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The North American monsoon, the dominant source of rainfall for much of the arid US Southwest, remains one of the least understood monsoon systems. The late Pleistocene evolution of this monsoon is poorly constrained, largely because glacial changes in winter rainfall obscure summer monsoon signatures in many regional proxy records. Here, we develop deglacial records of monsoon strength from isotopic analyses of leaf wax biomarkers in marine sediment cores. Reconstructions indicate a regional decrease in monsoon rainfall during the Last Glacial Maximum, and that the deglacial trajectory of the North American monsoon closely tracks changes in North American ice cover. In climate model simulations, North American ice cover shifts the westerlies southwards, favouring the mixing of cold, dry air into the US Southwest. This process, known as ventilation, weakens the monsoon by diluting the energy fluxes required for convection. As the ice sheet retreats northwards, the monsoon strengthens, and local ocean conditions may play a larger role in regulating its intensity. We conclude that on glacial–interglacial timescales, ice-sheet-induced reorganizations of atmospheric circulation have a dominant influence on the North American monsoon.

Southern African (SA) hydroclimate is largely shaped by the interactions of atmospheric circulations, e.g., Hadley Circulation, and oceanic elements, like the Benguela Upwelling System (BUS), Agulhas System, and Antarctic Circumpolar frontal system. Large-scale changes to the Meridional Temperature Gradient (MTG) influence both the atmospheric and oceanic components of the hydroclimate system, and thus impact hydroclimate over SA. We present a leaf wax hydroclimate record from ODP 1084, in the BUS, which reveals that changes in the isotopic signature of precipitation over SA coincide with a strengthening of the MTG across the Mid-Pleistocene Transition (MPT). We use HadCM3 simulations to demonstrate the sensitivity of winter rainfall to shifts in the MTG during the MPT. Given the ongoing impacts of climate change on water resources in SA, awareness of the relationship between rainfall and shifts in Hadley Circulation could provide insight into past water availability and aid regional adaptation efforts. Leaf wax isotopes and climate modeling show that Southern African rainfall and vegetation zones shifted in response to a stronger Meridional Temperature Gradient during the Mid-Pleistocene Transition.

Future projections of southwestern African hydroclimate are highly uncertain. However, insights from past warm climates, like the Pliocene, can reveal mechanisms of future change and help benchmark models. Using leaf wax hydrogen isotopes to reconstruct precipitation (δDp) from Namibia over the past 5 million years, we find a long‐term depletion trend (−50‰). Empirical mode decomposition indicates this trend is linked to sea surface temperatures (SSTs) within the Benguela Upwelling System, but modulated by Indian Ocean SSTs on shorter timescales. The influence of SSTs on reconstructed regional hydroclimate is similar to that observed during modern Benguela Nin∼$\\tilde{n}$ o events, which bring extreme flooding to the region. Isotope‐enabled simulations and PlioMIP2 results suggest that capturing a Benguela Nin∼$\\tilde{n}$ o‐like state is key to accurately simulating Pliocene, and future, regional hydroclimate. This has implications for future regional climate, since an increased frequency of Benguela Nin∼$\\tilde{n}$ os poses risk to the ecosystems and industries in the region. Plain Language Summary Rainfall in southwestern Africa will likely be impacted by human‐caused climate change, but climate models disagree on whether the region will get wetter or drier as the planet warms. Previous studies, which used plant pollen preserved in ocean sediment, tell us that southwestern Africa was wetter during the Pliocene, a warm period approximately 5.3 to 2.5 million‐years‐ago, and got drier over time as Earth cooled. This drying is thought to be caused by a concurrent decrease in temperatures within the eastern South Atlantic Ocean. In this study we measure hydrogen isotopes in ancient plant matter and use statistical tools which indicate that rainfall patterns in southwestern Africa are also impacted by changes in Indian Ocean temperatures. This combined Atlantic and Indian Ocean influence is similar to events that we observe in modern times where areas of arid southwestern Africa get short bouts of very strong rainfall when the coastal waters warm. The area that gets strong rainfall depends on where the warm water occurs along the western coast and whether there's also warmer‐ or colder‐than‐normal water in the Indian Ocean. If the Pliocene ocean temperature patterns resembled these events, we may need to do further studies to determine whether they will become more common in the future. Key Points Plio‐Pleistocene changes in the hydrogen stable isotopic signature of leaf waxes from Southern Africa are linked to Benguela temperatures Higher frequency shifts in the record are likely driven by Indian Ocean temperatures via a mechanism observed in the modern Isotope‐enabled simulations suggest that capturing this mechanism may be key to accurately simulating past and future regional hydroclimate

Despite tectonic conditions and atmospheric CO 2 levels ( pCO 2 ) similar to those of present-day, geological reconstructions from the mid-Pliocene (3.3-3.0 Ma) document high lake levels in the Sahel and mesic conditions in subtropical Eurasia, suggesting drastic reorganizations of subtropical terrestrial hydroclimate during this interval. Here, using a compilation of proxy data and multi-model paleoclimate simulations, we show that the mid-Pliocene hydroclimate state is not driven by direct CO 2 radiative forcing but by a loss of northern high-latitude ice sheets and continental greening. These ice sheet and vegetation changes are long-term Earth system feedbacks to elevated pCO 2 . Further, the moist conditions in the Sahel and subtropical Eurasia during the mid-Pliocene are a product of enhanced tropospheric humidity and a stationary wave response to the surface warming pattern, which varies strongly with land cover changes. These findings highlight the potential for amplified terrestrial hydroclimate responses over long timescales to a sustained CO 2 forcing. In contrast to future projections, paleoclimate records often find wetter subtropics in tandem with elevated CO 2 . Here, a compilation of proxies and simulations are used to reveal the climate dynamics and feedbacks responsible for generating wet subtropics during the mid-Pliocene.

The tropical Pacific climate has an outsized impact on global climate, yet future projections are poorly constrained. Data‐model comparisons from the mid‐Pliocene warm period (∼ ${\\sim} $3.3 million years ago) can help investigate warm climate dynamics and evaluate model behavior. Here we compare proxy records to PlioMIP2 models and a model with modified cloud albedo. Relative to modern, the mid‐Pliocene warm period records show subsurface warming across the tropical Pacific, strong eastern Pacific surface warming and weak western Pacific surface warming. Using clustering analyses to group model behavior relative to the proxy data, we find the model cluster with the best fit with the proxy data has enhanced warming in mid‐latitude thermocline source water regions which connect to the equator through the ventilated thermocline. Our study shows tropical ocean heat content during the mid‐Pliocene warm period was higher than today and has broad implications for the ocean's ability to absorb anthropogenic heat.

In August 2022, Death Valley, the driest place in North America, experienced record flooding from summertime rainfall associated with the North American monsoon (NAM). Given the socioeconomic cost of these type of events, there is a dire need to understand their drivers and future statistics. Existing theory predicts that increases in the intensity of precipitation is a robust response to anthropogenic warming. Paleoclimatic evidence suggests that northeast Pacific (NEP) sea surface temperature (SST) variability could further intensify summertime NAM rainfall over the desert southwest. Drawing on this paleoclimatic evidence, we use historical observations and reanalyzes to test the hypothesis that warm SSTs on the southern California margin are linked to more frequent extreme precipitation events in the NAM domain. We find that summers with above-average coastal SSTs are more favorable to moist convection in the northern edge of the NAM domain (southern California, Arizona, New Mexico, and the southern Great Basin). This is because warmer SSTs drive circulation changes that increase moisture flux into the desert southwest, driving more frequent precipitation extremes and increases in seasonal rainfall totals. These results, which are robust across observational products, establish a linkage between marine and terrestrial extremes, since summers with anomalously warm SSTs on the California margin have been linked to seasonal or multi-year NEP marine heatwaves. However, current generation earth system models (ESMs) struggle to reproduce the observed relationship between coastal SSTs and NAM precipitation. Across models, there is a strong negative relationship between the magnitude of an ESM’s warm SST bias on the California margin and its skill at reproducing the correlation with desert southwest rainfall. Given persistent NEP SST biases in ESMs, our results suggest that efforts to improve representation of climatological SSTs are crucial for accurately predicting future changes in hydroclimate extremes in the desert southwest.

The timing and mechanisms of past hydroclimate change in northeast Mexico are poorly constrained, limiting our ability to evaluate climate model performance. To address this, we present a multiproxy speleothem record of past hydroclimate variability spanning 62.5 to 5.1 ka from Tamaulipas, Mexico. Here we show a strong influence of Atlantic and Pacific sea surface temperatures on orbital and millennial scale precipitation changes in the region. Multiple proxies show no clear response to insolation forcing, but strong evidence for dry conditions during Heinrich Stadials. While these trends are consistent with other records from across Mesoamerica and the Caribbean, the relative importance of thermodynamic and dynamic controls in driving this response is debated. An isotope-enabled climate model shows that cool Atlantic SSTs and stronger easterlies drive a strong inter-basin sea surface temperature gradient and a southward shift in moisture convergence, causing drying in this region. A stalagmite hydroclimate record (Tamaulipas, Mexico) from 62.5 to 5.1 ka showed (1) Atlantic and Pacific temperatures impacted precipitation changes and (2) there were dry conditions during Heinrich Stadials, possibly because moisture shifted south.

Southwestern North America (SWNA), like many subtropical regions, is predicted to become drier in response to anthropogenic warming. However, during the Pliocene, when carbon dioxide was above pre‐industrial levels, multiple lines of evidence suggest that SWNA was much wetter. While existing explanations for a wet Pliocene invoke increases in winter rain, recent modeling studies hypothesize that summer rain may have also played an important role. Here, we present the first direct evidence for an intensified mid‐Pliocene monsoon in SWNA using leaf wax hydrogen isotopes. These new records provide evidence that the mid‐Pliocene featured an intensified and expanded North American Monsoon. Using proxies and isotope‐enabled model simulations, we show that monsoon intensification is linked to amplified warming on the southern California margin relative to the tropical Pacific. This mechanism has clear relevance for understanding present‐day monsoon variations, since we show that intervals of amplified subtropical warming on the California margin, as are seen during modern California margin heat waves, are associated with a stronger monsoon. Because marine heat waves are predicted to increase in frequency, the future may bring intervals of “Pliocene‐like” rainfall that co‐exist with intensifying megadrought in SWNA, with implications for ecosystems, human infrastructure, and water resources. Plain Language Summary The middle Pliocene, an interval approximately 3 million years ago, has long puzzled climate scientists. Despite having higher‐than‐preindustrial carbon dioxide levels, which should result in drier conditions in subtropical regions, some subtropical regions were wet during the Pliocene. In southwestern North America, there were large permanent lakes and plant and animal species that cannot exist in arid regions. We used measurements of hydrogen isotopes in ancient plant matter to show that wet conditions in the Pliocene southwest resulted from a stronger monsoon. This stronger monsoon was caused by changes in subtropical and tropical ocean temperatures in the eastern Pacific. This study presents the first direct evidence that monsoon changes caused wet conditions in the middle Pliocene. It also has relevance for the present, since we find evidence that present‐day changes in subtropical ocean temperatures can amplify the monsoon, via a mechanism that strongly resembles what happened in the Pliocene. Our study suggests that further studies of the Pliocene can shed light on how future monsoon changes may influence wildfire, landscapes, and water resources across the southwest. Key Points Leaf wax hydrogen isotopes preserved in ocean sediments reveal evidence of a stronger mid‐Pliocene monsoon in southwestern North America Isotope‐enabled simulations show that a stronger monsoon resulted from a diminished east Pacific subtropical‐tropical temperature gradient This mechanism is relevant to understanding present‐day monsoon variability in response to California margin marine heat waves

Significance Researchers have long invoked drought to explain the demise of many pre-Columbian Mesoamerican sites. However, the climatic history of many regions of Mesoamerica remains poorly understood. This includes the region around Cantona, a large fortified city in highland Mexico that was abandoned between 900 CE and 1050 CE. We used stable isotopes and elemental concentrations from lake sediments to reconstruct past climate, and found evidence of regional aridity between 500 CE and 1150 CE. In the initial phase of drought, Cantona’s population grew, possibly as a result of regional political instability. However, by 1050 CE, long-term environmental stress likely contributed to the city’s abandonment. Our work highlights the interplay of environmental and political factors in past human responses to climate change. There is currently no consensus on the importance of climate change in Mesoamerican prehistory. Some invoke drought as a causal factor in major cultural transitions, including the abandonment of many sites at 900 CE, while others conclude that cultural factors were more important. This lack of agreement reflects the fact that the history of climate change in many regions of Mesoamerica is poorly understood. We present paleolimnological evidence suggesting that climate change was important in the abandonment of Cantona between 900 CE and 1050 CE. At its peak, Cantona was one of the largest cities in pre-Columbian Mesoamerica, with a population of 90,000 inhabitants. The site is located in the Cuenca Oriental, a semiarid basin east of Mexico City. We developed a subcentennial reconstruction of regional climate from a nearby maar lake, Aljojuca. The modern climatology of the region suggests that sediments record changes in summer monsoonal precipitation. Elemental geochemistry (X-ray fluorescence) and δ ¹⁸O from authigenic calcite indicate a centennial-scale arid interval between 500 CE and 1150 CE, overlaid on a long-term drying trend. Comparison of this record to Cantona’s chronology suggests that both the city’s peak population and its abandonment occurred during this arid period. The human response to climate change most likely resulted from the interplay of environmental and political factors. During earlier periods of Cantona’s history, increasing aridity and political unrest may have actually increased the city’s importance. However, by 1050 CE, this extended arid period, possibly combined with regional political change, contributed to the city’s abandonment.

Despite tectonic conditions and atmospheric CO2 levels (pCO2) similar to those of present-day, geological reconstructions from the mid-Pliocene (3.3-3.0 Ma) document high lake levels in the Sahel and mesic conditions in subtropical Eurasia, suggesting drastic reorganizations of subtropical terrestrial hydroclimate during this interval. Here, using a compilation of proxy data and multi-model paleoclimate simulations, we show that the mid-Pliocene hydroclimate state is not driven by direct CO2 radiative forcing but by a loss of northern high-latitude ice sheets and continental greening. These ice sheet and vegetation changes are long-term Earth system feedbacks to elevated pCO2. Further, the moist conditions in the Sahel and subtropical Eurasia during the mid-Pliocene are a product of enhanced tropospheric humidity and a stationary wave response to the surface warming pattern, which varies strongly with land cover changes. These findings highlight the potential for amplified terrestrial hydroclimate responses over long timescales to a sustained CO2 forcing.