2 at the two baseline sites, as well as over northern high latitudes. These agreements illustrate a good ability of CAS-ESM2.0 in simulating carbon-climate interactions, even though uncertainties remain in the processes involved. This paper reports an important stage of the development of CAS-ESM with the coupling of carbon and climate, which will provide significant scientific support for climate research and China's goal of carbon neutrality. [ABSTRACT FROM AUTHOR]

During the transition from the Last Glacial Maximum (LGM) to the Holocene, terrestrial carbon sequestration occurred primarily in boreal forests and forest soils largely on landscapes that had been covered by ice sheets. Major processes operating during this period included radiative warming from rising concentrations of atmospheric CO2 (degassing oceans and oxidation of permafrost); increased seasonal warming associated with axial precession; melting of alpine glaciers and ice sheets; exposure of new land surfaces; and sequestration of carbon in expanding terrestrial vegetation and soils. We examine mechanisms of warming that melted glacial ice; temporal and spatial availability of newly exposed landscapes; rates at which plant colonization and soil development occurred; estimates of terrestrial carbon sequestration; and how those processes interacted with one another. Data from the West Antarctic Ice Sheet Divide ice core show that from 18 to 11 cal ka bp the concentration of atmospheric CO2 rose by ≈80 ppmv (≈170 Gt C); published estimates of net terrestrial carbon sequestration (following photosynthesis) are considerably higher (450–1250 Gt C). Thus, accumulation of carbon in terrestrial vegetation and soils played an important role in modulating atmospheric CO2 and, indirectly, Earth's climate during Termination 1, and possibly during earlier Quaternary ice ages. [ABSTRACT FROM AUTHOR]

In the context of past and present climate change, the Southern Ocean (SO) has been identified as a crucial region modulating the concentration of atmospheric CO2. The sustained upwelling of carbon-rich deep waters and inefficient nutrient utilization at the surface of the SO leads to an outgassing of natural CO2, while anthropogenic CO2 is entrained to depth during the formation of Antarctic Bottom water (AABW), Antarctic intermediate water (AAIW) and sub-Antarctic mode water (SAMW). Changes to the SO circulation resulting from both dynamic and buoyancy forcing can alter the rate of upwelling as well as formation and subsequent transport of AABW, AAIW and SAMW, thus impacting the air-sea CO2 exchange in the SO. Models of all complexity robustly show that stronger southern hemispheric (SH) westerlies enhance SO upwelling, thus leading to stronger natural CO2 outgassing, with a sensitivity of 0.13 GtC/yr for a 10% increase in SH westerly windstress. While the impact of changes in the position of the SH westerly winds was previously unclear, recent simulations with high-resolution ocean/sea-ice/carbon cycle models show that a poleward shift of the SH westerlies also enhances natural CO2 outgassing with a sensitivity of 0.08GtC/yr for a 5° poleward shift. While enhanced AABW transport reduces deep ocean natural DIC concentration and increases surface natural DIC concentration, it acts on a multi-decadal timescale. Future work should better constrain both the natural and anthropogenic carbon cycle response to changes in AABW and the compound impacts of dynamic and buoyancy changes on the SO marine carbon cycle. [ABSTRACT FROM AUTHOR]

The marine biological carbon pump (BCP) stores carbon in the ocean interior, isolating it from exchange with the atmosphere and thereby coregulating atmospheric carbon dioxide (CO2). As the BCP commonly is equated with the flux of organic material to the ocean interior, termed "export flux," a change in export flux is perceived to directly impact atmospheric CO2, and thus climate. Here, we recap how this perception contrasts with current understanding of the BCP, emphasizing the lack of a direct relationship between global export flux and atmospheric CO2. We argue for the use of the storage of carbon of biological origin in the ocean interior as a diagnostic that directly relates to atmospheric CO2, as a way forward to quantify the changes in the BCP in a changing climate. The diagnostic is conveniently applicable to both climate model data and increasingly available observational data. It can explain a seemingly paradoxical response under anthropogenic climate change: Despite a decrease in export flux, the BCP intensifies due to a longer reemergence time of biogenically stored carbon back to the ocean surface and thereby provides a negative feedback to increasing atmospheric CO2. This feedback is notably small compared with anthropogenic CO2 emissions and other carbon‐climate feedbacks. In this Opinion paper, we advocate for a comprehensive view of the BCP's impact on atmospheric CO2, providing a prerequisite for assessing the effectiveness of marine CO2 removal approaches that target marine biology. [ABSTRACT FROM AUTHOR]

Atmospheric CO2 variability on the glacial–interglacial (G–IG) timescale reflects a balance between oceanic and terrestrial processes involving carbon uptake and release. The Southern Ocean CO2 uptake is considered as an important modulator for the G–IG atmospheric CO2 variability, while the role of tropical ocean ventilation remains enigmatic. We present critical evidence for CO2 ventilation from the tropical Indian Ocean through the reconstruction of the Arabian Sea‐surface pCO2 for the past ∼136 ka utilizing boron isotope (δ11B) record of planktic foraminifera, Globigerinoides ruber. Our site in the Arabian Sea presently acts as a significant source of CO2. The reconstructed ΔpCO2 (ΔpCO2 = pCO2 Seawater − pCO2 Atmosphere) record shows an enhanced CO2 degassing up to ∼50 ppm during the major cooling events, such as the Last Glacial Maximum, Younger Dryas, and Heinrich‐Stadials. Our investigation based on multiproxy records of sea‐surface temperature, salinity, and productivity suggests that the northward invasion and shoaling of southern source CO2‐rich water, coupled with stronger upwelling, resulted in CO2 degassing during these cold intervals. This finding is in align with the tropical Atlantic which also demonstrated an enhanced CO2 degassing during the cold intervals; however, most of the upwelled CO2 was consumed as the water moved away from the upwelling sites. Therefore, our finding, when considered alongside tropical Atlantic records, suggests that tropical oceans played a minor role in reducing atmospheric CO2 levels during the cold intervals of the last glacial cycle, supporting the prevailing hypothesis. Key Points: Foraminifera δ11B‐based CO2 record of Arabian Sea surface for the past ∼136 kaEnhanced CO2 degassing during the cold climate events, that is, Heinrich‐Stadials, Younger Dryas, and Last Glacial MaximumThis degassing was caused by more export of southern‐sourced water and its upwelling [ABSTRACT FROM AUTHOR]

Comparing satellite retrieval with high-precision ground observations is an essential component for the validation of CO2 satellite products. The initial stage of assessing the bias in retrieval products from satellite and ground sources involves establishing a geographical connection between observations that are temporally and spatially proximate. The primary aim of this paper is to evaluate the influence of variations in neighborhood definitions and colocation methods on the assessment of satellite products and provide quantitative references. To achieve this, a series of experiments were conducted involving the Global Total Column Carbon Observation Network (TCCON) and the OCO-2 satellite. Various spatial-temporal neighborhoods and colocation methods were considered in these experiments. The results indicate that spatial neighborhoods exert a more substantial influence on bias compared to temporal neighborhoods. In the mid-latitudes of the Northern Hemisphere, there is an observed linear increase trend between the difference of OCO-2 and TCCON observations and the spatial neighborhood, with an average increase of 0.32 ppm as the neighborhood size changes from 1 ° to 10 ° . Regarding colocation methods, the simple spatiotemporal geographic constraints tend to overlook changes in the atmospheric state to a certain extent. The target geographic constraint method reduces the bias by 2% to 5% by increasing the proportion of OCO-2 observations targeting TCCON while the method of introducing T700 potential temperature reduces by 2% to 13% by screening the gradient of CO2 concentration change. Moreover, an evident correlation exists between the bias and their corresponding latitudes, with a 0.20 ppm increase in bias observed for every 10 ° increment in latitudes in the Northern Hemisphere. The bias of TCCON and OCO-2 shows a pronounced seasonal regularity, with the highest in summer. The study also discusses the selection of spatiotemporal matching with low satellite coverage, the bias distribution, and the attribution of bias to the natural wind field. [ABSTRACT FROM AUTHOR]

We provide a 1‐year dataset of atmospheric surface CO2, CH4 and H2O concentrations and δ13C‐CO2 values from an Australian savanna site. These semi‐arid ecosystems act as carbon sinks in wet years but the persistence of the sink in dry years is uncertain. The dataset can be used to constrain uncertainties in modelling of greenhouse gas budgets, improve algorithms for satellite measurements and characterize the role of vegetation and soil in modulating atmospheric CO2 concentrations. We found pronounced seasonal variations in daily mean CO2 concentrations with an increase (by 5–7 ppmv) after the first rainfall of the wet season in early December with peak concentrations maintained until late January. The CO2 increase reflected the initiation of rapid microbial respiration from soil and vegetation sources upon initial wetting. As the wet season progressed, daily CO2 concentrations were variable, but generally decreased back to dry season levels as CO2 assimilation by photosynthesis increased. Mean daily concentrations of CH4 increased in the wet season by up to 0.2 ppmv relative to dry season levels as the soil profile became waterlogged after heavy rainfall events. During the dry season there was regular cycling between maximum CO2/minimum δ13C‐CO2 at night and minimum CO2/maximum δ13C‐CO2 during the day. In the wet season diel patterns were less regular in response to variable cloud cover and rainfall. CO2 isotope data showed that in the wet season, surface CO2 was predominantly a two‐component mixture influenced by C3 plant assimilation (day) and soil/plant respiration (night), while regional background air from higher altitudes represented an additional CO2 source in the dry season. Higher wind speeds during the dry season increased vertical mixing compared to the wet season. In addition, night‐time advection of high‐altitude air during low temperature conditions also promoted mixing in the dry season. [ABSTRACT FROM AUTHOR]

Carbon release during continental rifting is thought to regulate atmospheric CO2 levels. Supercontinent dispersal‐induced extensional tectonics is similar during the Late Ediacaran and Jurassic, while they exhibit different increases in atmospheric CO2 concentration. The underlying mechanism of distinct CO2 emissions remains to be understood. Here, we conduct petrological‐thermomechanical modeling to show that metamorphic decarbonation and melting of carbonates that are derived from subducted sediments are ubiquitous during continental extension. We find that the hotter lithosphere and deeper storage of these carbonates cause more significant amounts of rift‐related CO2 release through volcanoes and faults. They may cause ∼12%–77% larger decarbonation efficiency, providing an efficient driving mechanism for a ∼31% larger increase in atmospheric CO2 levels during the Late Ediacaran than throughout the Jurassic. The rapid eruption and deposition of recycled carbonatite volcanic ash may contribute to the production of Late Ediacaran marine carbonates with the largest negative δ13C (−12‰). Plain Language Summary: The Late Ediacaran and Jurassic periods witnessed rapid increases in atmospheric CO2 concentration and significant negative carbon isotope excursions in marine carbonates during the Earth's history, but the dynamics of their changes remain enigmatic. In this study, we present high‐resolution petrological‐thermomechanical models of how these processes occur. Our results indicate that the extensional tectonics drives trans‐crustal faulting and thereby results in remobilized subcontinental lithospheric carbonates of subducted sedimentary origin (SLCSS) to melt and migrate upward along these faults. The efficiency of CO2 release from these carbonates via metamorphic reaction decarbonation is promoted by the deeper storage and hotter lithosphere, which can reach up to ∼12%–77% larger. Recycled carbonatite volcanic ash may rapidly erupt and deposit in shallow marine habitats following the emission of CO2 enriched in 13C, resulting in the formation of large negative δ13C in Late Ediacaran and Jurassic. We suggest that metamorphic CO2 degassing of remobilized subcontinental lithospheric mantle carbonates of subducted sedimentary origin (SLMCS) combined with a hotter lithosphere may be responsible for a 31% greater increase in atmospheric CO2 and the largest negative δ13C of −12‰ in the Late Ediacaran. Key Points: Numerical modeling of decarbonation of subducted sedimentary carbonates during continental riftingThermal structure of the continental lithosphere and carbon storage depth control the efficiency of CO2 degassingDecarbonation of stored sedimentary carbonates during continental rifting accounts for CO2 concentrations and negative δ13C in Late Ediacaran [ABSTRACT FROM AUTHOR]

Diurnal variation of atmospheric CO2 and its driving factors are studied using high‐precision in situ mixing ratio measurements and stable carbon isotopic CO2 (δ13C‐CO2) for the first time over Shadnagar, a suburban site in India, from November 2018 to October 2019. The annual mean mixing ratios of atmospheric CO2 and δ13C‐CO2 are observed to be 414.76 ± 4.26 ppm and −11.19 ‰ ± 1.63 ‰. Maximum diurnal variability of atmospheric CO2 was observed in summer monsoon (17.30 ± 9.29 ppm), and the minimum was noticed in winter (7.19 ± 0.11 ppm), indicating strong seasonality in diurnal variability of amplitude. To characterize the atmospheric CO2 sources, the Miller‐Tans model was implemented separately using the CO2 and its δ13C‐CO2 isotopic data during day and night. However, a strong source signature of δ13C‐CO2 was observed during the night‐time of summer monsoon with a slope of −37.42‰ ± 0.73‰, obtained using a reduced major axis regression. The identified source indicates possible emissions from fossil fuel combustion. Further, a Lagrangian back trajectory analysis was performed to study the influence of transport on the observed CO2 mixing ratios. The model trajectory confirms the influence of the Indian summer monsoon on the variations of atmospheric CO2 mixing ratios. Our study shows that fossil fuel emissions are one of the major contributors of CO2 in the study location (about ∼2 to 4 ppm) with biogenic fluxes adding a clear seasonality ranging from −8 to 4 ppm. Plain Language Summary: At a suburban site in India, the present study looked at the diurnal variation of seasonal atmospheric CO2 and its stable isotopes, 13C‐CO2. We used the Miller‐Tans model on seasonal data to capture the driving processes of atmospheric CO2 during day and night hours. A high source/sink signature of 13C was identified during the night of the summer monsoon. The presence of C3 ecosystem respiration and combustion is attributed as the source. Our research provided clear evidence that, in addition to short‐ and long‐range anthropogenic impacts, changes in the isotopic composition of atmospheric CO2 during photosynthesis and biogenic respiration are related to changes in atmospheric CO2 concentration. Key Points: The study focused on the diurnal variability of atmospheric CO2 mixing ratio and δ13C‐CO2 on a seasonal basisDetermination of δ13C signature of atmospheric CO2 sources using Miller‐Tans modelInvestigated the influence of transport on seasonal variability of atmospheric CO2 using the Lagrangian transport model and inventory fluxes [ABSTRACT FROM AUTHOR]

The temporal variation of greenhouse gas concentrations in China during the COVID-19 lockdown in China is analyzed in this work using high resolution measurements of near surface △CO 2 , △CH 4 and △CO concentrations above the background conditions at Lin'an station (LAN), a regional background station in the Yangtze River Delta region. During the pre-lockdown observational period (IOP-1), both △CO 2 and △CH 4 exhibited a significant increasing trend relative to the 2011-2019 climatological mean. The reduction of △CO 2 , △CH 4 and △CO during the lockdown observational period (IOP-2) (which also coincided with the Chinese New Year Holiday) reached up to 15.0 ppm, 14.2 ppb and 146.8 ppb, respectively, and a reduction of △CO 2 /△CO probably due to a dramatic reduction from industrial emissions. △CO 2 , △CH 4 and △CO were observed to keep declining during the post-lockdown easing phase (IOP-3), which is the synthetic result of lower than normal CO 2 emissions from rural regions around LAN coupled with strong uptake of the terrestrial ecosystem. Interestingly, the trend reversed to gradual increase for all species during the later easing phase (IOP-4), with △CO 2 /△CO constantly increasing from IOP-2 to IOP-3 and finally IOP-4, consistent with recovery in industrial emissions associated with the staged resumption of economic activity. On average, △CO 2 declined sharply throughout the days during IOP-2 but increased gradually throughout the days during IOP-4. The findings showcase the significant role of emission reduction in accounting for the dramatic changes in measured atmospheric △CO 2 and △CH 4 associated with the COVID-19 lockdown and recovery. [Display omitted] [ABSTRACT FROM AUTHOR]

Terrestrial ecosystems provide crucial ecosystem services to human beings, and their functions are largely dependent on soil moisture availability. Although many studies have evaluated the effects of vegetation and climate changes on soil moisture at the ecosystem scale, changes in ecosystem water limitation remain poorly understood. This study evaluated the spatiotemporal changes of ecosystem water limitation in the Yellow River Basin (YRB)—the second largest river basin of China—during 1982–2016 and identified the major drivers by establishing ecosystem limitation index (ELI) using soil moisture, transpiration, net radiation, and air temperature. The results show a significant and widespread spatiotemporal increase in the water limitation of ecosystems in YRB during the 35‐year study period. Temporally, areas with positive ELI (water‐limited regime) exhibited a significant upward trend (p < 0.01). Spatially, above 75.0% of the total areas showed an upward trend in ELI. Almost all ecosystems showed significant upward trends in water limitation over this period. Further analysis using two different but comparable methods, partial correlation and Lindeman‐Merenda‐Gold, show that vegetation change was the major driver of changes in water limitation, with contributions of more than 35.0%. Air temperature and atmospheric CO2 changes contributed comparably to changes in positive ELI, followed by precipitation changes. These findings improve the understanding of the spatiotemporal pattern and underlying mechanisms of ecosystem water limitation in the greening and warming YRB. Plain Language Summary: Functions of terrestrial ecosystems are largely dependent on soil moisture availability. Plenty of past studies have been conducted to investigate the vegetation and climate effects on soil moisture. However, the ecosystem water limitation and its spatiotemporal changes are rarely explored. We used a correlation difference as the ecosystem limitation index (ELI) to investigate the ecosystem water limitation over the Yellow River Basin (YRB)—the second largest river basin of China during 1982–2016. We found a significant and widespread increase in the water limitation of all ecosystems in YRB during the 35‐year study period. In total, above 75.0% of the total areas showed an upward trend in ELI (increasing water limitation). We used two different methods to attribute ELI and found that vegetation change was the major driver of changes in water limitation, with contributions of above 35.0%. Air temperature and atmospheric CO2 changes contributed comparably to ELI, followed by precipitation changes. The findings highlight the key role of vegetation in ecosystem water limitations in the greening and warming YRB. Key Points: Widespread increasing ecosystem water limitation was found in Yellow River Basin during 1982–2016All ecosystem types showed significant increasing water limitation regimesVegetation was found to be the major driver of the increased ecosystem water limitation [ABSTRACT FROM AUTHOR]

Based on inferences from proxy records the Miocene (23.03–5.33 Ma) was a time of amplified polar warmth compared to today. However, it remains a challenge to simulate a warm Miocene climate and pronounced polar warmth at reconstructed Miocene CO2 concentrations. Using a state‐of‐the‐art Earth‐System‐Model, we implement a high‐resolution paleobathymetry and simulate Miocene climate at different atmospheric CO2 concentrations. We estimate global mean surface warming of +3.1°C relative to the preindustrial at a CO2 level of 450 ppm. An increase of atmospheric CO2 from 280 to 450 ppm provides an individual warming of ∼1.4°C, which is as strong as all other Miocene forcing contributions combined. Substantial changes in surface albedo are vital to explain Miocene surface warming. Simulated surface temperatures fit well with proxy reconstructions at low‐ to mid‐latitudes. The high latitude cooling bias becomes less pronounced for higher atmospheric CO2 concentrations. At such CO2 levels simulated Miocene climate shows a reduced polar amplification, linked to a breakdown of seasonality in the Arctic Ocean. A pronounced warming in boreal fall is detected for a CO2 increase from 280 to 450 ppm, in comparison to weaker warming for CO2 changes from 450 to 720 ppm. Moreover, a pronounced warming in winter is detected for a CO2 increase from 450 to 720 ppm, in contrast to a moderate summer temperature increase, which is accompanied by a strong sea‐ice concentration decline that promotes cloud formation in summer via enhanced moisture availability. As a consequence planetary albedo increases and dampens the temperature response to CO2 forcing at a warmer Miocene background climate. Key Points: At a CO2 level of 450 ppm, a Miocene simulation shows a global mean surface warming of +3.1°C relative to the preindustrial stateAtmospheric CO2 increase from 280 to 450 ppm causes a warming of ∼1.4°C, which is as strong as all other forcing factors combinedAt higher atmospheric CO2 levels, the Miocene climate shows a reduced polar amplification linked to a breakdown of seasonality in the Arctic [ABSTRACT FROM AUTHOR]

The ecological integrity of US national parks and other protected areas are under threat in the Anthropocene. For Yellowstone National Park (YNP), the impacts that global change has already had on the park's capacity to sustain its large migratory herds of wild ungulates is incompletely understood. Here we examine how two understudied components of global change, the historical increase in atmospheric CO2 and the spread of nonnative, invasive plant species, may have altered the capacity of YNP to provide forage for ungulates over the last 200‐plus years. We performed two experiments: (1) a growth chamber study that determined the growth rates of important invasive and native YNP grasses that are forages for ungulates under preindustrial (280 ppm) versus modern (410 ppm) CO2 levels and (2) a field study that compared the effect of defoliation (clipping) on the shoot growth of invasive and native mesic grassland plants under ambient CO2 conditions in 2019. The growth chamber experiment revealed that modern CO2 increased the growth rates of both invasive and native grasses, and invasive grasses grew faster regardless of CO2 conditions. The field results showed a continuum of positive to negative responses of shoot growth to defoliation, with a subgroup of invasive species responding most positively. Altogether the results indicated that the historical increase in CO2 and the spread of invasive species, some of which were planted to provide forage for ungulates in the early and mid‐1900s, have likely increased the capacity of forage production in YNP. However, rising CO2 has also resulted in regional warming and increased aridity in YNP, which will likely reduce grassland productivity. The challenge for global change biologists and park managers is to determine how competing components of global change have already affected and will increasingly affect forage dynamics and the sustainability of Yellowstone's iconic ungulate herds in the Anthropocene. [ABSTRACT FROM AUTHOR]

We explore the ability of the atmospheric CO2 record since 1900 to constrain the source of CO2 from land use and land cover change (hereafter "land use"), taking account of uncertainties in other terms in the global carbon budget. We find that the atmospheric constraint favors land use CO2 flux estimates with lower decadal variability and can identify potentially erroneous features, such as emission peaks around 1960 and after 2000, in some published estimates. Furthermore, we resolve an offset in the global carbon budget that is most plausibly attributed to the land use flux. This correction shifts the mean land use flux since 1900 across 20 published estimates down by 0.35 PgC year−1 to 1.04 ± 0.57 PgC year−1, which is within the range but at the low end of these estimates. We show that the atmospheric CO2 record can provide insights into the time history of the land use flux that may reduce uncertainty in this term and improve current understanding and projections of the global carbon cycle. [ABSTRACT FROM AUTHOR]

Climate change and rising atmospheric CO 2 levels are critical factors influencing agricultural productivity, particularly in Morocco, where cereal crops are essential for food security. The primary objective of this study is to evaluate the combined effects of atmospheric CO 2 variations and climatic changes on cereal yields up to 2099 using the CARAIB dynamic vegetation model. This evaluation is driven by four future scenarios based on the Euro-CORDEX initiative's regional climate models under the Representative Concentration Pathway 8.5. As part of this evaluation, the study also validates the CARAIB model for Morocco's major cereal crops: soft wheat, durum wheat, and barley, in order to ensure the model's accuracy in simulating crop responses under projected environmental conditions. The CARAIB model effectively simulated historical cereal yield trajectories across major farming regions in Morocco from 2000 to 2016, demonstrating its robust predictive capability. Our future projections suggest that elevated CO 2 levels might initially sustain cereal yields at approximately 90 % of current levels until 2050. This trend indicates that the increase in atmospheric CO 2 may exert a moderating influence on the negative impacts of other environmental stressors on crop yields. However, despite this initial buffering effect, the overall yield trend from the present until 2099 indicates a decrease for most combinations of crop, zone, and climate model, even with the CO 2 fertilization effect, except in some cases, the model exhibits slight increases or stabilization in yields. Additionally, the CARAIB model predicts potential yield shocks of 10–35 % below current levels from the 2080 s onwards, primarily due to periodic droughts. This variation underscores the complexity of the interplay between CO 2 fertilization and climatic changes, emphasizing the urgent need for Morocco to develop adaptive agricultural strategies for long-term sustainability in the face of climatic challenges. • Evaluated CO 2 /climate change impacts on Morocco's cereal yields with CARAIB. • Validated CARAIB for Morocco's cereals: soft wheat, durum wheat, and barley. • Forecasted cereal yields stay near 90 % of current levels by 2050 due to CO 2 rise. • Long-term yield decrease expected, despite CO 2 buffer, significant after 2080. • Urged adaptive strategies for Moroccan agriculture against climate change. [ABSTRACT FROM AUTHOR]

Covalent organic frameworks (COFs) and ionic liquids (ILs) as raw materials exhibit effective catalytic performance and adsorption capabilities in the reaction of epoxides and atmospheric CO 2 , respectively. Nonetheless, bonding modes between ILs and COFs have restricted their utility, thereby constraining the in-depth study of synergistic catalytic mechanisms over these composite catalysts. Herein, a heterogeneous composite ([DBUH] 2 Cys@COF-X) were successfully prepared by the facile acid-base neutralization method, incorporating varying amounts of amino acid ILs ([DBUH] 2 Cys) into the pore channels of sulfonic acid-based COF (TpPa-SO 3 H). [DBUH] 2 Cys@COF-3 efficiently catalyzed epichlorohydrin with a yield of 96.38 % without any solvent or co-catalyst under atmospheric CO 2. Notably, larger sterically hindered epoxides were selectively blocked on the surface of [DBUH] 2 Cys@COF-3, enabling [DBUH] 2 Cys@COF-3 to act as a gatekeeper for selectively catalyzing small sterically hindered epoxides due to [DBUH] 2 Cys was distributed within the pore channels of [DBUH] 2 Cys@COF-3. Through DFT calculations, comprehensive understanding of the synergistic activation conferred of -COO- and [DBUH]+ towards CO 2 and epoxides in [DBUH] 2 Cys@COF-3 and the ring-opening mechanism was clarified. This work presents a fresh outlook on the catalyst design by the facile acid-base neutralization method for efficiently catalyzing small-hindered epoxides under atmospheric CO 2. [Display omitted] • Amino acid ionic liquids immobilized onto covalent organic frameworks were prepared by the acid-base neutralization method. • [DBUH] 2 Cys@COF-3 efficiently catalyzed epichlorohydrin with the yield of 96.38% under atmospheric CO 2. • The porous structure of [DBUH] 2 Cys@COF-3 inhibited large steric hindrance epoxides and selectively catalyzed small ones. • The catalytic mechanism of CO 2 cycloaddition over [DBUH] 2 Cys@COF-3 was deeply explored. [ABSTRACT FROM AUTHOR]

Radiocarbon (Δ14C) was measured for four years (2019–2022) in Zagreb (Croatia) and in Cvetković village near Jastrebarsko (Zagreb County, Croatia) to see whether there are differences between the city site and the rural one because of the fossil fuel combustion. The δ13C CO2 was measured as grab samples once in a month in period December 2020–November 2022. The bomb-produced 14C has been globally distributed across the planet, but the combustion of fossil fuels that do not contain 14C causes a local effect of lowering Δ14C. Zagreb is considered to be a location with heavy fossil fuel combustion as compared to the Cvetković (rural site). Monthly 14C activity at Zagreb is constantly below the 14C activity at Cvetković. Mean 14C activity at Zagreb (Δ14C Zagreb = −18.4 ± 2.6 ‰) is lower than that in Cvetković (Δ14C Cve = −2.9 ± 2.1 ‰) due to fossil fuel combustion in the city of Zagreb. This is especially pronounced during winter when the mean value in Zagreb is Δ14C Zagreb = −26.0 ± 4.3 ‰ and in Cvetković Δ14C Cve = −5.9 ± 3.4 ‰. Natural gas consumption was used as the proxy for fossil fuel combustion, and it shows better correlation with Δ14C in Zagreb than in Cvetković. The Δ14C difference, Δ14C Cve ‒ Δ14C Zagreb , becomes statistically negligible when natural gas consumption is small. No difference is observed on δ13C CO2 ; in Zagreb mean δ13C CO2 is −11.0 ± 1.3 ‰, and in Cvetković −11.4 ± 1.4 ‰. Lower δ13C CO2 values are observed in winter (Zagreb −11.9 ± 1.1 ‰, −12.2 ± 1.5 ‰ Cvetković) than in summer (−10.1 ± 0.8 ‰ vs. −10.4 ± 1.0 ‰) at both locations. Together with higher Δ14C in Cvetković, it indicates that at the area of Cvetković biogenic samples of modern origin (biomass, wood) as energy source for heating is more pronounced. • Δ14C of atmospheric CO 2 in Zagreb was constantly lower than in village Cvetković. • Overall fossil fuel contribution in the city of Zagreb is 1.9 %, in winter 2.4 %. • Natural gas consumption better correlates with the Zagreb Δ14C data. • δ13C CO2 data are the same in Zagreb and in Cvetković; both show seasonal variation. • Origin of CO 2 in Cvetković is burning of contemporaneous organic matter (biomass). [ABSTRACT FROM AUTHOR]

Heatwaves, expected to become more frequent, pose a significant threat to plant biomass production. This experiment was designed to estimate heatwave influence on Brassica juncea phytoremediation when superimposed on different CO 2 levels. A 7-day heatwave was generated during the species flowering stage. Heatwaves decreased all B. juncea dry weights. The lowest species dry weight was recorded when the heatwave was accompanied by 250 ppm CO 2 , in which the biomass significantly decreased by 40.0% relative to that of no heatwave under the same atmospheric CO 2 conditions. Heatwave superposition with 250 ppm CO 2 reduced the Cd content in B. juncea aerial parts by 28.1% relative to that of identical environmental conditions without heatwave, whereas the opposite result was observed under 550 ppm CO 2 conditions. The heatwave caused oxidative damage to B. juncea under all CO 2 conditions, as manifested by increased malondialdehyde levels in the plant shoots. With heatwave superposition, antioxidant enzyme activity was enhanced by exposure to 400 and 550 ppm CO 2. Considering biomass yield generation and Cd uptake capacity, heatwave superposition decreased the B. juncea phytoremediation effects, and high atmospheric CO 2 conditions could alleviate detrimental effects to a certain extent. This study uniquely examines the combined effects of heatwaves and varying CO 2 levels on phytoremediation, providing microscopic insights into oxidative damage and enzyme activity, highlighting the potential for CO 2 enrichment to mitigate heatwave impacts, and offering comprehensive analysis for future agricultural practices and environmental management. [Display omitted] • Heatwaves caused the most pronounced B. juncea biomass decrease at 250 ppm CO 2. • Elevated CO 2 levels enhanced Cd phytoremediation effect, especially at 550 ppm CO 2. • Heatwaves and CO 2 interaction increased antioxidant activity but decreased Cd removal effect. [ABSTRACT FROM AUTHOR]

This article is a Commentary on Cassan et al. (2023), 239: 992–1004. [ABSTRACT FROM AUTHOR]

During the late Miocene, global cooling occurred alongside the establishment of near‐modern terrestrial and marine ecosystems. Significant (3°C–5°C) sea surface cooling from 7.5 to 5.5 Ma is recorded by proxies at midlatitudes to high latitudes, yet the magnitude of tropical cooling and the role of atmospheric carbon dioxide (pCO2) in driving this trend are debated. Here, we present a new orbital‐resolution sea surface temperature (SST) record spanning the late Miocene to earliest Pliocene (9–5 Ma) from the eastern equatorial Indian Ocean (International Ocean Discovery Program Site U1443) based on Mg/Ca ratios measured in tests of the planktic foraminifer Trilobatus trilobus. Our SST record reveals a 3.2°C decrease from 7.4 to 5.8 Ma, significantly increasing previous estimates of late Miocene tropical cooling. Analysis of orbital‐scale variability shows that before the onset of cooling, SST variations were dominated by precession‐band (19–23 kyr) variability, whereas tropical temperature became highly sensitive to obliquity (41 kyr) after 7.5 Ma, suggesting an increase in high‐latitude forcing. We compare a revised global SST database with new paleoclimate model simulations and show that a pCO2 decrease from 560 to 300 ppm, in the range suggested by pCO2 proxy records, could explain most of the late Miocene sea surface cooling observed at Site U1443. Using our new Site U1443 record as representative of tropical SST evolution, estimated meridional SST gradients suggest a much more modest increase over the late Miocene than previously suggested, in agreement with modeled meridional SST gradients. Plain Language Summary: The late Miocene is an interesting time period for paleoclimatologists because the Earth underwent important climatic and ecological changes that led to the establishment of our modern climate. An important cooling of global surface oceans was recorded by tracers used to reconstruct past temperature; however, the amplitude of this cooling in the tropics and the role of atmospheric carbon dioxide (CO2) in driving it are unclear. We present a new reconstruction of sea surface temperatures from the eastern equatorial Indian Ocean based on the temperature‐dependent ratio of magnesium to calcium measured in fossil shells of zooplankton (Foraminifera). Our results reveal a cooling (3.2°C) higher than previous estimates of tropical ocean cooling (1°C–2.5°C). To understand the role of atmospheric CO2 in driving this cooling, we simulated Miocene climate using a complex model and find that an atmospheric CO2 decrease from 560 to 300 ppm could explain most of the reconstructed surface ocean cooling. We also find that ocean surface temperature gradients between the tropics (using our new reconstruction) and the northern high latitudes shows a more modest increase over the late Miocene than suggested by previous studies, in agreement with new and existing climate model results. Key Points: New late Miocene orbital‐resolution Mg/Ca sea surface temperature record from the eastern equatorial Indian Ocean spanning 9–5 MaTropical cooling is >3°C; new model simulations suggest that a pCO2 decrease from 560 to 300 ppm could account for most of this coolingIncrease in meridional sea surface temperature gradients over the late Miocene more modest than previously suggested [ABSTRACT FROM AUTHOR]

Earth's long‐term climate has been profoundly influenced by the episodic assembly and breakup of supercontinents at intervals of ca. 500 m.y. This reflects the cycle's impact on global sea level and atmospheric CO2 (and other greenhouse gases), the levels of which have fluctuated in response to variations in input from volcanism and removal (as carbonate) by the chemical weathering of silicate minerals. Supercontinent amalgamation tends to coincide with climatic cooling due to drawdown of atmospheric CO2 through enhanced weathering of the orogens of supercontinent assembly and a thermally uplifted supercontinent. Conversely, breakup tends to coincide with increased atmospheric CO2 and global warming as the dispersing continental fragments cool and subside, and weathering decreases as sea level rises. Supercontinents may also influence global climate through their causal connection to mantle plumes and large igneous provinces (LIPs) linked to their breakup. LIPs may amplify the warming trend of breakup by releasing greenhouse gases or may cause cooling and glaciation through sulfate aerosol release and drawdown of CO2 through the chemical weathering of LIP basalts. Hence, Earth's long‐term climatic trends likely reflect the cycle's influence on sea level, as evidenced by Pangea, whereas its influence on LIP volcanism may have orchestrated between Earth's various climatic states. [ABSTRACT FROM AUTHOR]

Increases in terrestrial water‐use efficiency (WUE) have been reported in many studies, pointing to potential changes in physiological forcing of global carbon and hydrological cycles. However, gains in WUE are of uncertain magnitude over longer (i.e. >10 years) periods of time largely owing to difficulties in accounting for structural and physiological acclimation. 13C signatures (i.e. δ13C) of plant organic matter have long been used to estimate WUE at temporal scales ranging from days to centuries. Mesophyll conductance is a key uncertainty in estimated WUE owing to its influence on diffusion of CO2 to sites of carboxylation. Here we apply new knowledge of mesophyll conductance to 464 δ13C chronologies in tree‐rings of 143 species spanning global biomes. Adjusted for mesophyll conductance, gains in WUE during the 20th century (0.15 ppm year−1) were considerably smaller than those estimated from conventional modelling (0.26 ppm year−1). Across the globe, mean sensitivity of WUE to atmospheric CO2 was 0.15 ppm ppm−1. Ratios of internal‐to‐atmospheric CO2 (on a mole fraction basis; ci/ca) in leaves were mostly constant over time but differed among biomes and plant taxa—highlighting the significance of both plant structure and physiology. Together with synchronized responses in stomatal and mesophyll conductance, our results suggest that ratios of chloroplastic‐to‐atmospheric CO2 (cc/ca) are constrained over time. We conclude that forest WUE may have not increased as much as previously suggested and that projections of future climate forcing via CO2 fertilization may need to be adjusted accordingly. [ABSTRACT FROM AUTHOR]

The increase in CO2 concentration in the atmosphere has prompted the research community to find solutions for this environmental problem, which causes climate change and global warming. CO2 removal through the use of negative emissions technologies could lead to global emission levels becoming net negative towards the end of this century. Among these negative emissions technologies, direct air capture (DAC), in which CO2 is captured directly from the atmosphere, could play an important role. The captured CO2 can be removed in the long term and through its storage can be used for chemical processes, allowing closed carbon cycles in the short term. For DAC, different technologies have been suggested in the literature, and an overview of these is proposed in this work. Absorption and adsorption are the most studied and mature technologies, but others are also under investigation. An analysis of the main key performance indicators is also presented here and it is suggested that more efforts should be made to develop DAC at a large scale by reducing costs and improving efficiency. An additional discussion, addressing the social concern, is indicated as well. [ABSTRACT FROM AUTHOR]

BACKGROUND Elevated CO2 usually reduces levels of proteins and essential micronutrients in crops. The adoption of early maturing varieties may minimize the deleterious effect of climate change on farming activities. Legumes stand out for their high nutritional quality, so the objective was to study whether the atmospheric CO2 concentration affected the growth, yield, and food quality of early maturing cultivars of peas, snap beans, and faba beans. Plants grew in greenhouses either at ambient (ACO2, 392 μmol mol−1) or under elevated (ECO2, 700 μmol mol−1) CO2 levels. Minerals, proteins, sugars, and phenolic compounds were measured in grains of peas and faba beans, and in pods of snap beans. RESULTS: The effect of ECO2 depended on legume species, being more evident for food quality than for vegetative growth and yield. The ECO2 increased Fe and P in faba bean grains, and Ca in snap bean pods. Under ECO2, grains of pea and faba bean increased levels of proteins and phenolics, respectively, and the sugars‐to‐protein ratio decreased in pods of snap beans. CONCLUSION: Early maturing varieties of legumes appear to be an interesting tool to cope with the negative effects that a long exposure to rising CO2 can exert on food quality. © 2021 The Authors. Journal of The Science of Food and Agriculture published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry. [ABSTRACT FROM AUTHOR]

Observations of ice sheet stability in Antarctica in the past, during significantly warmer climatic periods than today, are hampered by a paucity of continental outcrops but they are essential if we are to test and more fully understand established concepts of ice sheet growth and decay and robustly predict Earth's future during rapid global warming. In this paper we use the unique terrestrial record preserved in the three most southerly volcanoes on Earth to reconstruct Antarctica's environmental landscape in the Transantarctic Mountains for three time slices (at 20.1, 19.31 and c. 17 Ma). We demonstrate that the East Antarctic Ice Sheet margin was highly dynamic and fluctuated greatly during the Early Miocene. The period included a much-reduced ice cover that left the southern Transantarctic Mountains virtually ice-free, although a few tidewater glaciers reached the shores of the Early Miocene Ross Sea (Sheridan Bluff; 20.1 Ma); and a substantially thicker, much more voluminous ice mass (represented by volcanism at Mount Early; 19.31 Ma). An intermediate stage of ice retreat is also indicated (at c. 17 Ma), when the East Antarctic Ice Sheet had again withdrawn to behind the Transantarctic Mountains. It is represented by a subglacial volcanic centre identified remotely by aerogeophysics and sampled only as glacial erratics. The associated ice cover at c. 17 Ma had a relatively low surface elevation but it was confluent with thin ice domes that covered the adjacent mountains. Although the ice margin probably did not extend down to the Ross Sea coast, several glaciers fed icebergs into the Ross Sea. The large ice sheet indicated by the Mount Early volcanic centre is the only known physical record of a significant glacial event that took place at 19.31 ± 0.32 Ma ('Mount Early glacial'). The records examined at the other two volcanic sites correspond to eruptions during periods of substantially reduced ice in Antarctica, of which that at 20.1 ± 0.4 Ma is the most remarkable ('Sheridan Bluff interglacial'). The causes of the extreme variations in ice sheet thickness and extent indicated are uncertain but they occurred during a period of rapidly fluctuating atmospheric CO 2 compositions. Although Milankovitch astronomical forcing probably also played a significant part, the results suggest that the East Antarctic Ice Sheet was highly susceptible to comparatively slight changes in CO 2 during the warm Early Miocene. • Transantarctic Mountains landscapes reconstructed using glaciovolcanic sequences for three time slices in the Early Miocene • Large, deep, unfrozen freshwater lake identified at 87°S and c. 2000 m elevation, not predicted by current modelling • Very large ice sheet recognized, more expanded than today, which overtopped the Transantarctic Mountains • Ross Sea margin of the East Antarctic Ice Sheet was highly dynamic between 20 and 17 Ma • Ice sheet margin was highly sensitive to comparatively small changes in CO 2 [ABSTRACT FROM AUTHOR]

• The stronger susceptibility and shorter response time of vegetation productivity to drought were most observed in grasslands and dry regions. • Global drought-sensitive regions have experienced a predominately increasing susceptibility and shortening response time during the last two decades (2001-2021). • Elevated atmospheric CO 2 largely contribute to increasing susceptibility especially in grasslands and drylands. Drought generally causes the significant reduction of vegetation productivity. Most studies focus on the vegetation response to gradual water variabilities instead of anomalies in water availability, however, the spatiotemporal patterns of vegetation response to extreme water deficits during drought are still unclear. Here, based on leaf area index (LAI) and solar-induced chlorophyll fluorescence (SIF), we employed coincidence analysis to identify the susceptibility and response time of vegetation productivity to drought, and further investigated the spatiotemporal changes of them during 2001-2021 across the globe. We found stronger susceptibility and shorter response time both in areas occupied by more grasses and in more arid regions. Temporally, we revealed a predominately increasing susceptibility (P < 0.05) and shortening response time (P < 0.05) in drought-sensitive regions from 2001 to 2021. These changes have been majorly attributable to the elevated atmospheric CO 2 , which tended to strengthen the susceptibility in areas covered by more grasses and in arid regions. Our finding highlighted the increasing risk of decline in vegetation production under drought particular in grasslands and drylands. [ABSTRACT FROM AUTHOR]

We conducted a study to fully understand carbon dioxide (CO2) adsorption to a cylinder's internal surface and fractionation of CO2 and air during the preparation of standard mixtures of atmospheric CO2 levels through a multistep dilution. The CO2 molar fractions in standard mixtures prepared by diluting pure CO2 with air three times deviated by -0.207 ± 0.060 µmol mol-1 on average from the gravimetric 15 values which were calculated from masses of source materials by evaluating their CO2 molar fractions based on standard mixtures by diluting the pure CO2 with the air only once. It indicates that the deviation is larger than a compatibility goal of 0.1 µmol mol-1, which has been recommended by the World Meteorological Organization (WMO). The deviations were consistent with those calculated from the fractionation factors of 0.99968 ± 0.00010 and 0.99975 ± 0.00004 estimated in mother-daughter transfer 20 experiment that transfer CO2/Air mixtures from a cylinder to another evacuated receiving cylinder and by applying the Rayleigh model to the increase in CO2 molar fractions in source gas as pressure depleted from 11.5 MPa to 1.1 MPa. Both fractionation factors also agree within their uncertainties. Additionally, the mother-daughter transfer experiments showed that the deviation was caused by the fractionation of CO2 and air in the process of transferring a source gas (a CO2/Air mixture with a higher CO2 molar fraction 25 than that in the prepared gas mixture). The fact that the CO2 molar fraction weakened significantly as the transfer speed decreased suggested that the main factor of the fractionation could be thermal diffusion. However, experiments exiting a CO2 in air mixture (CO2/Air mixture) from a cylinder were conducted to evaluate the CO2 adsorption to the internal surface of the cylinder. As the cylinder pressure was reduced from 11.0 to 0.1 MPa, the CO2 molar fractions in the mixture flow leaving from the cylinder increased the CO2 molar fractions by 0.16 ± 0.04 µmol mol-1. By applying 30 the Langmuir adsorptiondesorption model to the measured data, the amount of CO2 adsorbed on the internal surfaces of a 10 L aluminum cylinder when preparing a standard mixture with atmospheric CO2 level was estimated to be 0.027 ± 0.004 µmol mol-1 at 11.0 MPa. [ABSTRACT FROM AUTHOR]

We use data on the following climate variables for the period of the last 798 thousand years: global ice volume ( Ice t ), atmospheric carbon dioxide level ( CO 2 , t ), and Antarctic land surface temperature ( Temp t ). Those variables are cyclical and are driven by the following strongly exogenous orbital variables: eccentricity of the Earth's orbit, obliquity, and precession of the equinox. We introduce score-driven ice-age models which use robust filters of the conditional mean and variance, generalizing the updating mechanism and solving the misspecification of a recent climate–econometric model (benchmark ice-age model). The score-driven models control for omitted exogenous variables and extreme events, using more general dynamic structures and heteroskedasticity. We find that the score-driven models improve the performance of the benchmark ice-age model. We provide out-of-sample forecasts of the climate variables for the last 100 thousand years. We show that during the last 10–15 thousand years of the forecasting period, for which humanity influenced the Earth's climate, (i) the forecasts of Ice t are above the observed Ice t , (ii) the forecasts of CO 2 , t level are below the observed CO 2 , t , and (iii) the forecasts of Temp t are below the observed Temp t . The forecasts for the benchmark ice-age model are reinforced by the score-driven models. [ABSTRACT FROM AUTHOR]

Changes in ocean carbon inventory are considered the likely primary driver of declining glacial atmospheric pCO2 through the Mid‐Pleistocene Transition (MPT). Here, we present global core‐top calibrations of the "size‐normalized weight" (SNW) of planktonic foraminifera for estimating paleo‐deep‐water Δ[CO32−]. Then, we apply this approach to reconstruct deep‐water Δ[CO32−] in the western tropical Pacific since ∼1.4 Ma. At ∼1.0–0.9 Ma, a rapid weakening of North Atlantic Deep Water (NADW) resulted in a transient increase of ∼10 μmol kg−1 in deep‐water Δ[CO32−], after which deep‐water Δ[CO32−] declined at ∼0.9–0.7 Ma. Glacial and interglacial deep‐water Δ[CO32−] reveals stepwise declines of about 7 and 10 μmol kg−1, respectively, after MIS 20 (0.79–0.81 Ma) and MIS 19 (0.76–0.79 Ma), reflecting increased DIC content in the deep Pacific. We infer that a sustained increase in deep Pacific carbon storage following the MPT was linked to enhanced oceanic stratification and greater influence of CO2‐rich Southern Ocean‐sourced waters. Plain Language Summary: Cyclic continental glaciation has been a primary feature of the Earth's climate system for the past 2 million years. At around 900,000 years ago, the climate system shifted from shorter, less intense to longer, stronger cycles. This climate shift, known as the Mid‐Pleistocene Transition (MPT), was marked by a decline of atmospheric pCO2. Previous geological records have shown that atmospheric CO2 was stored in the deep Atlantic Ocean at that time. The present study provides evidence for greater carbon storage in the deep Pacific Ocean. We infer that enhanced global deep‐ocean carbon storage during the MPT was due to expansion of sea ice in the Southern Ocean. Increased sea‐ice cover reduced outgassing of CO2 from the deep ocean. CO2‐rich deep waters from the Southern Ocean were transported around the world, making the Pacific Ocean an important carbon reservoir during the MPT. Key Points: Global core‐top SNW‐Δ[CO32−] calibrations are developedGlacial and interglacial deep Pacific Δ[CO32−] decreased by 7 and 10 μmol kg−1, respectively, during the MPTDeep Pacific carbon storage increased during the MPT due to enhanced Southern Ocean stratification [ABSTRACT FROM AUTHOR]

Widespread changes in forest structure and distribution have been documented in northern Patagonia over the past century. We employed LPJ-GUESS, a dynamic global vegetation model (DGVM) to investigate the role of climate, atmospheric carbon dioxide (CO2), and fire on simulated forest cover during the twentieth century. Our objective was to assess the drivers responsible for forest change to temperature, precipitation, radiation, fire and atmospheric CO2 Results: Simulations using observed changes in climate and CO2 from 1930 to 2010, showed an increase in forest cover under changing climate and CO2, because of higher carbon assimilation and net primary production. The model results were compared with a remote-sensing-derived biomass map and 'greening' indices from the normalized difference vegetation index. Model simulations and satellite data both show increased greening at high and low elevations. In contrast, simulations using pre-industrial climate and CO2 conditions resulted in a decrease in fire frequency and lower simulated biomass than is reflected by present-day vegetation. Conclusion: Our simulations shows that climate is the primary driver and CO2 fertilization is the secondary driver of forest expansion in northern Patagonia. We suggest that rising CO2 mitigates climate-induced drought stress due to increases in water-use efficiency. [ABSTRACT FROM AUTHOR]

The world's oceans have played an important role in sequestering atmospheric carbon dioxide through solubility and the action of algae. Fixation of atmospheric carbon dioxide by photoautotrophic algal cultures has the potential to diminish the release of carbon dioxide into the atmosphere, thereby helping to alleviate the trend toward global warming. This work investigates the role of algae in controlling the level of atmospheric carbon dioxide. Partial Rank Correlation Coefficients (PRCCs) technique is used to address how the concentration of atmospheric carbon dioxide is affected by changes in a specific parameter disregarding the uncertainty over the rest of the model parameters. Parameters related to algal growth are shown to significantly reduce the level of atmospheric CO2. Further, we explore the dynamics of nonautonomous system by incorporating the seasonal variations of some ecologically important model parameters. Our nonautonomous system exhibits globally attractive positive periodic solution, and also the appearance of double periodic solution is observed. Moreover, by letting the seasonally forced parameters as almost periodic functions of time, we show almost periodic behavior of the system. Our findings suggest that the policy makers should focus on continuous addition of nutrients in the ocean to accelerate the algal growth thereby reducing the level of carbon dioxide in the atmosphere. [ABSTRACT FROM AUTHOR]

Model intercomparison studies of coupled carbon‐climate simulations have the potential to improve our understanding of the processes explaining the pCO2 drawdown at the Last Glacial Maximum (LGM) and to identify related model biases. Models participating in the Paleoclimate Modeling Intercomparison Project (PMIP) now frequently include the carbon cycle. The ongoing PMIP‐carbon project provides the first opportunity to conduct multimodel comparisons of simulated carbon content for the LGM time window. However, such a study remains challenging due to differing implementation of ocean boundary conditions (e.g., bathymetry and coastlines reflecting the low sea level) and to various associated adjustments of biogeochemical variables (i.e., alkalinity, nutrients, dissolved inorganic carbon). After assessing the ocean volume of PMIP models at the pre‐industrial and LGM, we investigate the impact of these modeling choices on the simulated carbon at the global scale, using both PMIP‐carbon model outputs and sensitivity tests with the iLOVECLIM model. We show that the carbon distribution in reservoirs is significantly affected by the choice of ocean boundary conditions in iLOVECLIM. In particular, our simulations demonstrate a ∼250 GtC effect of an alkalinity adjustment on carbon sequestration in the ocean. Finally, we observe that PMIP‐carbon models with a freely evolving CO2 and no additional glacial mechanisms do not simulate the pCO2 drawdown at the LGM (with concentrations as high as 313, 331, and 315 ppm), especially if they use a low ocean volume. Our findings suggest that great care should be taken on accounting for large bathymetry changes in models including the carbon cycle. Key Points: Ocean volume is a dominant control on Last Glacial Maximum (LGM) carbon sequestration and must be accurately represented in modelsAdjusting the alkalinity to account for the relative change of volume at the LGM induces a large increase of oceanic carbon (of ∼250 GtC)Paleoclimate Modeling Intercomparison Project‐carbon models standardly simulate high LGM CO2 levels (over 300 ppm) despite a larger proportion of carbon in the ocean at LGM than pre‐industrial [ABSTRACT FROM AUTHOR]

Previous studies have reported that human influences are required to explain the observed global warming using a linear model (LM) ΔT=λLMΔF that relates change in solar forcing ΔF to change in global mean temperature (GMT) ΔT. This model has the shortcoming of assuming a given ΔF causes the same ΔT irrespective of the value of the initial global warming rate (dT/dy)i. Analysis of the GMT data showed that this warming rate has been increasing linearly since steady state ((dT/dy)o=0 for year yo=1864.5) as given by dT/dy=(dT/dy)i+aT(y−yi), where aT=7.1954×10−5 °C/year2 is the secular GMT acceleration and y−yi is the number of years of the change. The secular solar forcing due to 18% of the 11 yr solar cycle forcing of 0.19 W/m2 (0.08% of Total Solar Irradiance) was expressed as ΔF=0.18×0.19(y−yi)/11. Defining the climate sensitivity as λ=Δ(dT/dy)/ΔF=aT/(0.18×0.19/11)=0.023143 °C/year per W/m2 removed the shortcoming of the LM and integration of the model for dT/dy above and then simplifying gave a secular GMT‐solar forcing model given by ΔT=(dT/dy)i(y−yi)+(λ2/(2aT))(ΔF)2 that explained all of the observed global warming and increase in atmospheric CO2, sea level and ocean heat content. Therefore, for this nonlinear empirical model, invoking human influences to explain climate change was not required. The annual GMT model predicts a pause in global warming until 2040. Key Points: For a nonlinear empirical mathematical model, solar forcing explained all of the observed global warmingsFor doubling of the annual atmospheric CO2 concentration, the global mean temperature (GMT) increased by 1.4°C and the sea level by 0.37 mThe empirical model for the 30 yr GMT moving trends predicts deceleration to less than 0.1°C/decade for period 2007–2037 [ABSTRACT FROM AUTHOR]

The ACT‐America project is a NASA Earth Venture Suborbital‐2 mission designed to study the transport and fluxes of greenhouse gases. The open and freely available ACT‐America data sets provide airborne in situ measurements of atmospheric carbon dioxide, methane, trace gases, aerosols, clouds, and meteorological properties, airborne remote sensing measurements of aerosol backscatter, atmospheric boundary layer height and columnar content of atmospheric carbon dioxide, tower‐based measurements, and modeled atmospheric mole fractions and regional carbon fluxes of greenhouse gases over the Central and Eastern United States. We conducted 121 research flights during five campaigns in four seasons during 2016–2019 over three regions of the US (Mid‐Atlantic, Midwest and South) using two NASA research aircraft (B‐200 and C‐130). We performed three flight patterns (fair weather, frontal crossings, and OCO‐2 underflights) and collected more than 1,140 h of airborne measurements via level‐leg flights in the atmospheric boundary layer, lower, and upper free troposphere and vertical profiles spanning these altitudes. We also merged various airborne in situ measurements onto a common standard sampling interval, which brings coherence to the data, creates geolocated data products, and makes it much easier for the users to perform holistic analysis of the ACT‐America data products. Here, we report on detailed information of data sets collected, the workflow for data sets including storage and processing of the quality controlled and quality assured harmonized observations, and their archival and formatting for users. Finally, we provide some important information on the dissemination of data products including metadata and highlights of applications of ACT‐America data sets. Plain Language Summary: We describe the data collected and produced by the Atmospheric Carbon and Transport – America mission, including airborne and tower‐based measurements of greenhouse gases (e.g., carbon dioxide and methane) and modeled atmospheric mole fractions and regional carbon fluxes of greenhouse gases over North America. In this paper, we briefly describe the data collections and archival including the instruments and methodology used to generate, manage, and distribute the data, and the significance of these new measurements for the study of the North American carbon cycle. Key Points: Atmospheric Carbon and Transport – America (ACT‐America) provides a unique, weather‐oriented collection of atmospheric CO2, CH4, trace gases, and meteorological properties measurementsACT‐America data are free and open to the public from the Oak Ridge National Laboratory Distributed Active Archive Center (ORNL DAAC)ACT‐America data are uniquely suited to improve the accuracy and precision of regional inverse greenhouse gas (GHG) flux estimates [ABSTRACT FROM AUTHOR]

Sphagnum mosses account for most accumulated dead organic matter in peatlands. Therefore, understanding their responses to increasing atmospheric CO2 is needed for estimating peatland C balances under climate change. A key process is photorespiration: a major determinant of net photosynthetic C assimilation that depends on the CO2 to O2 ratio. We used climate chambers to investigate photorespiratory responses of Sphagnum fuscum hummocks to recent increases in atmospheric CO2 (from 280 to 400 ppm) under different water table, temperature, and light intensity levels. We tested the photorespiratory variability using a novel method based on deuterium isotopomers (D6S/D6R ratio) of photosynthetic glucose. The effect of elevated CO2 on photorespiration was highly dependent on water table. At low water table (−20 cm), elevated CO2 suppressed photorespiration relative to C assimilation, thus substantially increasing the net primary production potential. In contrast, a high water table (~0 cm) favored photorespiration and abolished this CO2 effect. The response was further tested for Sphagnum majus lawns at typical water table levels (~0 and −7 cm), revealing no effect of CO2 under those conditions. Our results indicate that hummocks, which typically experience low water table levels, benefit from the 20th century's increase in atmospheric CO2. With isotope data, we show that recent increases in atmospheric CO2 have suppressed photorespiration relative to carbon assimilation in Sphagnum peat mosses, particularly for hummocks with typically low water table levels. High water table levels favor photorespiration and abolish this CO2 effect. [ABSTRACT FROM AUTHOR]

The continuously increasing concentrations of atmospheric CO2 is predicted to affect biological processes at many levels of organisms. Yet, no study exists in the literature attempting to describe that the elevated atmospheric CO2 (eCO2) concentration may cause an evolutionary response on nucleotide sequences of ribosomal DNA of Myzus persicae [(Sulzer) (Hemiptera: Aphididae)]. Here, we provide a preliminary study to understand how the insect ribosomal DNA sequences are influenced under the elevated CO2 levels after several generations. Four M. persicae populations were established for 35 days under ambient CO2 (a CO2) (400 ppm), e CO2 (600 ppm), e CO2 (800 ppm) and e CO2 (1000 ppm) at 29ºC in moisture-controlled greenhouse chambers. Intraspecific variation of M. periscae was assessed by the sequencing and analyzing the internal transcribed spacer (ITS) region of Myzus persicae (Sulzer) under elevated atmospheric CO2 pressure. Based on our results, the phylogenetic analysis of ITS sequences differentiated the individuals grown at 800 ppm CO2 level. The alignment of ITS sequences of all specimens revealed several single-nucleotide substitutions on the nucleotide sequence of M. persicae samples grown at 800 ppm CO2 level. Overall results show that the elevated atmospheric CO2 levels could be a powerful evolutionary force than expected on M. persicae reared on eggplants. [ABSTRACT FROM AUTHOR]

In the early 20th century, after contributing major advances in calculating radiation forcing on planetary bodies, Milutin Milankovitch the Serbian mathematician took up the challenge of explaining why Earth has experienced recurrent episodes of glaciation. Influenced by the ideas of his predecessors, Milankovitch developed a theory that centered on the notion that summertime temperature at high northern latitudes is the most important influence on the advance and retreat of glaciations. The calculations revealed a periodicity in summer insolation that had a reasonable correspondence with what was then known about the occurrence of ice ages. From that was born the elemental foundation of the orbital theory of the ice ages. That theory evolved over the next three decades while retaining the fundamental tenant that summer season insolation at the higher northern latitudes determines Earth's climate variability. Scientists of the day were skeptical, and it was not until the 1960s that new techniques became available to test the temporal predictions of Milankovitch's theory. The orbital theory gained support in the 1950s and 60s when methods for paleoclimate reconstructions documented an orbital-like recurrence pattern of cold and warm climate conditions spanning the past 2.5 million years. Accompanying the documentation of Earth's climate rhythmicity from marine archives have been advances in other areas, including ice core records of atmospheric CO 2 that pose challenges to the original orbital theory, namely what role have variations in atmospheric CO 2 played in dictating the transitions from warm to cold and, what caused orbital scale variations in greenhouse gas concentrations? In this contribution we review the current state of knowledge about the Earth's carbon cycle on glacial/interglacial timescales and explore how new information has begun to shed light on the long-standing goal to understand Earth's natural climate rhythmicity. The findings presented here highlight the need to expand research on Earth's geologic processes that influence the carbon budget on glacial timescales. And with this comes a new hypothesis that incorporates geologic processes in orbital scale climate cycles. • The original Milankovitch theory for ice ages must be modified to include processes operating on orbital timescales that affect atmospheric CO 2. • The prevailing ocean only model for explaining Pleistocene CO 2 variability cannot be validated with available data. • Recent findings reveal carbon leakage from geologic reservoirs during the last glacial and deglacial cycle. • A hypothesis is proposed that argues glaciations were not terminated but were interrupted, in part by transient episodes of carbon leakage to the ocean/atm. [ABSTRACT FROM AUTHOR]

Global sea level fell progressively during the Pliocene in response to the growth of ice sheets at high northern latitudes. However, not until ∼2.7 Ma did the climate system actually generate and maintain major ice sheets in the Northern Hemisphere. A decline in atmospheric CO 2 levels (p CO 2) is thought to be responsible for the Pleistocene ice ages. However, the link between p CO 2 and Pliocene glaciations, or their delay, is not well understood, mainly because the magnitude of p CO 2 remains uncertain beyond the maximum age of ice cores (∼0.8 Ma). Here, we develop a carbonate clumped isotope (∆ 47) geothermometer specifically for pedogenic carbonate in Chinese loess, which we use to reconstruct temperatures in northern China, and p CO 2 , for the past 7.24 Ma. The palaeotemperature results show an oscillatory cooling trend over time in the East Asian monsoon region, in good agreement with coeval tropical sea surface temperature records. The p CO 2 reconstruction using the pedogenic carbonate palaeobarometer reveals low values (150–325 ppmv) during the intervals of 7.24–4.3 Ma and 2.6–0 Ma. In contrast, there is a three-step rise in p CO 2 up to 535 ppmv from 4.3 to 2.6 Ma, coincident with an oscillatory decrease in sea level and in benthic δ13C. We suggest that the Pliocene CO 2 rise was causally related to the sea-level fall and associated organic carbon oxidation on exposed continental shelves, thereby delaying the onset of the ice age. This implies a negative feedback mechanism induced by long-term cooling since the Late Miocene, which may have delayed the onset of the ice age but did not prevent it. • A carbonate clumped isotope (∆ 47) geothermometer was developed for Chinese loess. • Palaeotemperature reconstruction shows an oscillatory cooling trend since 7.24 Ma. • Low p CO 2 during 7.24–4.3 and 2.6–0 Ma estimated from calcic paleosols. • Sea-level falls during 4.3–2.6 Ma led to CO 2 release from continental shelves. • Cooling-induced p CO 2 rise as a negative feedback mechanism for the delayed ice age. [ABSTRACT FROM AUTHOR]

The significance of investigations into CO 2 adsorption from the air has been steadily on the rise in recent years. CeO 2 is known as an effective catalyst for CO 2 transformation and adsorbent of CO 2 , however, the study on the adsorption of low concentrations of CO 2 on CeO 2 and the effect of water vapor on the adsorption of CO 2 on CeO 2 is very limited. Herein, the adsorption behavior of low concentration of CO 2 , particularly atmospheric CO 2 (0.04 vol%), on CeO 2 and the effect of water vapor on the CO 2 adsorption were investigated by in situ FT-IR, mass spectroscopies, and DFT calculations. Low concentrations of CO 2 can be effectively adsorbed on CeO 2 , and typical four CO 2 adspecies such as bidentate carbonate, hydrogen carbonate, monodentate carbonate, and polydentate carbonate were formed on CeO 2. The CO 2 adsorption amount on CeO 2 was increased by 20% by the presence of water vapor compared with that in the absence of water vapor. Based on FT-IR analyses and DFT calculations, the adsorption strength of CO 2 is comparable to that of water on CeO 2 , and the water adspecies will assist the adsorption of CO 2 via hydrogen bonding, leading to the increase of CO 2 adsorption amount. [Display omitted] • Adsorption behavior of atmospheric CO 2 on CeO 2 was studied by FTIR and mass spectroscopy. • Atmospheric CO 2 was effectively adsorbed on CeO 2 , giving similar adspecies to that with pure CO 2. • CO 2 adsorption amount on CeO 2 was increased by 20% in the presence of water vapor. • The adsorbed water contributes to the increase of CO 2 adsorption amount based on DFT calculations. [ABSTRACT FROM AUTHOR]

• Carbon isotope discrimination (Δ13C) depends on phylogeny in C 3 woody plants. • Key differences identified between angiosperm and gymnosperm (large global dataset, n = 7098). • Δ13C is more sensitive to O 2 :CO 2 ratio in angiosperms; and to vapor pressure deficit in gymnosperms. • Variation of several per mil (‰) is possible, in geological contexts. • Paleo-pCO 2 is underestimated at typical VPD levels (1 kPa). For most of the Phanerozoic Eon, Earth's woody vegetation has been dominated by C 3 plants - predominantly gymnosperms - with angiosperms only emerging as the dominant plant group as CO 2 declined during the Cenozoic (66 Ma onward). At present, differences in carbon isotope discrimination (Δ 13 C) between angiosperm and gymnosperm plants are relatively small (2–3 ‰), but an increasing body of evidence points to larger differences across geological times (up to 6–7 ‰), potentially associated with varying environmental conditions and atmospheres (i.e. concentrations of atmospheric carbon dioxide, [CO 2 ], and oxygen, [O 2 ] could have ranged from ∼ 180 to 1100 ppm, and ∼ 15 to 25%, respectively, across the past 250 Ma). Yet, differences in Δ 13 C between the two plant groups, and their potential link to climatic and environmental changes, have not yet been fully explored and understood. Here, we combine a comprehensive ab initio model of discrimination, with a recent model of plant eco-physiology based on least-cost optimality theory, to show how differences in Δ 13 C between angiosperms and gymnosperms arise. We train the comprehensive model using a very large (n > 7000) database of leaf and tree ring data spanning the past 110 years. We find that averaged differences in Δ 13 C between angiosperm and gymnosperms decrease modestly with atmospheric [O 2 ]:[CO 2 ] ratios, and increase strongly with vapor pressure deficit (D). These relationships can be explained by three key physiological differences: (1) the ratio of cost factors for transpiration to carboxylation (higher in angiosperms); (2) the ratio of mesophyll to stomatal conductances of CO 2 (lower in gymnosperms); and (3) differences in photorespiration. In particular, the amount of CO 2 released from photorespiration per oxygenation reaction, λ , is generally lower in gymnosperms than in angiosperms. As a result of these factors, Δ 13 C is more sensitive to [CO 2 ] in angiosperms, and to D in gymnosperms. We propose a simplified empirical model to account for this behaviour, and test it against isotopic data from leaves, tree rings and previously-published plant chamber experiments, along with geological data from the Cenozoic. Overall, these data agree with our model over a range of [O 2 ]:[CO 2 ] ratios from 100 to 650 mol mol−1 (equivalent to a CO 2 range around 323–2100 ppm at 21% O 2), and D levels between 0.45 and 1.1 kPa (R2 = 0.51, RMSE = 1.49 ‰). Our simplified empirical model offers a new explanation for secular trends in the geological record, and suggests a way forward to improve paleo-[CO 2 ] proxies based on terrestrial discrimination models by incorporating the effects of [O 2 ], phylogeny, and photorespiration. Lastly, the framework predicts that the average difference in Δ 13 C between woody C 3 plant groups will increase in the future if both [CO 2 ] and global D continue to rise as suggested by projections. [ABSTRACT FROM AUTHOR]

The ocean's "biological pump" significantly modulates atmospheric carbon dioxide levels. However, the complexity and variability of processes involved introduces uncertainty in interpretation of transient observations and future climate projections. Much research has focused on "parametric uncertainty," particularly determining the exponent(s) of a power‐law relationship of sinking particle flux with depth. Varying this relationship's functional form introduces additional "structural uncertainty." We use an ocean biogeochemistry model substituting six alternative remineralization profiles fit to a reference power‐law curve, to systematically characterize structural uncertainty, which, in atmospheric pCO2 terms, is roughly 50% of parametric uncertainty associated with varying the power‐law exponent within its plausible global range, and similar to uncertainty associated with regional variation in power‐law exponents. The substantial contribution of structural uncertainty to total uncertainty highlights the need to improve characterization of biological pump processes, and compare the performance of different profiles within Earth System Models to obtain better constrained climate projections. Plain Language Summary: The ocean's "biological pump" regulates atmospheric carbon dioxide levels and climate by transferring organic carbon produced at the surface by phytoplankton to the ocean interior via "marine snow," where the organic carbon is consumed and respired by microbes. This surface to deep transport is usually described by a power‐law relationship of sinking particle concentration with depth. Uncertainty in biological pump strength can be related to different variable values ("parametric" uncertainty) or the underlying equations ("structural" uncertainty) that describe organic matter export. We evaluate structural uncertainty using an ocean biogeochemistry model by systematically substituting six alternative remineralization profiles fit to a reference power‐law curve. Structural uncertainty makes a substantial contribution, about one‐third in atmospheric pCO2 terms, to total uncertainty of the biological pump, highlighting the importance of improving biological pump characterization from observations and its mechanistic inclusion in climate models. Key Points: Six alternative flux profiles fit to a Martin Curve yield large differences in atmospheric carbonStructural uncertainty comprises one‐third of total uncertainty in the ocean's biological pumpVarying particle attenuation with depth may account for half of the biological pump's overall carbon drawdown [ABSTRACT FROM AUTHOR]

Increasing water‐use efficiency (WUE), the ratio of carbon gain to water loss, is a key mechanism that enhances carbon uptake by terrestrial vegetation under rising atmospheric CO2 (ca). Existing theory and empirical evidence suggest a proportional WUE increase in response to rising ca as plants maintain a relatively constant ratio between the leaf intercellular (ci) and ambient (ca) partial CO2 pressure (ci/ca). This has been hypothesized as the main driver of the strengthening of the terrestrial carbon sink over the recent decades. However, proportionality may not characterize CO2 effects on WUE on longer time‐scales and the role of climate in modulating these effects is uncertain. Here, we evaluate long‐term WUE responses to ca and climate from 1901 to 2012 CE by reconstructing intrinsic WUE (iWUE, the ratio of photosynthesis to stomatal conductance) using carbon isotopes in tree rings across temperate forests in the northeastern USA. We show that iWUE increased steadily from 1901 to 1975 CE but remained constant thereafter despite continuously rising ca. This finding is consistent with a passive physiological response to ca and coincides with a shift to significantly wetter conditions across the region. Tree physiology was driven by summer moisture at multi‐decadal time‐scales and did not maintain a constant ci/ca in response to rising ca indicating that a point was reached where rising CO2 had a diminishing effect on tree iWUE. Our results challenge the mechanism, magnitude, and persistence of CO2's effect on iWUE with significant implications for projections of terrestrial productivity under a changing climate. [ABSTRACT FROM AUTHOR]

Inter‐annual to decadal variability in the strength of the land and ocean carbon sinks impede accurate predictions of year‐to‐year atmospheric carbon dioxide (CO2) growth rate. Such information is crucial to verify the effectiveness of fossil fuel emissions reduction measures. Using a multi‐model framework comprising prediction systems initialized by the observed state of the physical climate, we find a predictive skill for the global ocean carbon sink of up to 6 years for some models. Longer regional predictability horizons are found across single models. On land, a predictive skill of up to 2 years is primarily maintained in the tropics and extra‐tropics enabled by the initialization of the physical climate. We further show that anomalies of atmospheric CO2 growth rate inferred from natural variations of the land and ocean carbon sinks are predictable at lead time of 2 years and the skill is limited by the land carbon sink predictability horizon. Plain Language Summary: Variations of the natural land and ocean carbon sinks in response to climate variability strongly regulate year‐to‐year variations in the growth rate of atmospheric carbon dioxide (CO2). Information on the near‐term evolution of the carbon sinks and CO2 in the atmosphere is necessary to understand where the anthropogenic carbon would go in response to emission reduction efforts addressing global warming mitigation. Predictions of this near‐term evolution would thus assist policy‐relevant analysis and carbon management activities. Here we use a set of prediction systems based on Earth system models to establish predictive skills of the ocean and land carbon sinks and to infer predictability of atmospheric CO2 growth rate. We show predictability horizons of up to 6 years for some models for the globally integrated ocean carbon sink, with even higher regional predictive skill. Variations of the land carbon sink are predictable up to 2 years and limit predictability of changes in atmospheric CO2 growth rate at lead time of 2 years. Our study thereby demonstrates an emerging capacity of the initialized simulations for predicting the global carbon cycle and the planet's breath maintained by variations of atmospheric CO2. Key Points: Predictive skill of the global ocean carbon sink due to initialization is up to 6 years for some models, with longer regional predictability in single modelsPredictive skill due to initialization for the land carbon sink of up to 2 years is primarily maintained in the tropics and extra‐tropicsAnomalies of atmospheric CO2 growth rate are predictable up to 2 years and are limited by the land carbon sink predictability horizon [ABSTRACT FROM AUTHOR]

There is general concern that the rapid increase in atmospheric CO2 concentration will lead to reduced stomatal conductance and subsequent increases in leaf temperature. Such an increase in leaf temperature is expected to adversely impact a plethora of processes connected to leaf metabolism and microbial/fungal communities on leaves. A model is proposed that combines the leaf energy balance with leaf gas exchange and photosynthesis to explore such issues. The model represents a hybrid ecological/physiological approach described by systems of equations based on steady‐state leaf‐gas exchange theories and leaf energy/radiation balance, equilibrium thermodynamics within the leaf, stomatal data, and atmospheric CO2 concentration. The model allows separating air from leaf temperatures thereby permitting exploration of the dependence of leaf cooling or heating for any combination of environmental conditions (e.g., wind velocity, atmospheric humidity, and atmospheric CO2 level), anatomic leaf properties (e.g., leaf size), and physiologic quantities (e.g., assimilation rate and transpiration rate). The model permits to distinguish whether leaf cooling or heating is to be expected if these parameters are varied. Based on model calculations, it is shown that leaf temperature is far more impacted by leaf size or wind speed than reduction in stomatal conductance caused by elevated atmospheric CO2. The model results are consistent with measurements of leaf cooling and heating. [ABSTRACT FROM AUTHOR]

A unifying theory interpreting the cause of ice-age cycles remains elusive, in view of the near-synchronous interhemispheric major climate shifts but anti-phased summer solar insolation signatures between hemispheres. Determining the timing and magnitude of mountain glaciations helps in shedding new light on this long-standing unsolved puzzle. Yet, robust glacial chronologies are limited for the pre-Last Glaciation time periods, impeding a full understanding of glacial histories and climatic mechanisms behind them in mountainous regions, such as the Tibetan Plateau (TP). Here, we report thirty-five new 10Be exposure-ages from moraine boulders in the Niqingqu Valley, eastern Haizishan Plateau, which adds to the existing glacial chronologies of the Hengduan Mountains, southeastern TP. The dating results indicate that four centennial to millennial-scale glacial activities occurred around 168.4 ka in the studied valley. Despite being indistinguishable in age, these high-frequency glacial fluctuations allow us to draw a conclusion that glaciers in the studied valley achieved the marine isotope stage 6 (MIS 6) glacial maximum prior to the Penultimate Glacial Maximum (PGM), inconsistent with Northern Hemisphere. We attribute the MIS 6 glacial maximum identified here to the response of glaciers to a combination of low-level atmospheric CO 2 content and low northern high-latitude summer solar insolation/high summer-monsoon precipitation. The consistent local PGM and low CO 2 levels suggest that low atmospheric CO 2 contents are likely a necessary prerequisite for the MIS 6 glaciation. • Thirty-five 10Be ages define multiple MIS 6 glacial behaviors. • The MIS 6 glacial maxima pre-date the Penultimate Glacial Maximum (PGM). • Summer insolation and monsoon precipitation possibly contribute to MIS-6 advance. • Low atmospheric CO 2 levels are likely the essential prerequisite of MIS 6 glaciation. [ABSTRACT FROM AUTHOR]

• The U/Th ratios in the sediments at Sites U1505 and U1438 were controlled by the redox conditions on the seafloor. • The seafloor redox conditions were attributed to the changes in the structure of the Pacific water mass driven by the middle Miocene cooling. • Increasing deepwater carbon storage in the Pacific Ocean contributed to the decline in atmospheric CO 2 during the middle Miocene. The climate transition during the middle Miocene promoted a series of environmental evolutions. However, the response of deep-sea redox conditions to this climate transition remains unclear. Here, we reconstruct the redox conditions in the subtropical Northwest Pacific during the Middle Miocene Climate Transition (MMCT) using the uranium and thorium data from drill cores at different water depths. Our findings indicate that the oxygenation level in the bathypelagic site (U1505) was weak at the early MMCT (14.6–14.1 Ma), elevated at the middle MMCT (14.1–13.3 Ma), and weak again at the late MMCT (13.3–12.8 Ma). In contrast, the oxygenation level in the abyssopelagic site (U1438) was strong in the early MMCT and weak in the middle and late MMCT. Combined with the changes in local productivity and ocean circulation, we argue that the varying oxygenation evolution trends at different water depths are probably related to the change in water mass structure driven by climate cooling from the early to the middle MMCT. However, local export production may be the main controlling factor during the late MMCT. Moreover, the decrease in deep-sea oxygenation across the MMCT also reflects the deepening of the respired carbon pool in the subtropical Northwest Pacific, which possibly contributed to the decline in atmospheric CO 2 at that time. [ABSTRACT FROM AUTHOR]

Background and aim: Some studies have shown that an increasing atmospheric CO2 concentration reduces plant transpiration while others have demonstrated that it interacts with nutrients in soil to enhance plant transpiration and plant growth including an increase in leaf area. However, none of these studies has focused on plant-induced suction in vegetated soils. The objective of this study is to quantify transpiration induced soil suction by Schefflera heptaphylla in NPK (Nitrogen, Phosphorus, Potassium) nutrient supplied, heavily compacted silty sand under two different atmospheric CO2 concentrations (400 ppm and 1000 ppm). Method: Three replicates of the plant were grown in NPK nutrient supplied silty sand and their plant characteristics and soil matric suction were measured under 400 and 1000 ppm atmospheric CO2 for three months. Results: Due to the supply of nitrogen-rich nutrient in silty sand, leaf area index (LAI) of plant increased by 22% under 1000 ppm CO2 compared to 400 ppm CO2. Thus, the larger LAI induced higher soil suction substantially since no significant difference in soil suction was found between current and elevated atmospheric CO2. LAI of Schefflera heptaphylla could be a reliable parameter to understand and predict soil suction as verified by strong correlation coefficient (R2 = 0.85–0.98; P value < 0.1) of peak induced suction and atmospheric CO2 concentration. Conclusion: This study reveals that the use of vegetation in soil structures needs proper management with additional supply of nutrients to thrive under future atmospheric condition and to induce suction hence improve shear strength in vegetated soil structures. [ABSTRACT FROM AUTHOR]