Paleoclimate research in the Maya region of Mesoamerica provides compelling evidence of drought during key periods of cultural transition in Maya society. These include the transition from the ...Preclassic to the Classic, and from Classic to the Postclassic. Previous research emphasized a causal relationship between drought and cultural change, or so-called “collapse” in the Maya region. Recent advances in the range and precision of climate-sensitive proxies and the development of new archives have enabled quantitative reconstructions of past hydroclimate, as well as providing evidence of high impact, short-duration events, such as tropical cyclones. Simultaneously, archaeological research has unearthed widespread evidence of technologies used by the Maya to exert control over water resources in urban, rural, and agricultural settings. Evidence suggests that many of these water features were in use for multiple generations, possibly centuries, and many were constructed during the Terminal Preclassic and Terminal Classic periods. We suggest that, given the availability of new archaeological and paleoclimate records, these data can be combined to identify the full complexity of Maya adaptation to hydroclimate variability to emphasize adaptation and resilience to both water scarcity and over-abundance (e.g., flooding). Such syntheses, which can offer lessons for present-day efforts to grapple with regional climate change, will benefit from additional studies in data-poor zones of the Maya region, as well as public archiving of paleoclimate and archaeological data.
Past climates inform our future Tierney, Jessica E; Poulsen, Christopher J; Montañez, Isabel P ...
Science (American Association for the Advancement of Science),
11/2020, Letnik:
370, Številka:
6517
Journal Article
Recenzirano
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As the world warms, there is a profound need to improve projections of climate change. Although the latest Earth system models offer an unprecedented number of features, fundamental uncertainties ...continue to cloud our view of the future. Past climates provide the only opportunity to observe how the Earth system responds to high carbon dioxide, underlining a fundamental role for paleoclimatology in constraining future climate change. Here, we review the relevancy of paleoclimate information for climate prediction and discuss the prospects for emerging methodologies to further insights gained from past climates. Advances in proxy methods and interpretations pave the way for the use of past climates for model evaluation-a practice that we argue should be widely adopted.
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 gradients
The pattern of Pliocene warmth supports a weakening of Walker circulation under higher CO2
The summer rainfall climate of East Asia underwent large and abrupt changes during past climates, in response to precessional forcing, glacial–interglacial cycles as well as abrupt changes to the ...North Atlantic during the Last Glacial. However, current interpretations of said changes are typically formulated in terms of modulation of summer monsoon intensity, and do not account for the known complexity in the seasonal evolution of East Asian rainfall, which exhibits sharp transition from the Spring regime to the Meiyu, and then again from the Meiyu to the Summer regime.
We explore the interpretation that East Asian rainfall climate undergoes a modulation of its seasonality during said paleoclimate changes. Following previous suggestions we focus on role of the westerly jet over Asia, namely that its latitude relative to Tibet is critical in determining the stepwise transitions in East Asian rainfall seasons. In support of this linkage, we show from observational data that the interannual co-variation of June (July–August) rainfall and upper tropospheric zonal winds show properties consistent with an altered timing of the transition to the Meiyu (Summer), and with more northward-shifted westerlies for earlier transitions.
We similarly suggest that East Asian paleoclimate changes resulted from an altered timing in the northward evolution of the jet and hence the seasonal transitions, in particular the transition of the jet from south of the Plateau to the north that determines the seasonal transition from Spring rains to the Meiyu. In an extreme scenario – which we speculate the climate system tended towards during stadial (cold) phases of D/O stadials and periods of low Northern Hemisphere summer insolation – the jet does not jump north of the Plateau, essentially keeping East Asia in prolonged Spring conditions.
We argue that this hypothesis provides a viable explanation for a key paleoproxy signature of D/O stadials over East Asia, namely the heavier mean δ18O of precipitation as recorded in speleothem records. The southward jet position prevents the low-level monsoonal flow – which is isotopically light – from penetrating into the interior of East Asia; as such, precipitation there will be heavier, consistent with speleothem records. This hypothesis can also explain other key evidences of East Asian paleoclimate changes, in particular the occurrence of dusty conditions during North Atlantic stadials, and the southward migration of the Holocene optimal rainfall.
•We explore role of seasonal rainfall transitions in East Asian paleoclimate change.•Seasonal regimes determined by meridional position of westerlies relative to Tibet.•East Asian paleoclimate changes reflect systematic meridional shifts to westerlies.•Modern-day analogs and model simulations support this hypothesis.•Hypothesis may partly explain cave records of East Asian paleoclimate.
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
The North American Monsoon (NAM) is a critical component of the hydrologic cycle in southwestern North America. However, our understanding of the long‐term evolution of this monsoon system is ...incomplete due to a paucity of long‐term paleoclimatic records from the core and periphery of the NAM domain. C. Routson et al. (2022, https://doi.org/10.1029/2022gl099772) leverage a new compilation of Holocene proxy records to generate a new composite index of NAM evolution over the Holocene. They identify a relationship between NAM strength and the latitudinal temperature gradient over the Holocene. The authors suggest that shifts in the strength of the westerlies altered NAM convection via ventilation, a process whereby the large‐scale circulation imports low moist entropy air into the monsoon domain. These results highlight the importance of paleoclimatic records, especially from past “warm” intervals, for generating new insights about the evolution of regional monsoons.
Plain Language Summary
The North American Monsoon is a circulation that brings summer rain to regions of the US southwest and western Mexico. Despite the fact that this circulation provides a majority of the precipitation in many parts of the arid southwest, we do not fully understand the factors that govern long‐term changes in this monsoon. A new paper by C. Routson et al. (2022, https://doi.org/10.1029/2022gl099772) uses proxy indicators of past summer rainfall, stretching from Central America to Arizona, to reconstruct changes in the monsoon over the geologic epoch known as the Holocene, or the last 11,000 years of Earth history. The results of this study suggest that Holocene changes in the temperature difference between the equator to pole, known as the “latitudinal temperature gradient,” exerted an important influence over monsoon strength in the southwest. This has implications for the future of the summer monsoon, especially as the high latitudes of the northern hemisphere are expected to warm in the future.
Key Point
The strength North American Monsoon tracks changes in the position of the midlatitude westerlies over the Holocene
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 CO
levels (pCO
) 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
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
. 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
forcing.
Abstract
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.
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