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Stratospheric dynamics and chemistry can impact the tropospheric climate through changing radiatively active atmospheric constituents and stratosphere‐troposphere interactions. The impact of stratospheric dynamics and chemistry on the Last Glacial Maximum (LGM) climate is not well‐studied and remains an uncertain aspect of glacial‐interglacial climate change. Here we perform coupled LGM simulations using the Community Earth System Model version 2 (CESM2), with a high‐top atmosphere—the Whole Atmosphere Community Climate Model version 6 with a middle atmosphere chemistry mechanism (WACCM6ma). The CESM2(WACCM6ma) LGM simulations show a weaker stratospheric circulation than the preindustrial, 10%–35% less tropospheric ozone and 10%–50% more ozone in the lower stratosphere. These dynamical and chemical changes cause slightly more cooling (<5%) in LGM surface and tropospheric temperatures than parallel simulations using a low‐top atmosphere without active chemistry. Results from our model suggest that stratospheric dynamics and chemistry may have little direct effect on the glacial‐interglacial climate change. Plain Language Summary: The Last Glacial Maximum (LGM), the peak of the last ice age about 20,000 years ago, has been used frequently to calculate the Earth's climate sensitivity and to evaluate Earth System Models (ESM). These applications of the LGM climate rely on an accurate understanding of the LGM temperature and its relationship with changes in Earth's orbits and the subsequent physical, chemical, and biological interactions in the system. How the changes in stratospheric dynamics and chemistry impact the LGM temperature is important but not well studied. Here we address this question using a state‐of‐the‐art ESM that can simulate explicitly stratospheric dynamics and chemistry. A comparison with parallel simulations using the same ESM but without the capability of stratospheric dynamics and chemistry shows that stratospheric processes exert only a minor impact on the changes in the LGM surface temperature (approximately −0.21°C in global annual mean, or about <5% of the total change). Our results, if confirmed by other ESMs, suggest that stratospheric dynamics and chemistry do not affect much glacial‐interglacial climate change, and that ESMs without stratospheric dynamics and chemistry are sufficient for climate sensitivity and model evaluation studies using the LGM. Key Points: We present coupled Last Glacial Maximum (LGM) simulations using CESM2(WACCM6ma) with a high‐top atmosphere and compare them with parallel low‐top simulationsThe high‐top LGM simulations show weaker stratospheric circulation and substantial ozone changes in the troposphere and lower stratosphereThe active stratospheric dynamics and chemistry in CESM2(WACCM6ma) cause little change (<5%) in LGM surface and tropospheric temperatures [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]

The Community Earth System Model version 2 (CESM2) simulates a high equilibrium climate sensitivity (ECS > 5°C) and a Last Glacial Maximum (LGM) that is substantially colder than proxy temperatures. In this study, we examine the role of cloud parameterizations in simulating the LGM cooling in CESM2. Through substituting different versions of cloud schemes in the atmosphere model, we attribute the excessive LGM cooling to the new CESM2 schemes of cloud microphysics and ice nucleation. Further exploration suggests that removing an inappropriate limiter on cloud ice number (NoNimax) and decreasing the time‐step size (substepping) in cloud microphysics largely eliminate the excessive LGM cooling. NoNimax produces a more physically consistent treatment of mixed‐phase clouds, which leads to an increase in cloud ice content and a weaker shortwave cloud feedback over mid‐to‐high latitudes and the Southern Hemisphere subtropics. Microphysical substepping further weakens the shortwave cloud feedback. Based on NoNimax and microphysical substepping, we have developed a paleoclimate‐calibrated CESM2 (PaleoCalibr), which simulates well the observed twentieth century warming and spatial characteristics of key cloud and climate variables. PaleoCalibr has a lower ECS (∼4°C) and a 20% weaker aerosol‐cloud interaction than CESM2. PaleoCalibr represents a physically more consistent treatment of cloud microphysics than CESM2 and is a valuable tool in climate change studies, especially when a large climate forcing is involved. Our study highlights the unique value of paleoclimate constraints in informing the cloud parameterizations and ultimately the future climate projection. Plain Language Summary: The Community Earth System Model version 2 (CESM2) shows a much higher equilibrium climate sensitivity (ECS > 5°C) than its predecessor models (≤4°C), which, if true, implies a greater future warming than previously thought and a more severe challenge for climate adaptation and mitigation. It is critical to determine whether the high ECS is realistic and what causes its increase. In a previous study, we suggested that the high ECS is likely unrealistic because CESM2 simulates excessive cooling for an ice age climate—the Last Glacial Maximum (LGM; ∼21,000 years ago). In this study, we investigate which aspects of CESM2 are responsible for the extreme LGM cooling and the high ECS. We find that the simulated LGM climate is very sensitive to treatments of cloud microphysical processes, and that removing an inappropriate limiter on cloud ice number and using a smaller time‐step size in the microphysics largely eliminate the excessive LGM cooling. With these microphysical modifications, CESM2 simulates a much lower ECS (∼4°C) and matches present‐day observations well. Our study suggests that an ECS > 5°C is likely unrealistic and highlights the importance of using past climates to inform and validate the model development including the treatment of clouds. Key Points: Excessive Last Glacial Maximum (LGM) cooling and an ECS > 5°C in Community Earth System Model version 2 are attributed to cloud microphysical processes including ice nucleationA new configuration (PaleoCalibr) is developed that removes an inappropriate cloud‐ice‐number limiter and decreases microphysical timestepPaleoCalibr simulates realistic LGM and modern climates, a lower ECS (3.9°C), and a weaker shortwave cloud feedback [ABSTRACT FROM AUTHOR]

During the Last Interglacial, approximately 129 to 116 ka (thousand years ago), the Arctic summer climate was warmer than the present, and the Greenland Ice Sheet retreated to a smaller extent than its current state. Previous model‐derived and geological reconstruction estimates of the sea‐level contribution of the Greenland Ice Sheet during the Last Interglacial vary widely. Here, we conduct a transient climate simulation from 127 to 119 ka using the Community Earth System Model (CESM2), which includes a dynamic ice sheet component (the Community Ice Sheet Model, CISM2) that is interactively coupled to the atmosphere, land, ocean, and sea ice components. Vegetation distribution is updated every 500 years based on biomes simulated using a monthly climatology to force the BIOME4 equilibrium vegetation model. Results show a substantial retreat of the Greenland Ice Sheet, reaching a minimum extent at 121.9 ka, equivalent to a 3.0 m rise in sea level relative to the present day, followed by gradual regrowth. In contrast, a companion simulation employing static vegetation based on pre‐industrial conditions shows a much smaller ice‐sheet retreat, highlighting the importance of the changes in high‐latitude vegetation distribution for amplifying the ice‐sheet response. Plain Language Summary: Arctic summer climates during the Last Interglacial period (approximately 129‐116 thousand years ago) were warmer than the present, and part of the Greenland Ice Sheet likely melted, contributing substantially to global sea‐level rise. Estimates of how much the ice sheet melted range widely. We use a coupled climate model that represents different parts of the Earth system (atmosphere, ocean, sea ice, land) with an evolving Greenland Ice Sheet model to simulate the period from 127‐119 thousand years ago. Our results show that the Greenland Ice Sheet reached its minimum size 121.9 thousand years ago, losing an equivalent of 3.0 m of sea level compared to the present day. We also show that changes in Arctic vegetation distribution during the Last Interglacial have a strong influence on the climate, and therefore how much the Greenland Ice Sheet retreats. Key Points: A simulation from 127 to 119 ka with CESM2‐CISM2 reveals retreat and incipient regrowth of the Greenland Ice Sheet for the Last InterglacialThe Greenland Ice Sheet reaches a minimum extent at 121.9 ka with a contribution of 3.0 m sea‐level equivalent relative to present daySimulated forest and tundra expansion in Canada and Greenland leads to greater warming and ice loss than with a pre‐industrial land cover [ABSTRACT FROM AUTHOR]

The upper end of the equilibrium climate sensitivity (ECS) has increased substantially in the latest Coupled Model Intercomparison Projects phase 6 with eight models (as of this writing) reporting an ECS > 5°C. The Community Earth System Model version 2 (CESM2) is one such high‐ECS model. Here we perform paleoclimate simulations of the Last Glacial Maximum (LGM) using CESM2 to examine whether its high ECS is realistic. We find that the simulated LGM global mean temperature decrease exceeds 11°C, greater than both the cooling estimated from proxies and simulated by an earlier model version (CESM1). The large LGM cooling in CESM2 is attributed to a strong shortwave cloud feedback in the newest atmosphere model. Our results indicate that the high ECS of CESM2 is incompatible with LGM constraints and that the projected future warming in CESM2, and models with a similarly high ECS, is thus likely too large. Plain Language Summary: Equilibrium climate sensitivity (ECS) is one of the most important metrics in climate science. It measures the amount of global warming over hundreds of years after a doubling of the atmospheric CO2 concentration. An ECS range of 1.5°C–4.5°C has been consistently supported by climate models over the past 40 years. However, this has changed in the latest generation of climate models with eight (as of this writing) showing an ECS > 5°C. Such a high ECS implies that future warming will be much greater than previously thought for the same amount of greenhouse gas emissions, making it more challenging for human and natural systems to adapt. This study examines whether the ECS in one "high‐ECS" model—Community Earth System Model version 2 (CESM2)—is realistic. Our approach is to perform a paleoclimate simulation of the culmination of the last glacial period, which was colder and had a lower atmospheric CO2 level than today. We find that the magnitude of cooling in the CESM2 simulation is much larger than supported by the observational evidence, indicating the model's ECS is too large. We further find that the high ECS of CESM2 is caused by the treatment of clouds in the model. Key Points: Community Earth System Model version 2 simulates an Last Glacial Maximum (LGM) global temperature at least 5°C colder than the proxy estimate, indicating its equilibrium climate sensitivity is too highThe large LGM cooling is caused by a strong shortwave cloud feedback in the new atmosphere modelThe shortwave cloud feedback in LGM simulations is connected to that in abrupt 4×CO2 simulations from the present‐day climate [ABSTRACT FROM AUTHOR]

Results are presented and compared for the Community Earth System Model version 2 (CESM2) simulations of the middle Holocene (MH, 6 ka) and Last Interglacial (LIG, 127 ka). These simulations are designated as Tier 1 experiments (midHolocene and lig127k) for the Coupled Model Intercomparison Project phase 6 (CMIP6) and the Paleoclimate Modeling Intercomparison Project phase 4 (PMIP4). They use the low‐top, standard 1° version of CESM2 contributing to CMIP6 DECK, historical, and future projection simulations, and to other modeling intercomparison projects. The midHolocene and lig127k provide the opportunity to examine the responses in CESM2 to the orbitally induced changes in the seasonal and latitudinal distribution of insolation. The insolation anomalies result in summer warming over the Northern Hemisphere continents, reduced Arctic summer minimum sea ice, and increased areal extent of the North African monsoon. The Arctic remains warm throughout the year. These changes are greater in the lig127k than midHolocene simulation. Other notable changes are reduction of the Niño3.4 variability and Drake Passage transport and a small increase in the Atlantic Meridional Overturning Circulation from the piControl to midHolocene to lig127k simulation. Comparisons to paleo‐data and to simulations from previous model versions are discussed. Possible reasons for mismatches with the paleo‐observations are proposed, including missing processes in CESM2, simplifications in the CMIP6 protocols for these experiments, and dating and calibration uncertainties in the data reconstructions. Plain Language Summary: The modeling of past climates, using physically based tools and comparing to paleo‐observations, is increasingly seen as a strong test of the models that are used for the projection of future climate changes. We report on simulations by one of the latest large‐scale climate models, the Community Earth System Model version 2 (CESM2), for the two most recent warm epochs: the current interglacial—the Holocene, which began 11,650 years ago ‐ and the previous interglacial—the Last Interglacial, which extended from 129,000 to 116,000 years ago. The simulations find summer warming over the Northern Hemisphere continents, reduced Arctic summer sea ice, and increased areal extent of the North African monsoon. Cryospheric and oceanic feedbacks drive year‐round Arctic warmth. The CESM2 middle Holocene and Last Interglacial simulations are an important resource for the community participating in the Coupled Model Intercomparison Project phase 6 (CMIP6). Key Points: Orbital configurations for 6 ka and 127 ka force enhanced Northern Hemisphere summer insolation versus the preindustrial, with greater anomalies at 127 kaThis forcing in CESM2 yields summer warming over Northern Hemisphere continents, reduced Arctic minimum sea ice, and increased extent of the North African monsoonThe middle Holocene and Last Interglacial simulations provide out‐of‐sample tests of CESM2 [ABSTRACT FROM AUTHOR]

We investigate hydrology during a past climate slightly warmer than the present: the last interglacial (LIG). With daily output of preindustrial and LIG simulations from eight new climate models we force hydrological model PCR‐GLOBWB and in turn hydrodynamic model CaMa‐Flood. Compared to preindustrial, annual mean LIG runoff, discharge, and 100‐yr flood volume are considerably larger in the Northern Hemisphere, by 14%, 25%, and 82%, respectively. Anomalies are negative in the Southern Hemisphere. In some boreal regions, LIG runoff and discharge are lower despite higher precipitation, due to the higher temperatures and evaporation. LIG discharge is much higher for the Niger, Congo, Nile, Ganges, Irrawaddy, and Pearl and lower for the Mississippi, Saint Lawrence, Amazon, Paraná, Orange, Zambesi, Danube, and Ob. Discharge is seasonally postponed in tropical rivers affected by monsoon changes. Results agree with published proxies on the sign of discharge anomaly in 15 of 23 sites where comparison is possible. Plain Language Summary: It is still uncertain how the water cycle will respond to a warmer climate in the coming decades. To increase our understanding of the relationships between climate and hydrology, we study the past climate of the last interglacial, which was slightly warmer than the present. We present the results of a modeling approach, showing that while Northern Hemisphere precipitation was higher during the last interglacial, discharge of rivers was even higher, and floods were even larger. On the contrary, in the Southern Hemisphere precipitation, discharge and floods were lower. We show that, for some regions, precipitation, discharge, and floods do not have the same direction of change. The seasonal timing of discharge also changes for some large basins of the Northern Hemisphere. Finally, for 23 sites, we compare our results to geological evidence. These results form a useful term of comparison to both projections of the future and geological studies of past hydrology. Key Points: We present the first modeling of hydrology and floods for the last interglacialBoreal precipitation and runoff are higher than preindustrial, and boreal river discharge and flood volume are (in %) even higherMost of the discharge occurs later in the year in large river basins of the Northern Hemisphere [ABSTRACT FROM AUTHOR]

Three new equilibrium mid‐Pliocene (MP) simulations are implemented with the Community Climate System Model version 4 (CCSM4) and Community Earth System Model versions 1.2 (CESM1.2) and 2 (CESM2). All simulations are carried out with the same boundary and forcing conditions following the protocol of Pliocene Model Intercomparison Project Phase 2 (PlioMIP2). These simulations reveal amplified MP climate change relative to the preindustrial going from CCSM4 to CESM2, seen in global and polar averages of surface warming, sea ice reduction in both the Arctic and the Antarctic, and weakened Hadley circulation. The enhanced global mean warming arises from enhanced Earth system sensitivity (ESS) to not only CO2 change but also changes in boundary conditions primarily from vegetation and ice sheets. ESS is amplified by up to 70% in CCSM4 and up to 100% in CESM1.2 and CESM2 relative to the equilibrium climate sensitivity of respective models. Simulations disagree on several climate metrics. Different from CCSM4, both CESM1.2 and CESM2 show reduction of cloud cover, and weakened Walker circulation accompanied by an El Niño‐like mean state of the tropical Pacific in MP simulations relative to the preindustrial. This El Niño‐like mean state is consistent with paleo‐observational sea surface temperatures, suggesting an improvement upon CCSM4. The performances of MP simulations are assessed with a new compilation of observational MP sea surface temperature. The model‐data comparison suggests that CCSM4 is not sensitivity enough to the MP forcings, but CESM2 is likely too sensitive, especially in the tropics. Plain Language Summary: Our knowledge of past climate evolves with both new paleo‐observations and advancements in modeling past climates. Using the mid‐Pliocene (MP, 3.205 million years ago) as an example, we demonstrate how to implement geological reconstructions of past topography, bathymetry, and vegetation distribution in Earth system models (ESMs); how to initialize these experiments; and, finally, the new knowledge learnt from simulations with three consecutive versions of the ESMs from the same model lineage. In our simulations, the MP climate warms substantially more than the estimated amount of warming that only consider changes in CO2 radiative forcing. The simulated MP climate features strongly amplified polar warmth, massive loss of Arctic and Antarctic summer sea ice, and weakened Northern Hemispheric cell of the Hadley circulation. Interestingly, two newer versions of ESMs are more sensitive to not only CO2 changes but also changes in biome range and ice sheets than the earlier version. Paleo‐observations suggest that MP global warming is underestimated by the previous versions of models but may be overestimated by the latest version. Key Points: PlioMIP2 simulations are completed with Earth System Models from the NCAR family: CCSM4, CESM1, and CESM2Simulated mid‐Pliocene climate by CESM2 features greater changes in many climate metrics than simulations by previous versionsCESM1 and CESM2 match paleo‐observations better than CCSM4, yet CESM2 likely overestimates the Earth System Sensitivity [ABSTRACT FROM AUTHOR]

Spinning up a highly complex, coupled Earth system model (ESM) is a time consuming and computationally demanding exercise. For models with interactive ice sheet components, this becomes a major challenge, as ice sheets are sensitive to bidirectional feedback processes and equilibrate over glacial timescales of up to many millennia. This work describes and demonstrates a computationally tractable, iterative procedure for spinning up a contemporary, highly complex ESM that includes an interactive ice sheet component. The procedure alternates between a computationally expensive coupled configuration and a computationally cheaper configuration where the atmospheric component is replaced by a data model. By periodically regenerating atmospheric forcing consistent with the coupled system, the data atmosphere remains adequately constrained to ensure that the broader model state evolves realistically. The applicability of the method is demonstrated by spinning up the preindustrial climate in the Community Earth System Model Version 2 (CESM2), coupled to the Community Ice Sheet Model Version 2 (CISM2) over Greenland. The equilibrium climate state is similar to the control climate from a coupled simulation with a prescribed Greenland ice sheet, indicating that the iterative procedure is consistent with a traditional spin‐up approach without interactive ice sheets. These results suggest that the iterative method presented here provides a faster and computationally cheaper method for spinning up a highly complex ESM, with or without interactive ice sheet components. The method described here has been used to develop the climate/ice sheet initial conditions for transient, ice sheet‐enabled simulations with CESM2‐CISM2 in the Coupled Model Intercomparison Project Phase 6 (CMIP6). Plain Language Summary: Experiments with Earth system models typically use the preindustrial (1850 CE) climate as a reference point when examining the climate response to a given experiment scenario. A preindustrial simulated climate state is therefore important to develop and represent consistently, which often requires long and computationally expensive spin‐up or equilibration simulations. The latest generation Earth system models include time‐evolving ice sheet components, which complicate the task of generating a self‐consistent simulated preindustrial climate. Additional complexity arises because ice sheets interact with the rest of the climate system through complex processes and feedbacks and respond slowly to climate change over many millennia. This equilibration timescale is computationally intractable using traditional spin‐up/equilibration simulation techniques. To circumvent this challenge, we present a novel method for generating an internally consistent climate state that is suitable for models with interactive ice sheet components. This method uses fewer computational resources than traditional simulation methods, while generating a climate consistent with more expensive methods. We demonstrate the viability of the method by generating the preindustrial control climate in the Community Earth System Model Version 2 (CESM2), which includes an interactive Greenland ice sheet. Key Points: We describe a computationally tractable, iterative procedure for spinning up a coupled Earth system‐ice sheet modelEquilibrium state from the iterative procedure is similar to a more expensive traditional model spin‐up with prescribed ice sheetsThe procedure is used for developing initial conditions for transient, fully coupled simulations in the Coupled Model Intercomparison Project phase 6 [ABSTRACT FROM AUTHOR]

Prolonged periods of low light and cold temperatures at Earth's surface are hypothesized effects of the end‐Cretaceous asteroid impact. However, debate remains about the causes and consequences of this "impact winter." We perform simulations of the Chicxulub impact with an Earth system model that can simulate the evolution of extreme aerosol loading to quantify the climatic responses to emissions (soot, sulfur, and dust) associated with the impact winter. We show that all impact winter emissions can drastically reduce surface temperature and precipitation. However, only soot emission from impact‐driven firestorms is capable of reducing light to below the photosynthetic threshold for many months. Therefore, our results suggest that widespread fires may have been necessary to produce the observed pattern of marine extinction across the Cretaceous‐Paleogene boundary. We identify polar coasts and the surrounding open oceans as regions likely to have experienced the least climatic, and biotic, disruption from the impact winter. Plain Language Summary: Over 75% of all species went extinct across the Cretaceous‐Paleogene boundary (66 million years ago). This extinction coincided with a massive asteroid impact in the Yucatan Peninsula, which emitted material high into the atmosphere and potentially caused widespread fires. These emissions from the impactor, impact site, and fires would have blocked sunlight from reaching the surface, resulting in an "impact winter" that likely contributed to the mass extinction. However, details of the impact winter have been difficult to quantify. Here we use a state‐of‐the‐art Earth system model to simulate the climate responses to emission of dust and sulfur from the asteroid impact and soot from fires. We find that all simulated emission scenarios reduce sunlight at the surface, cause global cooling, and disrupt the hydrologic cycle. However, only soot emission from fires is able to reduce sunlight, and therefore prevent the growth of primary producers, for long enough to potentially starve large marine organisms. Our results suggest that widespread fires and the resulting soot emission may have been an important component of the extinction. Further, in all emission scenarios we find that the impact winter was least disruptive to the climate in the high‐latitude regions, potentially providing refugia for life. Key Points: Compared to sulfur and dust, soot emission from fires produces the largest reduction in surface light, temperature, and precipitationA prolonged reduction in surface light from soot emission could help explain the marine extinction by starvationHigh‐latitude coastal regions experienced the least climatic, and likely biotic, disruption from the impact winter [ABSTRACT FROM AUTHOR]

Monsoon responses to eruptions over the last millennium (LM) are examined in an ensemble of climate simulations as a function of eruption hemisphere. A composite analysis reveals a particularly strong sensitivity of monsoon rainfall in the year following Northern Hemisphere (NH) extratropical eruptions. Additional analysis focusing on the 18th century eruption of Mt. Laki and idealized simulations representing an analogue Southern Hemisphere eruption (SH‐Laki) reveal monsoon responses that are approximately symmetric across hemispheres, despite exhibiting asymmetries in other aspects of the climate response. We conclude that 1) latitudinally mirrored eruptions result in approximately symmetric monsoon responses, 2) disproportionate weakening (strengthening) of NH (SH) monsoons by NH eruptions over the LM resulted in part from their relatively high latitudes, and 3) uncertainty in eruption latitude fundamentally limits our ability to accurately simulate associated monsoon and tropical precipitation responses in nature. Key Points: In idealized climate model experiments, hemispherically mirrored extratropical volcanic forcing is found to drive symmetric monsoon responsesThe relatively high latitude of past northern hemisphere eruptions contributes to the relative severity of their monsoon responsesUncertain aerosol latitudinal structures fundamentally limit our ability to simulate monsoon responses to volcanism in nature [ABSTRACT FROM AUTHOR]

Forcings and feedbacks controlling the seasonally sea ice‐free Arctic Ocean during the mid‐Piacenzian Warm period (3.264–3.025 Ma, MPWP), a period when CO2 level, geography, and topography were similar to present day, remain unclear given that many complex Earth System Models with comparatively higher skills at simulating twentieth century Arctic sea ice tend to produce perennial Arctic sea ice for this period. We demonstrate that explicitly simulating aerosol‐cloud interactions and the exclusion of industrial pollutants from model forcing conditions is key to simulating seasonally sea ice‐free Arctic Ocean of MPWP. The absence of industrial pollutants leads to fewer and larger cloud droplets over the high‐latitude Northern Europe and North Pacific, which allows greater absorption of solar radiation at the surface during the early summer. This enhanced absorption triggers the seasonally runaway sea ice surface albedo feedback that gives rise to September sea ice‐free Arctic Ocean and strongly amplified northern high‐latitude surface warmth. Key Points: A newer Earth system model simulates September Sea ice‐free Arctic Ocean of mid‐Piacenzian with realistic boundary and forcing conditionsThe absence of industrial pollutants causes cloud dimming during May and June in the North Pacific and northern EuropeEarly summer warming causes the seasonally runaway ice albedo‐surface temperature feedback leading to September Arctic sea ice‐free state [ABSTRACT FROM AUTHOR]

With CO2 concentrations similar to today (410 ppm), the Pliocene Epoch offers insights into climate changes under a moderately warmer world. Previous work suggested a low zonal sea surface temperature (SST) gradient in the tropical Pacific during the Pliocene, the so‐called "permanent El Niño." Here, we recalculate SSTs using the alkenone proxy and find moderate reductions in both the zonal and meridional SST gradients during the mid‐Piacenzian warm period. These reductions are captured by coupled climate model simulations of the Pliocene, especially those that simulate weaker Walker circulation. We also produce a spatial reconstruction of mid‐Piacenzian warm period Pacific SSTs that closely resembles both Pliocene and future, low‐emissions simulations, a pattern that is, to a first order, diagnostic of weaker Walker circulation. Therefore, Pliocene warmth does not require drastic changes in the climate system—rather, it supports the expectation that the Walker circulation will weaken in the future under higher CO2. Plain Language Summary: The Pliocene Epoch is the most recent time in Earth history when CO2 levels exceeded 400 ppm. The climate was warmer than preindustrial times, with smaller ice sheets. Previous studies suggested that the Pacific ocean was stuck in a "permanent El Niño" during the Pliocene. However, climate model simulations do not predict that this would happen at CO2 levels near 400 ppm—unusual changes in climate, such as large changes in cloud cover or hurricane frequency, would be needed to explain it. In this work we reanalyze Pliocene sea surface temperature data and do not find evidence of a permanent El Niño. Our results suggest that difference in temperatures across the tropical Pacific was smaller than it is today, but only by about 1 °C. Climate model simulations agree with our new analysis, suggesting that higher CO2, along with small changes in ice, vegetation, and mountains, is enough to explain Pliocene climate. We also show that the sea surface temperature patterns in the Pliocene Pacific Ocean look similar to those that climate models predict under a low‐emissions climate change scenario. The similarity suggests that the Pliocene can help us understand how the tropics respond to an ongoing increase in CO2. Key Points: Pliocene SSTs calculated from the alkenone proxy do not support a "permanent El Niño"Pliocene model simulations can reproduce proxy‐inferred SST patterns and gradientsThe pattern of Pliocene warmth supports a weakening of Walker circulation under higher CO2 [ABSTRACT FROM AUTHOR]

Reconstructing deep time climate often relies on a limited number of data points, which can hinder inference of the large‐scale climate state. Here we use an Earth system model with idealized boundary conditions to simulate climate responses to paleogeographic changes associated with the breakup of a supercontinent. After the supercontinent splits, weaker tropical easterlies occur in the larger ocean basin, which dampens the Walker circulation and warms the equatorial ocean through reduced upwelling. Additionally, cloud formation increases across the midlatitude ocean, causing locally cooler sea surface temperatures. Over land, there is dramatic tropical cooling post breakup due to enhanced moisture and cloud formation. Consequently, the latitudinal temperature gradient responses over land and ocean oppose each other, which makes inferring large scale climate changes from spatially spare proxy records particularly challenging in this scenario. Our findings emphasize the tight coupling between geography and planetary scale climate dynamics in the tropics and subtropics. Plain Language Summary: The gradual opening of the Atlantic Ocean by seafloor spreading led to one of the most dramatic changes in land/sea distribution of the past few hundred million years. We show that this opening of an ocean basin causes tropical humidification and large‐scale reorganization of tropical circulation as well as global and regional temperature change, especially on land. These responses have implications for interpretation of past temperature reconstructions. Key Points: Simulated the climate responses to the splitting of a supercontinent with a coupled Earth system modelGoing from one to two continents cools global climate especially over land, weakens Walker circulation, and strengthens Hadley circulationContrasting responses over land and sea and across longitudes makes it difficult to infer past temperature change from sparse records [ABSTRACT FROM AUTHOR]

Abstract: Speleothem records from the South Asian summer monsoon (SASM) region display variability in the ratio of 18O and 16O (δ18O) in calcium carbonate at orbital frequencies. The dominant mode of variability in many of these records reflects cycles of precession. There are several potential explanations for why SASM speleothem records show a strong precession signal, including changes in temperature, precipitation, and circulation. Here we use an Earth system model with water isotope tracers and water‐tagging capability to deconstruct the precession signal found in SASM speleothem records. Our results show that cycles of precession‐eccentricity produce changes in SASM intensity that correlate with local temperature, precipitation, and δ18O. However, neither the amount effect nor temperature differences are responsible for the majority of the SASM δ18O variability. Instead, changes in the relative moisture contributions from different source regions drive much of the SASM δ18O signal, with more nearby moisture sources during Northern Hemisphere summer at aphelion and more distant moisture sources during Northern Hemisphere summer at perihelion. Further, we find that evaporation amplifies the δ18O signal of soil water relative to that of precipitation, providing a better match with the SASM speleothem records. This work helps explain a significant portion of the long‐term variability found in SASM speleothem records. [ABSTRACT FROM AUTHOR]

Abstract: Variations in terrestrial oxygen‐isotope reconstructions from ice cores and speleothems have been primarily attributed to climatic changes of surface air temperature, precipitation amount, or atmospheric circulation. Here we demonstrate with the fully coupled isotope‐enabled Community Earth System Model an additional process contributing to the oxygen‐isotope variations during glacial meltwater events. This process, termed “the direct meltwater effect,” involves propagating large amounts of isotopically depleted meltwater throughout the hydrological cycle and is independent of climatic changes. We find that the direct meltwater effect can make up 15–35% of the δ18O signals in precipitation over Greenland and eastern Brazil for large freshwater forcings (0.25–0.50 sverdrup (106 m3/s)). Model simulations further demonstrate that the direct meltwater effect increases with the magnitude and duration of the freshwater forcing and is sensitive to both the location and shape of the meltwater. These new modeling results have important implications for past climate interpretations of δ18O. [ABSTRACT FROM AUTHOR]

It has been difficult to access projections of global-scale climate change with high temporal resolution spaning the late Pleistocene and Holocene. This has limited our ability to discern how climate fluctuations have affected species' range dynamics and extinction processes, turn-over in ecological communities and changes in genetic diversity. PaleoView is a new freeware tool, which provides a comprehensive but easy-to-use way to generate and view paleoclimate data at temporal and spatial resolutions suitable for detecting biotic responses to major climate shifts since the last glacial maximum. Regional to global scale simulations of temperature, precipitation, humidity and mean sea level pressure can be generated from PaleoView as gridded or time series data at time intervals as short as a decade for any period during the last 21 000 yr. They can be viewed using a built-in geographical user interface or saved as data files. Modelled climate reconstructions are based on daily simulation output from the Community Climate System Model ver. 3 (CCSM3). This global coupled atmosphere-ocean-sea ice-land general circulation model accurately reproduces major climatic features associated with the most recent deglaciation event, and predicts present-day patterns of climate conditions with verified hindcast skill. By providing a portal for readily accessing climate reconstructions at high temporal resolutions, PaleoView can help to better establish the consequences of past climate fluctuations on macro-ecological patterns of biological and genetic diversity. [ABSTRACT FROM AUTHOR]