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Global warming is increasing permanently, because the concentration of CO2 in the atmosphere is rising continuously. According to National Oceanographic and Atmospheric Administration, the concentration of CO2 in the atmosphere was 407 ppm in June 2016 and 413 ppm in April 2017 as a last record for now. If the effects of other greenhouse gases, such as CH4, N2O, SF6, NF3, chlorofluorocarbons, hydrofluorocarbons, perfluorocarbons are added, the effective concentration may reach or exceed 550 ppm CO2-equivalent. According to the United Nations Intergovernmental Panel on Climate Change-2014 Climate Change Report, this is about two times higher than 278 ppm CO2 concentration in the pre-industrial year 1765. Thus, very urgent solutions must be found. The aim of this article is to suggest a vital, fast and very meticulous solution using NH3 gas in the atmosphere in order to decrease the atmospheric CO2 without delay. The laboratory experiments in the gas phase for (NH3+ CO2) reaction showed us that to use NH3 gas in the atmosphere will be a very fast, effective method for decreasing CO2 concentration of atmosphere. (NH3+ CO2) reaction is also quantitative in the cold atmosphere strata and there will be no more free ammonia in the atmosphere and no public health problem.

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]

Photosynthesis of wheat and maize declined when grown with NH4+ as a nitrogen (N) source at ambient CO2 concentration compared to those grown with a mixture of NO3– and NH4+, or NO3– as the sole N source. Interestingly, these N nutritional physiological responses changed when the atmospheric CO2 concentration increases. We studied the photosynthetic responses of wheat and maize growing with various N forms at three levels of growth CO2 levels. Hydroponic experiments were carried out using a C3 plant (wheat, Triticum aestivum L. cv. Chuanmai 58) and a C4 plant (maize, Zea mays L. cv. Zhongdan 808) given three types of N nutrition: sole NO3– (NN), sole NH4+ (AN) and a mixture of both NO3– and NH4+ (Mix-N). The test plants were grown using custom-built chambers where a continuous and desired atmospheric CO2 (Ca) concentration could be maintained: 280 μmol mol–1 (representing the pre-Industrial Revolution CO2 concentration of the 18th century), 400 μmol mol–1 (present level) and 550 μmol mol–1 (representing the anticipated futuristic concentration in 2050). Under AN, the decrease in net photosynthetic rate (Pn) was attributed to a reduction in the maximum RuBP-regeneration rate, which then caused reductions in the maximum Rubisco-carboxylation rates for both species. Decreases in electron transport rate, reduction of electron flux to the photosynthetic carbon [Je(PCR)] and electron flux for photorespiratory carbon oxidation [Je(PCO)] were also observed under AN for both species. However, the intercellular (Ci) and chloroplast (Cc) CO2 concentration increased with increasing atmospheric CO2 in C3 wheat but not in C4 maize, leading to a higher Je(PCR)/ Je(PCO) ratio. Interestingly, the reduction of Pn under AN was relieved in wheat through higher CO2 levels, but that was not the case in maize. In conclusion, elevating atmospheric CO2 concentration increased Ci and Cc in wheat, but not in maize, with enhanced electron fluxes towards photosynthesis, rather than photorespiration, thereby relieving the inhibition of photosynthesis under AN. Our results contributed to a better understanding of NH4+ involvement in N nutrition of crops growing under different levels of CO2.

We use data on the following climate variables for the period of the last 798 thousand years: global ice volume (Icet), atmospheric carbon dioxide level (CO2,t), and Antarctic land surface temperature (Tempt). 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 Icet are above the observed Icet, (ii) the forecasts of CO2,t level are below the observed CO2,t, and (iii) the forecasts of Tempt are below the observed Tempt. The forecasts for the benchmark ice-age model are reinforced by the score-driven models.

Independent methods for estimating local greenhouse gas emissions have been developed utilizing different instrumentation, sampling, and estimation techniques. Comparing independent estimates theoretically improves understanding of emission sources. However, each method estimates emissions with varying fidelity, complicating comparisons across methods, cities, and over time. It is thus difficult for decision-makers to judge how to use novel estimation methods, particularly when the literature implies a singular method is best. We review 650 articles to define the scope and contours of estimation methods, develop and apply an uncertainty typology, and describe the strengths and weaknesses of different approaches. We identify two prominent process-based estimation techniques (summing of utility bills and theoretical modeling), three techniques that attribute observed atmospheric CO _2 to source locations (eddy covariance footprinting, dispersion models, and regression), and methods that spatiotemporally distribute aggregate emissions using source proxies. We find that ‘ground truth’ observations for process-based method validation are available only at the aggregate scale and emphasize that validation at the aggregate scale does not imply a valid underlying spatiotemporal distribution. ‘Ground truth’ observations are also available post-combustion as atmospheric CO _2 concentrations. While dispersion models can spatially and temporally estimate upwind source locations, missing validation data by source introduces unknowable uncertainty. We find that many comparisons in the literature are made across methods with unknowable uncertainty, making it infeasible to rank methods empirically. We see promise in the use of regression for source attribution owing to its controlling for confounding emissions, flexibly accommodating different source proxies, explicitly quantifying uncertainty, and growing availability of CO _2 samples for modeling. We see developing cross-walks between land use and end-use sectors as an important step to comparing process-based methods with those attributing atmospheric CO _2 to sources. We suggest pooling data streams can produce better decision support resources for cities with proper attribution of empirical fidelity.

Photosynthesis of wheat and maize declined when grown with NH4+ as a nitrogen (N) source at ambient CO2 concentration compared to those grown with a mixture of NO3– and NH4+, or NO3– as the sole N source. Interestingly, these N nutritional physiological responses changed when the atmospheric CO2 concentration increases. We studied the photosynthetic responses of wheat and maize growing with various N forms at three levels of growth CO2 levels. Hydroponic experiments were carried out using a C3 plant (wheat, Triticum aestivum L. cv. Chuanmai 58) and a C4 plant (maize, Zea mays L. cv. Zhongdan 808) given three types of N nutrition: sole NO3– (NN), sole NH4+ (AN) and a mixture of both NO3– and NH4+ (Mix-N). The test plants were grown using custom-built chambers where a continuous and desired atmospheric CO2 (C a ) concentration could be maintained: 280 μmol mol–1 (representing the pre-Industrial Revolution CO2 concentration of the 18th century), 400 μmol mol–1 (present level) and 550 μmol mol–1 (representing the anticipated futuristic concentration in 2050). Under AN, the decrease in net photosynthetic rate (P n ) was attributed to a reduction in the maximum RuBP-regeneration rate, which then caused reductions in the maximum Rubisco-carboxylation rates for both species. Decreases in electron transport rate, reduction of electron flux to the photosynthetic carbon [ Je(PCR) ] and electron flux for photorespiratory carbon oxidation [ Je(PCO) ] were also observed under AN for both species. However, the intercellular (C i ) and chloroplast (C c ) CO2 concentration increased with increasing atmospheric CO2 in C3 wheat but not in C4 maize, leading to a higher Je(PCR)/ Je(PCO) ratio. Interestingly, the reduction of P n under AN was relieved in wheat through higher CO2 levels, but that was not the case in maize. In conclusion, elevating atmospheric CO2 concentration increased C i and C c in wheat, but not in maize, with enhanced electron fluxes towards photosynthesis, rather than photorespiration, thereby relieving the inhibition of photosynthesis under AN. Our results contributed to a better understanding of NH4+ involvement in N nutrition of crops growing under different levels of CO2. [ABSTRACT FROM AUTHOR]

We analyzed real-time measurements of atmospheric carbon dioxide (CO2), with total electricity production and nationwide restrictions phases in China, the United States of America, Europe, and India due to the novel coronavirus COVID-19 pandemic and its effects on atmospheric CO2. A decline of 3.7% in the global energy demand at about 150 million tonnes of oil equivalent (Mtoe) in the first quarter (Q1) of 2020 was recorded compared to Q1 2019 due to the cutback on international economic activities. Our results showed that: (1) electricity production for the same period in 2018, 2019, and 2020 shrunk at an offset of 9.20%, which resulted in a modest reduction (−1.79%) of atmospheric CO2 to the 2017–2018 CO2 level; (2) a non-seasonal, abrupt, and brief atmospheric CO2 decrease by 0.85% in mid-February 2020 could be due to Phase 1 restrictions in China. The results indicate that electricity production reduction is significant to the short-term variability of atmospheric CO2. It also highlights China’s significant contribution to atmospheric CO2, which suggests that, without the national restriction of activities, CO2 concentration is set to exceed 2019 by 1.79%. Due to the lockdown, it quickly decreased and sustained for two months. The results underscore atmospheric CO2 reductions on the monthly time scale that can be achieved if electricity production from combustible sources was slashed. The result could be useful for cost-benefit analyses on the decrease in electricity production of combustible sources and the impact of this reduction on atmospheric CO2. [ABSTRACT FROM AUTHOR]

Volcanic eruptions induce a dynamical response in the climate system characterized by short-term global reductions in both surface temperature and precipitation, as well as a response in biogeochemistry. The available observations of these responses to volcanic eruptions, such as to Pinatubo, provide a valuable method to compare against model simulations. Here, the Community Climate System Model Version 3 (CCSM3) reproduces the physical climate response to volcanic eruptions in a realistic way, as compared to direct observations from the 1991 eruption of Mount Pinatubo. The model's biogeochemical response to eruptions is smaller in magnitude than observed, but because of the lack of observations, it is not clear why or where the modeled carbon response is not strong enough. Comparison to other models suggests that this model response is much weaker over tropical land; however, the precipitation response in other models is not accurate, suggesting that other models could be getting the right response for the wrong reason. The underestimated carbon response in the model compared to observations could also be due to the ash and lava input of biogeochemically important species to the ocean, which are not included in the simulation. A statistically significant reduction in the simulated carbon dioxide growth rate is seen at the 90% level in the average of 12 large eruptions over the period 1870–2000, and the net uptake of carbon is primarily concentrated in the tropics, with large spatial variability. In addition, a method for computing the volcanic response in model output without using a control ensemble is tested against a traditional methodology using two separate ensembles of runs; the method is found to produce similar results in the global average. These results suggest that not only is simulating volcanoes a good test of coupled carbon–climate models, but also that this test can be performed without a control simulation in cases where it is not practical to run separate ensembles with and without volcanic eruptions.

The conversion of dissolved nutrients and carbon to organic matter by phytoplankton in the surface ocean, and its downward transport by sinking particles, produces a “biological pump” that reduces the concentration of atmospheric CO2. Global rates of organic matter export are a poor indicator of biological carbon storage however, because organic matter gets distributed across water masses with diverse pathways and timescales of return to the surface. Here we show that organic matter export and carbon storage can be related through a sequestration efficiency, which measures how long regenerated nutrients and carbon will be stored in the interior ocean before being returned to the surface. For the first time, we derive global maps of the sequestration efficiency of the biological pump at different residence time horizons. These maps reveal how regional patterns of organic matter export contribute to the biological pump, and how the biological pump responds to changes in biological productivity driven by climate change.

A data-constrained ocean circulation model is used to characterize the distribution of water masses and their ages in the global ocean. The model is constrained by the time-averaged temperature, salinity, and radiocarbon distributions in the ocean, as well as independent estimates of the mean sea surface height and sea surface heat and freshwater fluxes. The data-constrained model suggests that the interior ocean is ventilated primarily by water masses forming in the Southern Ocean. Southern Ocean waters, including those waters forming in the Antarctic and subantarctic regions, make up about 55% of the interior ocean volume and an even larger percentage of the deep-ocean volume. In the deep North Pacific, the ratio of Southern Ocean to North Atlantic waters is almost 3:1. Approximately 65% of interior ocean waters make first contact with the atmosphere in the Southern Ocean, further emphasizing the central role played by the Southern Ocean in the regulation of the earth’s climate. Results of the age analysis suggest that the mean ventilation age of deep waters is greater than 1000 yr throughout most of the Indian and Pacific Oceans, reaching a maximum of about 1400–1500 yr in the middepth North Pacific. The mean time for deep waters to be reexposed at the surface also reaches a maximum of about 1400–1500 yr in the deep North Pacific. Together these findings suggest that the deep North Pacific can be characterized as a “holding pen” of stagnant and recirculating waters.

The evolution of intermediate circulation in the northern Indian Ocean since the last deglaciation has been reconstructed from two marine cores located at intermediate depths off the southern tip of India (MD77‐191) and in the northern Bay of Bengal (BoB) (MD77‐176). Benthic foraminiferal δ13C, seawater carbonate ion concentration ([CO32−]) estimated from the Sr/Ca, and paleotemperature reconstructed on the basis of the Mg/Li of aragonite benthic species Hoeglundina elegans were used to trace the evolution of past intermediate‐deepwater masses and to constrain ocean‐atmosphere exchanges during the two‐stage increase in atmospheric CO2 across the last deglaciation. The intermediate water [CO32−] was mainly affected by changes in the ocean alkalinity inventory, associated with the modulation of atmospheric CO2 on glacial‐interglacial timescales. Higher benthic foraminiferal δ13C, depleted [CO32−], and decreased benthic‐planktonic 14C age offsets at intermediate water depths suggest a release of deep‐sea CO2 to the atmosphere through the Antarctic Intermediate Water (AAIW) in the Southern Ocean during the 17–15.2 and 12.6–10.5 cal kyr BP time intervals. In addition, the decreased H. elegans Mg/Li record seems to reflect an increased contribution of cold water mass during the 17–15.2 and 12.6–11.9 cal kyr BP intervals and throughout the Holocene. In contrast, two warm events occurred in the 15–13.3 and 11–10.3 cal kyr BP time intervals. During the late Holocene, a decrease in the intermediate water [CO32−] indicates a contribution to atmospheric CO2 rise since 8 cal kyr BP, due to the depleted global ocean alkalinity and/or the variations in surface productivity (at least for MD77‐191). Key Points: We produced high‐resolution records of benthic foraminiferal elemental ratios, δ18O, and δ13C from two cores from the northern Indian OceanEvaluation of past IWT from the Mg/Li ratio and the δ18O since last deglaciation exhibit millennial‐scale changesVariations of the IW Δ[CO32−] calculated from H. elegans Sr/Ca are related to atmospheric CO2 changes on glacial‐interglacial timescales [ABSTRACT FROM AUTHOR]

On interannual timescales the growth rate of atmospheric CO2 is largely controlled by the response of the land and ocean carbon sinks to climate variability. Yet, it is unknown to what extent this variability limits the predictability of atmospheric CO2 variations. Using perfect‐model Earth System Model simulations, we show that variations in atmospheric CO2 are potentially predictable for 3 years. We find a 2‐year predictability horizon for global oceanic CO2 flux with longer regional predictability of up to 7 years. The 2‐year predictability horizon of terrestrial CO2 flux originates in the tropics and midlatitudes. With the predictability of the isolated effects of land and ocean carbon sink on atmospheric CO2 of 5 and 12 years respectively, land dampens the overall predictability of atmospheric CO2 variations. Our research shows the potential of Earth System Model‐based predictions to forecast multiyear variations in atmospheric CO2. Plain Language Summary: The amount of anthropogenic carbon emissions absorbed by land and ocean from the atmosphere varies annually due to their sensitivity to climate. Therefore, the atmospheric CO2 growth rate does not strictly follow the emissions signal. Whether decadal prediction systems can also predict variations of atmospheric CO2 has not been shown yet but is crucial to inform policy makers about the efficiency of the implementation of the Paris Agreement. Using numerical Earth System simulations in an idealized prediction framework, we show that global atmospheric CO2 is predictable up to 3 years in advance. The global ocean and land CO2 fluxes are predictable for 2 years. The isolated effects of the land and ocean carbon sink on atmospheric CO2 are predictable for 5 and 12 years, respectively. Therefore, the land carbon cycle limits atmospheric CO2 predictability. Our study demonstrates that simulation‐based multiyear forecasts have the potential to predict natural atmospheric CO2 variations. Key Points: Global annual atmospheric CO2 variations are potentially predictable up to 3 years in advanceThe global oceanic CO2 flux is predictable for 2 years with regional predictability up to a decadeThe global land CO2 flux predictability of 2 years is dominated by the tropical forests [ABSTRACT FROM AUTHOR]

The urgency to address global climate change induced by greenhouse gas emissions is increasing. In particular, the rise in atmospheric CO2 levels is generating alarm. Technologies to remove CO2 from ambient air, or "direct air capture" (DAC), have recently demonstrated that they can contribute to "negative carbon emission." Recent advances in surface chemistry and material synthesis have resulted in new generations of CO2 sorbents, which may drive the future of DAC and its large‐scale deployment. This Review describes major types of sorbents designed to capture CO2 from ambient air and they are categorized by the sorption mechanism: physisorption, chemisorption, and moisture‐swing sorption. [ABSTRACT FROM AUTHOR]

A record of δ18Oc from the Indian sector of Southern Ocean and atmospheric CO2, and δ18O of European Project for Ice Coring in Antarctica (EPICA) and Greenland Ice Sheet Project 2 (GISP2) reveals that a coherent response between δ18O record of Antarctic ice core and the δ18Oc record from Southern Indian Ocean during the deglaciation with initial warming starting around 18 kyr BP which is in agreement with the raise of atmospheric CO2 during same time. A distinct asynchrony between the records of δ18Oc from the Southern Indian Ocean and δ18O of GISP2 during the last deglaciation is noticed. We report that Southern Ocean degassing played an important role in raising atmospheric CO2 through Atlantic Meridional Overturning Circulation (AMOC), which has an implication in triggering abrupt climate events through coupling of ocean and atmospheric processes. Highlights: A synchrony between the temperature variations between the Antarctica and the southern sector of the Indian Ocean are noticed during the deglaciation. Initiation of deglacial warming in the southern sector of Indian Ocean started around 18 Ka which coincides with rise of atmospheric CO2 during the same time. Degassing in the Southern Ocean played an important role in raising atmospheric CO2 during the deglaciation. Changes in AMOC variations contributed to the trigger of CO2 degassing from the deep Southern Ocean. [ABSTRACT FROM AUTHOR]

In this study, investigation of the seasonal cycle of the tropospheric CO2 concentration over India was carried out using the GEOS-Chem atmospheric transport model, Greenhouse gas Observation SATellite (GOSAT) retrievals, and in-situ measurements. The model simulation is highly coherent with the satellite and in-situ datasets, and it shows a distinct seasonal cycle of the tropospheric CO2 tendency over India with a negative phase (decreasing concentration) during April–August and a positive phase (increasing concentration) during September–March. The model diagnostics were analyzed to estimate budgets of the surface layer CO2, up to 650 hPa pressure level, for the two-phases of the seasonal cycle. A mean tendency, equivalent to −0.70 ppmv month−1, observed during April–August, which results from the loss of CO2 content in the surface layer through horizontal advection (−2.25 ppmv month−1) and vertical diffusion (−0.20 ppmv month−1), that dominates the gain from vertical advection (1.53 ppmv month−1). The negative contribution of horizontal advection in this period comes from the transport of CO2 depleted air-parcels over the oceanic region to India by the southwest monsoon winds and the positive contributions of vertical advection comes from upwelling of CO2 enriched air-parcels. The mean tendency, equivalent to 1.01 ppmv month−1, during September–March results from the gain through vertical advection (0.78 ppmv month−1) and horizontal advection (0.37 ppmv month−1) and a small contribution of vertical diffusion (−0.15 ppmv month−1). In this period, positive contribution of horizontal advection is due to the transport of CO2 enriched air-parcels from the southeast Asian region to India by north-east monsoon winds. At the annual scale, CO2 content of the surface layer over India has a net gain of 0.75 GtC that comes from 14.31 GtC through vertical advection that exceeds the loss due to horizontal advection (−11.10 GtC) and vertical diffusion processes (−2.46 GtC). This net gain is almost 85% higher than the input of 0.4 GtC through surface fluxes, which composed of 0.61 GtC anthropogenic emission and −0.21 GtC net terrestrial ecosystem exchanges. Additional sensitivity experiment was carried out to elucidate the semi-annual features of the seasonal cycle of CO2 for north India, in contrast to the annual characteristics of the seasonal cycle for south India in relation to the GOSAT observation. Highlights: Greenhouse gas Observation SATellite (GOSAT) L3B and L4B retrievals and in situ flux tower measurements were analysed to describe seasonal cycle of tropospheric CO2 over India; and GEOS-Chem atmospheric transport model diagnostics were used to examine the causes of the variability. The seasonal cycle over north India is composed of mixed signature of annual and semi-annual frequencies while south India experiences dominance of annual oscillation. However, the surface layer CO2 seasonal tendency has a major negative phase during April–August and a positive phase during September–March. The net negative tendency during April–August results from the loss of CO2 from the surface layer through horizontal advection and vertical diffusion processes that dominates the gain from vertical advection; while the net positive tendency during September–March results from the gain through vertical advection and horizontal advection and a small negative contribution from vertical diffusion. At annual scale, the surface layer over India experiences net positive gain of CO2 concentration, which is 85% more than the net input from the surface fluxes, and is mostly contributed by large-scale transport processes. Sensitivity experiments were carried out to elucidate the semi-annual features of the seasonal cycle of CO2 over north India in relation to the GOSAT observation. It turns out that the secondary trough during October–December on the background of net positive tendency during September–March results from the drawdown of CO2 by the terrestrial ecosystem uptake. [ABSTRACT FROM AUTHOR]

Tomkins et al. [A. G. Tomkins et al., Nature 533, 235-238 (2016)] suggested that iron oxides contained in 2.7-Ga iron micrometeorites can be used to determine the concentration of O2 in the Archean upper atmosphere. Specifically, they argued that the presence of magnetite in these objects implies that O2 must have been near present-day levels (~21%) within the altitude range where the micrometeorites were melted during entry. Here, we reevaluate their data using a 1D photochemical model. We find that atomic oxygen, O, is the most abundant strong oxidant in the upper atmosphere, rather than O2. But data from shock tube experiments suggest that CO2 itself may also serve as the oxidant, in which case micrometeorite oxidation really constrains the CO2/N2 ratio, not the total oxidant abundance. For an atmosphere containing 0.8 bar of N2, like today, the lower limit on the CO2 mixing ratio is ~0.23. This would produce a mean surface temperature of ~300 K at 2.7 Ga, which may be too high, given evidence for glaciation at roughly this time. If pN2 was half the present value, and warming by other greenhouse gases like methane was not a major factor, the mean surface temperature would drop to ~291 K, consistent with glaciation. This suggests that surface pressure in the Neoarchean may need to have been lower--closer to 0.6 bar--for CO2 to have oxidized the micrometeorites. Ultimately, iron micrometeorites may be an indicator for ancient atmospheric CO2 and surface pressure; and could help resolve discrepancies between climate models and existing CO2 proxies such as paleosols. [ABSTRACT FROM AUTHOR]

Pedogenic features of paleosols preserved in the lower and upper Cretaceous sedimentary strata of the western Sichuan Basin, China, were described and examined in terms of their macromorphology and micromorphology, mineralogy, and geochemistry. These Cretaceous paleosols (entisols, inceptisols, and aridisols) are analogous to modern soil orders based on soil taxonomy (Soil Survey Staff, 2014). A quantitative reconstruction of paleoclimates inferred from the geochemical composition of paleosol horizons, depth to carbonate nodules, and the stable oxygen isotopic composition of pedogenic carbonates in paleosols indicates that a cool‐arid climate associated with short‐term subhumid fluctuations prevailed in the early Cretaceous (Hauterivian) and a cool‐arid climate regime existed in the late Cretaceous (Maastrichtian) of the Sichuan Basin. Atmospheric CO2 concentrations estimated from the paleosol barometer indicated values between 156 ± 138 and 753 ± 339 ppmv for the Hauterivian and between 110 ± 71 and 235 ± 77 ppmv for the middle–late Maastrichtian. An overall coupling between atmospheric pCO2 and paleotemperature occurred in the Sichuan Basin, while the covariations are likely linked to global cooling events in the Maastrichtian and/or are likely associated with the Faraoni oceanic anoxic event (F‐OAE) during the late Hauterivian. Paleoclimate variations in the Sichuan Basin were controlled by regional paleogeography, the prevalence of subtropical high‐pressure systems, monsoonal circulation, and the atmospheric CO2 level in the early and late Cretaceous. [ABSTRACT FROM AUTHOR]

Climate change scenarios predict increasing atmospheric CO2 concentrations ([CO2]), temperatures and droughts in tropical regions. Individually, the effects of these climate factors on plants are well established, whereas experiments on the interactive effects of a combination of factors are rare. Moreover, how these environmental factors will affect tree species along a wet to dry gradient (e.g. along tropical forest–savanna transitions) remains to be investigated. We hypothesized that under the simulated environmental conditions, plant growth, physiological performance and survivorship would vary in a manner consistent with the species' positions of origin along this gradient. In a glasshouse experiment, we raised seedlings of three Eucalyptus species, each occurring naturally in a wet forest, savanna and forest–savanna ecotone, respectively. We evaluated the effect of drought, elevated temperature (4 °C above ambient glasshouse temperature of 22 °C) and elevated temperature in combination with elevated [CO2] (400 ppm [CO2] above ambient of 400 ppm), on seedling growth, survivorship and physiological responses (photosynthesis, stomatal conductance and water-use efficiency). Elevated temperature under ambient [CO2] had little effect on growth, biomass and plant performance of well-watered seedlings, but hastened mortality in drought-affected seedlings, affecting the forest and ecotone more strongly than the savanna species. In contrast, elevated [CO2] in combination with elevated temperatures delayed the appearance of drought stress symptoms and enhanced survivorship in drought-affected seedlings, with the savanna species surviving the longest, followed by the ecotone and forest species. Elevated [CO2] in combination with elevated temperatures also enhanced growth and biomass and photosynthesis in well-watered seedlings of all species, but modified shoot:root biomass partitioning and stomatal conductance differentially across species. Our study highlights the need for a better understand of the interactive effects of elevated [CO2], temperature and drought on plants and the potential to upscale these insights for understanding biome changes. [ABSTRACT FROM AUTHOR]

Radiocarbon (14C) concentrations of annual tree rings from an European beech tree (Fagus sylvatica) from Woods Hole, MA, USA were analyzed at National Ocean Sciences Accelerator Mass Spectrometry facility (NOSAMS) to construct a 14C bomb curve record from northeastern North America. The 14C concentration rises from a pre-bomb (1895–1955) average of Fraction Modern (F14C) = 0.9764 to a peak of 1.8639 in 1964. After 1964, F14C gradually decreases to 1.0611 in 2014. The annual tree-ring radiocarbon content agrees with the atmospheric radiocarbon content of the Northern Hemisphere and is very similar to the radiocarbon concentration of a red oak (Quercus rubra) tree located in Bear Mountain State Park in New York, USA. Intra-annual tree-ring pairs did not produce evidence of a seasonal effect on the radiocarbon concentration, but there were few samples and more study is warranted. [ABSTRACT FROM AUTHOR]

Understanding the spatiotemporal dynamics of atmospheric carbon dioxide (CO 2) is crucial for achieving carbon neutrality and safeguarding planetary well-being. Satellite-based carbon monitoring is increasingly instrumental in studying atmospheric CO 2 changes, offering a broad spatial view. However, current research lacks a comprehensive exploration of both day-to-day variations in atmospheric CO 2 at a finer-resolution grid and its coupled relationship with particulate pollution, owing to a substantial number of missing values in satellite-retrieved CO 2 data. To address these issues, this study presents an enhanced regression-based machine learning model, reconstructing full-coverage daily atmospheric CO 2 concentrations in China from 2015 to 2020 at a 0.01° spatial resolution. Utilizing spatiotemporal high-resolution column-averaged dry-air mole fraction of CO 2 (XCO 2) data from the Orbiting Carbon Observatory 2 (OCO-2) as the dependent variable and multi-source environmental factors as independent variables, we achieved overall, spatial, and temporal cross-validation R2 [RMSE] results of 0.98 [0.74 ppm], 0.95 [1.15 ppm], and 0.93 [1.44 ppm], respectively. The resulting full-coverage estimates revealed significant spatial heterogeneity over time, with the highest CO 2 levels primarily in spring in the North China Plain, and the lowest in summer in northern China and autumn in the Qinghai-Tibet Plateau and the south. National weekend averages of mean symmetrized residuals in XCO 2 surpassed weekday averages. Within megacities such as Shanghai and Wuhan, industry-intensive areas exhibited higher XCO 2 values compared to other urban areas within the same cities. Despite spatial and seasonal fluctuations, the country-average XCO 2 increased from 2015 to 2020 but has decelerated recently. A moderate correlation was observed between the spatial pattern of long-term CO 2 concentrations and particulate aerosols, viewed from the total atmospheric column perspective, with some regional discrepancies. The present modeling method advances carbon monitoring techniques, and the predictive data foster an enriched understanding of the carbon cycle, climate change, and sustainable development. • A full-coverage dataset of daily 1-km atmospheric CO 2 from 2015 to 2020 is generated. • The resulting dataset achieves high modeling accuracy (CV R2 = 0.98, RMSE = 0.74 ppm). • Atmospheric CO 2 varies significantly within and between cities, with seasonal fluctuations. • The national CO 2 increased but with spatial, seasonal, and interannual fluctuations. • Atmospheric CO 2 is correlated well with aerosol loads but varies by seasons and regions. [ABSTRACT FROM AUTHOR]

Recent upward revisions in key sink/source terms for fixed nitrogen (N) in the oceans imply a short residence time and strong negative feedbacks involving denitrification and N fixation to prevent large swings in the ocean N inventory over timescales of a few centuries. We tested the strength of these feedbacks in a global biogeochemical elemental cycling (BEC) ocean model that includes water column denitrification and an explicit N fixing phytoplankton group. In the northern Indian Ocean and over longer timescales in the tropical Atlantic, we find strong stabilizing feedbacks that minimize changes in marine N inventory over timescales of ∼30–200 years. In these regions high atmospheric dust/iron inputs lead to phosphorus limitation of diazotrophs, and thus a tight link between N fixation and surface water N/P ratios. Maintenance of the oxygen minimum zones in these basins depends on N fixation driven export. The stabilizing feedbacks in other regions are significant but weaker owing to iron limitation of the diazotrophs. Thus Fe limitation appears to restrict the ability of N fixation to compensate for changes in denitrification in the current climate, perhaps leading the oceans to lose fixed N. We suggest that iron is the ultimate limiting nutrient leading to nitrogen being the proximate limiting nutrient over wide regions today. Iron stress was at least partially alleviated during more dusty, glacial times, leading to a higher marine N inventory, increased export production, and perhaps widespread phosphorus limitation of the phytoplankton community. The increased efficiency of the biological pump would have contributed to the glacial drawdown in atmospheric CO2.

Photorespiration, or C2 photosynthesis, is generally considered a futile cycle that potentially decreases photosynthetic carbon fixation by more than 25%. Nonetheless, many essential processes, such as nitrogen assimilation, C1 metabolism, and sulfur assimilation, depend on photorespiration. Most studies of photosynthetic and photorespiratory reactions are conducted with magnesium as the sole metal cofactor despite many of the enzymes involved in these reactions readily associating with manganese. Indeed, when manganese is present, the energy efficiency of these reactions may improve. This review summarizes some commonly used methods to quantify photorespiration, outlines the influence of metal cofactors on photorespiratory enzymes, and discusses why photorespiration may not be as wasteful as previously believed.

Satellite-based atmospheric CO2 observations have provided a great opportunity to improve our understanding of the global carbon cycle. However, thermal infrared (TIR)-based satellite observations, which are useful for the investigation of vertical distribution and the transport of CO2, have not yet been studied as much as the column amount products derived from shortwave infrared data. In this study, TIR-based satellite CO2 products – from Atmospheric Infrared Sounder, Tropospheric Emission Spectrometer (TES), and Thermal And Near infrared Sensor for carbon Observation – and carbon tracker mole fraction data were compared with in situ Comprehensive Observation Network for Trace gases by AIrLiner (CONTRAIL) data for different locations. The TES CO2 product showed the best agreement with CONTRAIL CO2 data resulting in R2 ~ 0.87 and root-mean-square error ~0.9. The vertical distribution of CO2 derived by TES strongly depends on the geophysical characteristics of an area. Two different climate regions (i.e., southeastern Japan and southeastern Australia) were examined in terms of the vertical distribution and transport of CO2. Results show that while vertical distribution of CO2 around southeastern Japan was mainly controlled by horizontal and vertical winds, horizontal wind might be a major factor to control the CO2 transport around southeastern Australia. In addition, the vertical transport of CO2 also varies by region, which is mainly controlled by anthropogenic CO2, and horizontal and omega winds. This study improves our understanding of vertical distribution and the transport of CO2, both of which vary by region, using TIR-based satellite CO2 observations and meteorological variables.

We describe and implement a data selection algorithm aimed at identifying background atmospheric CO2 observations from in situ continuous measurements. Several selection criteria for detecting the background data have been developed and are currently used: the main objective of this work was to define a common methodology to extract the atmospheric background signal minimizing heterogeneities due to the use of different selection algorithms. The algorithm used in this study, (BaDS, Background Data Selection) was tested and optimized using data (from 2014 to 2018) from four Italian stations characterized by markedly different environmental conditions (i.e., mountain, coastal and marine): Plateau Rosa (PRS), Mt. Cimone (CMN), Capo Granitola (CGR) and Lampedusa (LMP). Their locations extend from the Alps to the central Mediterranean. The adopted algorithm proved to be effective in separating the local/regional from the background signal in the CO2 time series. About 6% of the data at LMP, 11% at PRS, 20–38% at CMN and 65% at CGR were identified as non-background. LMP and PRS can be used as reference sites for the central Mediterranean, while CMN and CGR were more impacted by regional sources and sinks. Finally, we discuss a possible application of BaDS screened data.

The worldwide lockdown in response to the COVID-19 pandemic in year 2020 led to an economic slowdown and a large reduction in fossil fuel CO _2 emissions (Le Quéré 2020 Nat. Clim. Change 10 647–53, Liu 2020 Nat. Commun. 11 ); however, it is unclear how much it would slow the increasing trend of atmospheric CO _2 concentration, the main driver of climate change, and whether this impact can be observed considering the large biosphere and weather variabilities. We used a state-of-the-art atmospheric transport model to simulate CO _2 , and the model was driven by a new daily fossil fuel emissions dataset and hourly biospheric fluxes from a carbon cycle model forced with observed climate variability. Our results show a 0.21 ppm decrease in the atmospheric column CO _2 anomaly in the Northern Hemisphere latitude band 0–45° N in March 2020, and an average of 0.14 ppm for the period of February–April 2020, which is the largest decrease in the last 10 years. A similar decrease was observed by the carbon observing satellite GOSAT (Yokota et al 2009 Sola 5 160–3). Using model sensitivity experiments, we further found that the COVID and weather variability are the major contributors to this CO _2 drawdown, and the biosphere showed a small positive anomaly. Measurements at marine boundary layer stations, such as Hawaii, exhibit 1–2 ppm anomalies, mostly due to weather and the biosphere. At the city scale, the on-road CO _2 enhancement measured in Beijing shows a reduction by 20–30 ppm, which is consistent with the drastically reduced traffic during the COVID lockdown. A stepwise drop of 20 ppm during the city-wide lockdown was observed in the city of Chengdu. The ability of our current carbon monitoring systems in detecting the small and short-lasting COVID signals at different policy relevant scales (country and city) against the background of fossil fuel CO _2 accumulated over the last two centuries is encouraging. The COVID-19 pandemic is an unintended experiment. Its impact suggests that to keep atmospheric CO _2 at a climate-safe level will require sustained effort of similar magnitude and improved accuracy, as well as expanded spatiotemporal coverage of our monitoring systems.

We used a thermodynamic climate model to compute the Northern Hemisphere temperature anomaly for the period 1980–2500. We obtained temperature anomalies for the ocean, the combined ocean-land area and the land. Two IPCC et al. (2007) CO 2 concentration scenarios were considered, the high RCP8.5 and the low RCP4.5, we also included two estimates of the Total Solar Irradiance (TSI). We found that in the RCP8.5 scenario the effect of the TSI is to prevent the temperature for having a runaway behavior after the year 2240, that otherwise it would have due to the high CO 2 emission; it is the TSI that makes the temperature anomalies to have an inflection and start decreasing. For the RCP4.5 scenario, without the solar effect, the temperature anomaly after 2075 presents an inflection and becomes constant after the year 2120, but the temperature anomaly has a clear decreasing trend when including the TSI. The TSI presents three future secular minima in the studied period. They introduce only perturbations on the general decreasing trend. The land presents the largest temperature anomalies as well as the most prominent changes, followed by the land-ocean and the ocean. We concluded that the TSI has a fundamental role in the temperature behavior over the long-term. • We obtained temperature anomalies using a high and low IPCC-5 CO 2 emission scenarios and the Total Solar Irradiance (TSI). • For high CO 2 emission, the inclusion of the TSI prevents the temperature from having a runaway behavior. • For low CO 2 emission with no TSI, the temperature anomaly after 2075 is constant, presenting a decreasing trend with the TSI. • The TSI presents three future secular minima, but they only introduce a perturbation in the general decreasing trend. • We concluded that the TSI has a fundamental role in the temperature behavior over the long-term. [ABSTRACT FROM AUTHOR]

Throughout the last glacial cycle millennial timescale variations in atmospheric CO2 occurred in parallel with perturbations in deep ocean circulation, which were themselves reflected by observable changes in surface conditions across the North Atlantic region. Here we use continuous proxy records to argue that an equivalent relationship has held throughout the last 800 kyr, that is, since before the first occurrence of Heinrich events (strictly defined). Our results highlight the importance of internal climate dynamics in amplifying external (insolation) forcing on the climate system to produce the large amplitude of glacial terminations (deglaciations) during the middle to late Pleistocene. We show that terminations are characterized by an interval of intense ice rafting followed by a subsequent and abrupt shift to anomalously warm surface conditions (with respect to the more gradually evolving background state), which we interpret to reflect an abrupt recovery of deep ocean circulation in the Atlantic. According to our synthesis, this is followed by a period of enhanced (or at least anomalous) overturning lasting thousands of years until equilibrium interglacial conditions are attained and during which atmospheric CO2 is likely to decrease. Our results therefore suggest that deglacial oscillations in ocean circulation can have a lasting influence on early interglacial climate and highlight the transient nature of atmospheric CO2 overshoots associated with the onset of some previous interglacials. Accordingly, we suggest that these intervals should be considered as a part of the deglacial process. This has implications for studies concerned with the evolution of atmospheric CO2 during interglacial periods including the Holocene. Key Points: The relationship between changes in atmospheric CO2 and surface conditions across the NE Atlantic has been consistent over the past 800 kyrThe ocean/atmosphere system may take thousands of years to reequilibrate following abrupt deglacial oscillations in ocean circulationInclusion of nonequilibrium intervals within interglacial comparisons may lead to artifacts in calculated trends in, for example, atmospheric CO2 [ABSTRACT FROM AUTHOR]

The proliferation of C 4 photosynthesis in land plants marks the most important ecological shift in tropical and subtropical terrestrial ecosystems during the Cenozoic. Despite the wealth of geochemical data indicating an increased C 4 biomass in the paleovegetation since the Late Miocene spatial and temporal variations of C 4 plants abundance are not well known. The driving force for their expansion such as CO 2 starvation, aridification and fire frequency are controversially discussed. Here we present an integrated review of the stable isotope stratigraphy, trace elements, palynology, lithological and phytolith record from early-middle Miocene lacustrine sediments in the Hoh Xil Basin. We report the first known silicified wood fossils from the genus Picea and indicate the Hoh Xil region might have already reached an elevation above 2500 m at least in the early Miocene, representing the early uplift stage of northern Tibetan Plateau. Our results show that the expansion of C 4 vegetation and the appearance of Picea in the interior of the Tibet during the early Miocene may be related to early uplift of the northern Tibetan Plateau and the consequent cooling event, P CO 2 decrease, and aridification. We suggest that the C 4 signal in vertebrate faunas predating the early Miocene is due to the ecology of the herbivores (who preferred plants with C 3 metabolism) rather than the lack of C 4 plants in the vegetation. Unlabelled Image • Two C 4 vegetation expansion events occurred in northern Tibetan Plateau during the early Miocene. • The two C 4 vegetation expansion events are correlative with Mi1b, Mi2b, respectively. • The fossils indicate the Hoh Xil region might have reached an elevation above 2500 m in the early Miocene. [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]

Climate change and the associated increase in atmospheric CO2 levels may affect the severity of plant diseases and threaten future crop yields. Here, we compared responses of the model plant Arabidopsis thaliana to leaf and root pathogens with hemi-biotrophic or necrotrophic infection strategies under pre-industrial, current, and future atmospheric CO2 conditions. Defenses against biotrophs are generally regulated by salicylic acid (SA) signaling, whereas jasmonic acid (JA) signaling controls defenses against necrotrophs. Under the CO2 conditions tested, basal expression of the JA-responsive marker gene PDF1.2 increased at increasing CO2 concentrations. The SA-responsive marker genes ICS1 and FRK1 showed an opposite behavior, being lower expressed under high CO2 and higher expressed under low CO2, respectively. Accordingly, plants showed enhanced resistance to the necrotrophic leaf pathogen Botrytis cinerea under high CO2, while resistance to the hemi-biotrophic leaf pathogen Pseudomonas syringae pv. tomato was reduced. The opposite was true for plants grown under low CO2. Disease severity caused by the soil-borne pathogens Fusarium oxysporum f.sp. raphani and Rhizoctonia solani was similar under all CO2 conditions tested. Collectively, our results stress the notion that atmospheric CO2 impacts the balance between SA- and JA-dependent defenses and concomitant resistance against foliar (hemi)biotrophic and necrotrophic pathogens. The direction of the CO2-mediated effects on SA- and JA-mediated defenses varies between reported studies, suggesting that the defense output is influenced by environmental context. These findings highlight that a wider dynamic range of climate change parameters should be studied simultaneously to harness plant traits for the development of future climate-resilient crops. [ABSTRACT FROM AUTHOR]