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The evolution of past global ice sheets is highly uncertain. One example is the missing ice problem during the Last Glacial Maximum (LGM, 26 000-19 000 years before present) – an apparent 8-28 m discrepancy between far-field sea level indicators and modelled sea level from ice sheet reconstructions. In the absence of ice sheet reconstructions, researchers often use marine δ 18 O proxy records to infer ice volume prior to the LGM. We present a global ice sheet reconstruction for the past 80 000 years, called PaleoMIST 1.0, constructed independently of far-field sea level and δ 18 O proxy records. Our reconstruction is compatible with LGM far-field sea-level records without requiring extra ice volume, thus solving the missing ice problem. However, for Marine Isotope Stage 3 (57 000-29 000 years before present) - a pre-LGM period - our reconstruction does not match proxy-based sea level reconstructions, indicating the relationship between marine δ 18 O and sea level may be more complex than assumed. The configuration of past ice sheets, and therefore sea level, is highly uncertain. Here, the authors provide a global reconstruction of ice sheets for the past 80,000 years that allows to test proxy based sea level reconstructions and helps to reconcile disagreements with sea level changes inferred from models.

Drought frequency and severity are projected to increase in the future, but the changes are expected to be unevenly distributed across the globe. Based on multi-model simulations under three different future emissions and shared socioeconomic pathways, we show that a significant drought intensification is expected in dry regions, whereby the severity depends on greenhouse gas emissions and development pathways. The drought hotspots are located in the sub-tropical regions where a moderate to extreme summer drought in today’s climate is expected to become a new normal by the end of the 21 st century under the warmest scenario. On average, under the warmest future scenario, the drought occurrence rate is projected to be 100% higher than that of the low emission scenario. Further, for the regions which are currently less affected by long-lasting droughts, such as the European continent, climate models indicate a significant increase in drought occurrence probability under the warmest future scenario.

Glacial climate is marked by abrupt, millennial-scale climate changes known as Dansgaard–Oeschger cycles. The most pronounced stadial coolings, Heinrich events, are associated with massive iceberg discharges to the North Atlantic. These events have been linked to variations in the strength of the Atlantic meridional overturning circulation. However, the factors that lead to abrupt transitions between strong and weak circulation regimes remain unclear. Here we show that, in a fully coupled atmosphere–ocean model, gradual changes in atmospheric CO 2 concentrations can trigger abrupt climate changes, associated with a regime of bi-stability of the Atlantic meridional overturning circulation under intermediate glacial conditions. We find that changes in atmospheric CO 2 concentrations alter the transport of atmospheric moisture across Central America, which modulates the freshwater budget of the North Atlantic and hence deep-water formation. In our simulations, a change in atmospheric CO 2 levels of about 15 ppmv—comparable to variations during Dansgaard–Oeschger cycles containing Heinrich events—is sufficient to cause transitions between a weak stadial and a strong interstadial circulation mode. Because changes in the Atlantic meridional overturning circulation are thought to alter atmospheric CO 2 levels, we infer that atmospheric CO 2 may serve as a negative feedback to transitions between strong and weak circulation modes. During glacial climates, the strength of the Atlantic overturning circulation has changed abruptly. Climate model simulations show that gradual changes in atmospheric CO 2 levels can trigger such events via atmospheric moisture transport.

Arctic temperature is one of the most uncertain aspects of mid‐Holocene (MH) climate change modeling, usually attributed to the different responses of different models to external forcing. However, in this study, we find that significant discrepancies (i.e., the noise is close to the signal in term of climate change) in the MH Arctic temperature changes can occur within the same model and for identical external forcing due to initial ocean condition perturbations. It is shown that initial ocean perturbations can affect the surface energy budget change through the uncertain cloud effect on shortwave radiation in boreal summer. The resulted uncertain change in summer surface heat flux alters the subsequent autumn and winter sea ice and contributes to significant differences in Arctic temperature via sea ice‐albedo feedback. This study suggests that internal uncertainty of an individual model is a non‐negligible source of overall uncertainty in simulating the MH Arctic temperature change. Plain Language Summary Proxies suggest that during the mid‐Holocene (MH) (∼6,000 years ago), the climate was warmer than in the preindustrial period. However, climate models generally suggested a colder MH climate. This discrepancy between data and model results could be attributed to either the biased interpretation of proxy records or the models' unrealistic response to external forcings. From the modeling perspective, the Arctic temperature change is one of the most uncertain aspects of MH simulation, which can hinder our understanding of MH climate change and the comparison with proxy data. Generally, the uncertainty in modeling Arctic temperatures is attributed to variations in the response to external forcings among different models. However, we have uncovered that this uncertainty also exists within a single model. The responses of Arctic temperatures differ significantly in identical experiments when different initial ocean states are used. Specifically, the influence of clouds on the climate is highly sensitive to the initial ocean conditions, leading to various effects on solar radiation, which in turn affects changes in Arctic sea ice to varying degrees, ultimately resulting in distinct Arctic temperature changes during the MH. This study underscores the importance of addressing internal uncertainties within individual climate models when simulating changes in Arctic temperatures. Key Points The Arctic temperature changes in the mid‐Holocene (MH) exhibit considerable discrepancy among the experiments with initial ocean perturbation The summer cloud effect on solar insolation plays a crucial role in modifying the Arctic sea ice change and hence the temperature response The internal uncertainty within a single model is non‐negligible in simulating the MH Arctic temperature change

Changes in the magnitude of millennial-scale climate variability (MCV) during the Late Pleistocene occur as a function of changing background climate state over tens of thousands of years, an indirect consequence of slowly varying incoming solar radiation associated with changes in Earth’s orbit. However, whether astronomical forcing can stimulate MCV directly (without a change in the background state) remains to be determined. Here we use a comprehensive fully coupled climate model to demonstrate that orbitally driven insolation changes alone can give rise to spontaneous millennial-scale climate oscillations under intermediate glacial conditions. Our results demonstrate that an abrupt transition from warm interstadial to cold stadial conditions can be triggered directly by a precession-controlled increase in low-latitude boreal summer insolation and/or an obliquity-controlled decrease in high-latitude mean annual insolation, by modulating North Atlantic low-latitude hydroclimate and/or high-latitude sea ice–ocean–atmosphere interactions, respectively. Furthermore, contrasting insolation effects over the tropical versus subpolar North Atlantic, exerted by obliquity or precession, result in an oscillatory climate regime, even within an otherwise stable climate. With additional sensitivity experiments under different glacial–interglacial climate backgrounds, we synthesize a coherent theoretical framework for climate stability, elaborating the direct and indirect (dual) control by Earth’s orbital cycles on millennial-scale climate variability during the Pleistocene. Millennial-scale climate oscillations can arise from orbital forcing alone during relatively stable glacial climate states, according to an analysis of high- and low-latitude climate proxy records as well as climate modelling.

During the Middle Miocene climate transition about 14 million years ago, the Antarctic ice sheet expanded to near-modern volume. Surprisingly, this ice sheet growth was accompanied by a warming in the surface waters of the Southern Ocean, whereas a slight deep-water temperature increase was delayed by more than 200 thousand years. Here we use a coupled atmosphere–ocean model to assess the relative effects of changes in atmospheric CO 2 concentration and ice sheet growth on regional and global temperatures. In the simulations, changes in the wind field associated with the growth of the ice sheet induce changes in ocean circulation, deep-water formation and sea-ice cover that result in sea surface warming and deep-water cooling in large swaths of the Atlantic and Indian ocean sectors of the Southern Ocean. We interpret these changes as the dominant ocean surface response to a 100-thousand-year phase of massive ice growth in Antarctica. A rise in global annual mean temperatures is also seen in response to increased Antarctic ice surface elevation. In contrast, the longer-term surface and deep-water temperature trends are dominated by changes in atmospheric CO 2 concentration. We therefore conclude that the climatic and oceanographic impacts of the Miocene expansion of the Antarctic ice sheet are governed by a complex interplay between wind field, ocean circulation and the sea-ice system. During the expansion of the Antarctic ice sheet about 14 million years ago, sea surface temperatures in the Southern Ocean rose. Climate model simulations suggest that this short-lived warming was related to changes in ocean–atmosphere circulation induced by the growth of the ice sheet.

Droughts and hot extremes, individually and in combination, are intensifying, driven by anthropogenic greenhouse gas emissions. However, a globally comparable and cross‐national assessment of the future risks posed by these events remains a critical gap. Our analysis shows that under current policies, leading to ∼2.7°C warming by 2100, 28.5% ± 9.3% of the global population (roughly 2.6 ± 0.9 billion people) may face heightened compound hot‐dry extremes. Based on present‐day per capita emissions, the cumulative lifetime emissions of ∼3.4 average global citizens (or ∼1.2 average US citizens) could expose one individual to these conditions by the end of century. Tropical island nations are expected to experience the most severe increases in compound hot‐dry extremes. More critically, low‐income countries, despite contributing minimally to global emissions, are projected to suffer more frequently than high‐income countries. These findings underscore the urgent need for equity‐focused, immediate policy action to address the socio‐economic disparities exacerbated by climate change.

Coinciding with global warming, Arctic sea ice has rapidly decreased during the last four decades and climate scenarios suggest that sea ice may completely disappear during summer within the next about 50–100 years. Here we produce Arctic sea ice biomarker proxy records for the penultimate glacial (Marine Isotope Stage 6) and the subsequent last interglacial (Marine Isotope Stage 5e). The latter is a time interval when the high latitudes were significantly warmer than today. We document that even under such warmer climate conditions, sea ice existed in the central Arctic Ocean during summer, whereas sea ice was significantly reduced along the Barents Sea continental margin influenced by Atlantic Water inflow. Our proxy reconstruction of the last interglacial sea ice cover is supported by climate simulations, although some proxy data/model inconsistencies still exist. During late Marine Isotope Stage 6, polynya-type conditions occurred off the major ice sheets along the northern Barents and East Siberian continental margins, contradicting a giant Marine Isotope Stage 6 ice shelf that covered the entire Arctic Ocean. Coinciding with global warming, Arctic sea ice has rapidly decreased during the last four decades. Here, using biomarker records, the authors show that permanent sea ice was still present in the central Arctic Ocean during the last interglacial, when high latitudes were warmer than present.

The Atlantic multidecadal oscillation (AMO) and its possible change during the Holocene are examined in this study, using long-term simulations of the earth system model Community Earth System Models (COSMOS). A quasi-persistent ∼55–80-yr cycle characterizing in the North Atlantic sea surface temperature is highly associated with the multidecadal variability of the Atlantic meridional overturning circulation (AMOC) during the Holocene. This mode can be found throughout the Holocene, indicating that the AMO is dominated by internal climate variability. Stronger-than-normal AMOC results in warmer-than-normal surface temperature spreading over almost the whole North Hemisphere, in particular the North Atlantic Ocean. During the warm phase of the AMO, more precipitation is detected in the North Atlantic low and high latitudes. It also generates a dipolar seesaw pattern in the sea ice anomaly. The results reveal that the influence of the AMO can be amplified by a more vigorous AMOC variability during the early Holocene in the presence of a remnant of the Laurentide Ice Sheet and when freshwater entered the North Atlantic Ocean. This conclusion could have potential application for the past AMO reconstruction and the future AMO estimation.

Paleo‐proxy data indicate that during the Last Glacial Maximum (LGM), the volume of Antarctic Bottom Water (AABW) in the Atlantic was nearly four times greater than it is today. We employed an ocean‐only model to simulate the galcial ocean and sea‐ice conditions. Our simulations reveal two key mechanisms driving its greater volume. First, while present‐day sea ice formation is driven largely by seasonal changes, the glacial mechanism is the substantial export of sea ice toward lower latitudes. The glacial sea ice formation was more than quadruple current levels, providing a steady source of Dense Shelf Water (DSW) crucial for AABW expansion. Second, weaker mixing between North Atlantic Deep Water (NADW) and AABW during the LGM allows the latter to maintain the colder, denser properties of its DSW origin. Together, these factors clarify how glacial conditions supported significantly greater AABW volumes, aligning well with paleo‐proxy evidence. Plain Language Summary During the last ice age, Antarctic Bottom Water (AABW)—the cold, dense water that spreads along the deep ocean floor—fills a much larger portion of the Atlantic Ocean than it does today. We employ ocean model simulations to examine why this happens. First, the movement of sea ice toward lower latitudes is much greater than it is in the modern ocean. This increased sea ice movement creates a steady, abundant supply of very salty, heavy water near the Antarctic coast, which then sinks to form AABW. Second, there is less mixing between the water formed in the north and the colder, denser AABW during the ice age. This helps the AABW remain cold and heavy, allowing it to spread more widely. Together, these factors could be the reasons why the conditions during the last ice age support a much larger volume of AABW, consistent with what we see in paleo climate records. Key Points Reduced mixing between NADW and AABW preserved AABW's colder, denser properties, allowing its broader expansion during the LGM Simulations show PD sea ice production arises from seasonal variations, whereas LGM production is driven by export Model results indicate glacial sea ice production exceeds four times today's level, one of the key factors for extensive AABW