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The total meridional heat transport (MHT) is relatively stable across different climates. Nevertheless, the strength of individual processes contributing to the total transport are not stable. Here we investigate the MHT and its main components especially in the atmosphere, in five coupled climate model simulations from the Deep‐Time Model Intercomparison Project (DeepMIP). These simulations target the early Eocene climatic optimum, a geological time period with high CO2 concentrations, analog to the upper range of end‐of‐century CO2 projections. Preindustrial and early Eocene simulations, at a range of CO2 levels are used to quantify the MHT changes in response to both CO2 and non‐CO2 related forcings. We found that atmospheric poleward heat transport increases with CO2, while oceanic poleward heat transport decreases. The non‐CO2 boundary conditions cause more MHT toward the South Pole, mainly through an increase in the southward oceanic heat transport. The changes in paleogeography increase the heat transport via transient eddies at the northern mid‐latitudes in the Eocene. The Eocene Hadley cells do not transport more heat poleward, but due to the warmer atmosphere, especially the northern cell, circulate more heat in the tropics, than today. The monsoon systems' poleward latent heat transport increases with rising CO2 concentrations, but this change is counterweighted by the globally smaller Eocene monsoon area. Our results show that the changes in the monsoon systems' latent heat transport is a robust feature of CO2 warming, which is in line with the currently observed precipitation increase of present day monsoon systems. Plain Language Summary: In the Earth's climate system both the atmosphere and the ocean are transporting heat through different processes from the tropics toward the poles. We investigate the transport of the atmosphere in several climate model set ups, which aim to simulate the very warm climate of the early Eocene (∼56–48 Myr ago). This period is relevant, because the atmospheric CO2 concentration was close to our pessimistic projection of CO2 concentration for the end of the century. In our study we separate the results into transport changes due to the different set up of the Eocene, and transport changes due to larger CO2 concentration values. We found that with rising CO2 values the atmosphere transports more heat from the tropics to the poles. The different location of the continents and seas is influencing the heat transport of the midlatitude cyclones. The Eocene tropical meridional overturning circulation's poleward heat transport does not increase, but it circulates more heat than today. The monsoon systems seem to be affecting a globally smaller area in the Eocene, but they are also more effective in transporting heat. This conclusion is in line with the observation, that current day monsoon systems' precipitation increases, as our CO2 concentration rises. Key Points: The latent heat transport of the monsoon increases through the Eocene higher CO2 concentration, but it is reduced by the Eocene topographyThe poleward heat transport of midlatitude cyclones is higher in the Northern Hemisphere in the Eocene, due to the different topographyThe Eocene northern Hadley cell circulates more heat, than in the present, while its net poleward heat transport is even less than today [ABSTRACT FROM AUTHOR]

Inconsistencies in the Eocene climates of East Asia have been revealed in both geological studies and simulations. Several earlier reconstructions showed an arid zonal band in mid‐latitude China, but others showed a humid climate in the same region. Moreover, previous Eocene modeling studies have demonstrated that climate models can simulate both scenarios in China. Therefore, it is essential to investigate the cause of this model spread. We conducted a series of experiments using Norwegian Earth System Model 1‐F and examined the impact of mountains in Southern China on the simulated Eocene climate. These mountains, including the Gangdese and Southeast Mountains, are located along the main path of water vapor transport to East Asia. Our results reveal that the Southeast Mountains play the dominant role in controlling the simulated precipitation in Eastern China during the Eocene. When the heights of the Southeast Mountains exceed ∼2,000 m, an arid zonal band appears in mid‐latitude China, whereas humid climates appear in Eastern China when the elevation of the Southeast Mountains is relatively low. Key Points: Southeast Mountains control simulated Eocene precipitation in Eastern ChinaWhen the Southeast Mountains are high, an arid zonal band appears in mid‐latitude ChinaThe early Eocene climate in East Asia is not monsoonal climate [ABSTRACT FROM AUTHOR]

The early Eocene (∼56–48 Myr ago) is characterized by high CO2 estimates (1,200–2,500 ppmv) and elevated global temperatures (∼10°C–16°C higher than modern). However, the response of the hydrological cycle during the early Eocene is poorly constrained, especially in regions with sparse data coverage (e.g., Africa). Here, we present a study of African hydroclimate during the early Eocene, as simulated by an ensemble of state‐of‐the‐art climate models in the Deep‐time Model Intercomparison Project (DeepMIP). A comparison between the DeepMIP pre‐industrial simulations and modern observations suggests that model biases are model‐ and geographically dependent, however, these biases are reduced in the model ensemble mean. A comparison between the Eocene simulations and the pre‐industrial suggests that there is no obvious wetting or drying trend as the CO2 increases. The results suggest that changes to the land sea mask (relative to modern) in the models may be responsible for the simulated increases in precipitation to the north of Eocene Africa. There is an increase in precipitation over equatorial and West Africa and associated drying over northern Africa as CO2 rises. There are also important dynamical changes, with evidence that anticyclonic low‐level circulation is replaced by increased south‐westerly flow at high CO2 levels. Lastly, a model‐data comparison using newly compiled quantitative climate estimates from paleobotanical proxy data suggests a marginally better fit with the reconstructions at lower levels of CO2. Plain Language Summary: Approximately 50 Myr ago, a period known as the early Eocene, atmospheric carbon dioxide levels were significantly higher than today, and were more similar to what they could be in the future, if efforts to reduce human greenhouse gas emissions are unsuccessful. However, rainfall changes during this period are less well understood, especially over data‐sparse regions such as Africa. Here, a collection of state‐of‐the‐art climate models are used to study African rainfall during this period, comparing the simulations first to present‐day African rainfall (to validate the models), second to varying levels of atmospheric carbon dioxide, and lastly to newly compiled reconstructions of early Eocene rainfall (from plant fossils). The main findings are that although the models can reproduce present‐day rainfall over Africa, and compare reasonably well with the reconstructions, there is no clear rainfall signal when atmospheric carbon dioxide is increased. Nevertheless, the combination of a different continental configuration, vegetation, topography, and atmospheric carbon dioxide leads to changing rainfall patterns, connected to temperature and low‐level wind changes. Key Points: State‐of‐the‐art climate models are used to study African hydroclimate during the early Eocene (approximately 50 Myr ago)With increasing levels of CO2, there are changes to African precipitation, due to dynamical changes such as low‐level circulationA comparison between the models and newly compiled climate estimates shows a marginally better match at lower levels of CO2 [ABSTRACT FROM AUTHOR]

Here, we compare the ocean overturning circulation of the early Eocene (47–56 Ma) in eight coupled climate model simulations from the Deep‐Time Model Intercomparison Project (DeepMIP) and investigate the causes of the observed inter‐model spread. The most common global meridional overturning circulation (MOC) feature of these simulations is the anticlockwise bottom cell, fed by sinking in the Southern Ocean. In the North Pacific, one model (GFDL) displays strong deepwater formation and one model (CESM) shows weak deepwater formation, while in the Atlantic two models show signs of weak intermediate water formation (MIROC and NorESM). The location of the Southern Ocean deepwater formation sites varies among models and relates to small differences in model geometry of the Southern Ocean gateways. Globally, convection occurs in the basins with smallest local freshwater gain from the atmosphere. The global MOC is insensitive to atmospheric CO2 concentrations from 1× (i.e., 280 ppm) to 3× (840 ppm) pre‐industrial levels. Only two models have simulations with higher CO2 (i.e., CESM and GFDL) and these show divergent responses, with a collapsed and active MOC, respectively, possibly due to differences in spin‐up conditions. Combining the multiple model results with available proxy data on abyssal ocean circulation highlights that strong Southern Hemisphere‐driven overturning is the most likely feature of the early Eocene. In the North Atlantic, unlike the present day, neither model results nor proxy data suggest deepwater formation in the open ocean during the early Eocene, while the evidence for deepwater formation in the North Pacific remains inconclusive. Plain Language Summary: The ocean's overturning circulation refers to the replenishment of the ocean's deep water by cold dense polar surface waters and its eventual return to the surface. It affects the climate through redistribution of heat across the globe and uptake of atmosphere carbon dioxide (CO2). Here, we explore the overturning circulation of the Early Eocene, a hot period 47–56 million years ago when atmosphere CO2 levels were similar to the "worst case" projections for the end of this century, in eight climate models setup up for that time. Our results, together with available ocean circulation sediment data for the time, indicate that during the early Eocene deep water originated predominantly from cold surface waters around Antarctica. The North Atlantic source of deep water that today contributes to European's relatively mild climate for its latitude, was completely absent at the time. Interestingly, even when the carbon dioxide in the Eocene model simulations was lowered to levels similar to today and before the industrial revolution, the North Atlantic source of deep water remains absent, indicating that it is the distribution of continents and ice‐sheets, rather than CO2 that is responsible for the difference between the modern and Eocene circulation. Key Points: This study evaluates the ocean's meridional overturning circulation during the early Eocene in eight models of the DeepMIP projectThe primary region of deep‐water formation depends both on the atmospheric freshwater flux and the strait geometry in the Southern OceanCompatible with proxy records, six of eight models show that deep waters predominantly originated from the south [ABSTRACT FROM AUTHOR]

We present results from an ensemble of eight climate models, each of which has carried out simulations of the early Eocene climate optimum (EECO, ∼ 50 million years ago). These simulations have been carried out in the framework of the Deep-Time Model Intercomparison Project (DeepMIP; http://www.deepmip.org , last access: 10 January 2021); thus, all models have been configured with the same paleogeographic and vegetation boundary conditions. The results indicate that these non-CO 2 boundary conditions contribute between 3 and 5 ∘ C to Eocene warmth. Compared with results from previous studies, the DeepMIP simulations generally show a reduced spread of the global mean surface temperature response across the ensemble for a given atmospheric CO 2 concentration as well as an increased climate sensitivity on average. An energy balance analysis of the model ensemble indicates that global mean warming in the Eocene compared with the preindustrial period mostly arises from decreases in emissivity due to the elevated CO 2 concentration (and associated water vapour and long-wave cloud feedbacks), whereas the reduction in the Eocene in terms of the meridional temperature gradient is primarily due to emissivity and albedo changes owing to the non-CO 2 boundary conditions (i.e. the removal of the Antarctic ice sheet and changes in vegetation). Three of the models (the Community Earth System Model, CESM; the Geophysical Fluid Dynamics Laboratory, GFDL, model; and the Norwegian Earth System Model, NorESM) show results that are consistent with the proxies in terms of the global mean temperature, meridional SST gradient, and CO 2 , without prescribing changes to model parameters. In addition, many of the models agree well with the first-order spatial patterns in the SST proxies. However, at a more regional scale, the models lack skill. In particular, the modelled anomalies are substantially lower than those indicated by the proxies in the southwest Pacific; here, modelled continental surface air temperature anomalies are more consistent with surface air temperature proxies, implying a possible inconsistency between marine and terrestrial temperatures in either the proxies or models in this region. Our aim is that the documentation of the large-scale features and model–data comparison presented herein will pave the way to further studies that explore aspects of the model simulations in more detail, for example the ocean circulation, hydrological cycle, and modes of variability, and encourage sensitivity studies to aspects such as paleogeography, orbital configuration, and aerosols. [ABSTRACT FROM AUTHOR]

Summary: Repetitive DNA sequences and some genes are epigenetically repressed by transcriptional gene silencing (TGS). When genetic mutants are not available or problematic to use, TGS can be suppressed by chemical inhibitors. However, informed use of epigenetic inhibitors is partially hampered by the absence of any systematic comparison. In addition, there is emerging evidence that epigenetic inhibitors cause genomic instability, but the nature of this damage and its repair remain unclear. To bridge these gaps, we compared the effects of 5‐azacytidine (AC), 2′‐deoxy‐5‐azacytidine (DAC), zebularine and 3‐deazaneplanocin A (DZNep) on TGS and DNA damage repair. The most effective inhibitor of TGS was DAC, followed by DZNep, zebularine and AC. We confirmed that all inhibitors induce DNA damage and suggest that this damage is repaired by multiple pathways with a critical role of homologous recombination and of the SMC5/6 complex. A strong positive link between the degree of cytidine analog‐induced DNA demethylation and the amount of DNA damage suggests that DNA damage is an integral part of cytidine analog‐induced DNA demethylation. This helps us to understand the function of DNA methylation in plants and opens the possibility of using epigenetic inhibitors in biotechnology. Significance Statement: We show that 2′‐deoxy‐5‐azacytidine is the most potent chemical inhibitor of DNA methylation in plants, and demonstrate that azacytidine‐like drugs cause DNA damage, which is signaled by ATM and ATR kinases and requires repair by homologous recombination. [ABSTRACT FROM AUTHOR]

The mid-Cretaceous period was one of the warmest intervals of the past 140 million years1–5, driven by atmospheric carbon dioxide levels of around 1,000 parts per million by volume6. In the near absence of proximal geological records from south of the Antarctic Circle, it is disputed whether polar ice could exist under such environmental conditions. Here we use a sedimentary sequence recovered from the West Antarctic shelf—the southernmost Cretaceous record reported so far—and show that a temperate lowland rainforest environment existed at a palaeolatitude of about 82° S during the Turonian–Santonian age (92 to 83 million years ago). This record contains an intact 3-metre-long network of in situ fossil roots embedded in a mudstone matrix containing diverse pollen and spores. A climate model simulation shows that the reconstructed temperate climate at this high latitude requires a combination of both atmospheric carbon dioxide concentrations of 1,120–1,680 parts per million by volume and a vegetated land surface without major Antarctic glaciation, highlighting the important cooling effect exerted by ice albedo under high levels of atmospheric carbon dioxide. Multi-proxy core data and model simulations support the presence of temperate rainforests near the South Pole during mid-Cretaceous warmth, indicating very high CO2 levels and the absence of Antarctic ice. [ABSTRACT FROM AUTHOR]

We present results from an ensemble of seven climate models, each of which has carried out simulations of the early Eocene climate optimum (EECO, ~50 million years ago). These simulations have been carried out in the framework of DeepMIP (www.deepmip.org), and as such all models have been configured with identical paleogeographic and vegetation boundary conditions. The results indicate that these non-CO2 boundary conditions contribute between 3 and 5 °C to Eocene warmth. Compared to results from previous studies, the DeepMIP simulations show reduced spread of global mean surface temperature response across the ensemble, for a given atmospheric CO2 concentration. In a marked departure from the results from previous simulations, at least two of the DeepMIP models (CESM and GFDL) are consistent with proxy indicators of global mean temperature, and atmospheric CO2, and meridional SST gradients. The best agreement with global SST proxies from these models occurs at CO2 concentrations of around 2400 ppmv. At a more regional scale the models lack skill in reproducing the proxy SSTs, in particular in the southwest Pacific, around New Zealand and south Australia, where the modelled anomalies are substantially less than indicated by the proxies. However, in these regions modelled continental surface air temperature anomalies are consistent with surface air temperature proxies, implying an inconsistency between marine and terrestrial temperatures in either the proxies or models in this region. Our aim is that the documentation of the large scale features and model-data comparison presented herein will pave the way to further studies that explore aspects of the model simulations in more detail, for example the ocean circulation, hydrological cycle, and modes of variability; and encourage sensitivity studies to aspects such as paleogeography and aerosols. [ABSTRACT FROM AUTHOR]

In greenhouse world conditions, low seasonality in the polar region can have significant effects on paleobotanical proxy data interpretation of environmental conditions in the Arctic region. Here we present two simulations with a Maastrichtian (~70 Ma) set-up that differ only by atmospheric CO2 levels, applying the Earth system model COSMOS in a coupled atmosphere-ocean configuration. In the first simulation with a CO2 level of 280 ppm (C-280) we observe a strong surface temperature contrast of ~20-25 °C between the summer and winter seasons over the Arctic Ocean. In the second experiment with a CO2 level of 1120 ppm (C-1120) the contrast is highly reduced to ~3 °C. Most of this seasonal temperature contrast reduction stems from relatively warm and sea ice free Arctic winter conditions in C-1120. The key to these winter conditions is the summer warming of the Arctic basin in C-1120 that effectively stores energy due to sea ice free conditions compared to ice covered conditions during summer in C-280. During the winter months, this heat reservoir and associated surface heat fluxes are sufficient to sustain relatively mild Arctic conditions and prevent sea ice formation during polar night. In this context, extremely low seasonality associated with relatively warm winters could have created unusual climatic conditions in the greenhouse Arctic. Therefore, Late Cretaceous sub/arctic plants could have been well adapted to such mild non-actualistic climate environments. [ABSTRACT FROM AUTHOR]