Fossil palm beetles refine upland winter temperatures in the Early Eocene Climatic Optimum

Eocene climate and associated biotic patterns provide an analog system to understand their modern interactions. The relationship between mean annual temperatures and winter temperatures—temperature seasonality—may be an important factor in this dynamic. Fossils of frost-intolerant palms imply low Eocene temperature seasonality into high latitudes, constraining average winter temperatures there to >8 °C. However, their presence in a paleocommunity may be obscured by taphonomic and identification factors for macrofossils and pollen. We circumvented these problems by establishing the presence of obligate palm-feeding beetles (Chrysomelidae: Pachymerina) at three localities (a fourth, tentatively) in microthermal to lower mesothermal Early Eocene upland communities in Washington and British Columbia. This provides support for warmer winter Eocene climates extending northward into cooler Canadian uplands.Eocene climates have received intense scrutiny in recent decades, providing an increasingly detailed analog system with which to assess the dynamics of modern global climate change, thereby allowing prediction of its long-term trends and biotic consequences. It was a time of increased global temperatures associated with elevated atmospheric carbon levels, particularly during episodic hyperthermal events such as the Early Eocene Climatic Optimum (EECO) (1, 2). The Eocene was also notable for greater climatic homogeneity, with low latitudinal temperature gradients coupled with low temperature seasonality extending to the poles, creating a global climate type unknown today outside of restricted regions. This is thought to have had far-ranging biological consequences, e.g., for intercontinental dispersal, global biodiversity patterns, community structures, and evolutionary trajectories (e.g., refs. 2–6). Here, we use the presence of insects that have obligate associations with floral climatic indicators to refine characterization of winter climate in a cool higher midlatitude upland during the EECO, and therefore the environmental context of these biotic patterns during an interval of global warmth.A key factor in these dynamics may have been warmer winter temperatures [coldest month mean temperature (CMMT)] relative to mean annual temperature (MAT) even in mid- and higher latitude regions of cooler MAT (3). Consequently—as in the modern tropics and some southern hemisphere midlatitude coastal areas—Eocene organisms across the globe would not have been burdened by the necessity to allocate resources to endure hostile extreme winter climates with its costs for metabolic and insulation adaptations, or escape its effects by an annual period of dormancy or expenditure of large amounts of energy in migration. This would allow the potential of near year-round reproduction, feeding, and growth, providing an explanatory context for observed differential Eocene diversity patterns. In the geologically brief time scale of our modern interval of global climate change, the partial release of such constraints by warming winters is seen to have had a large influence on natural communities, with resulting high economic and social impact, e.g., the current dramatic mountain pine beetle infestations of western North America (7, 8).     

Fifty million years of beetle evolution along the Antarctic Polar Front

Global cooling and glacial–interglacial cycles since Antarctica’s isolation have been responsible for the diversification of the region’s marine fauna. By contrast, these same Earth system processes are thought to have played little role terrestrially, other than driving widespread extinctions. Here, we show that on islands along the Antarctic Polar Front, paleoclimatic processes have been key to diversification of one of the world’s most geographically isolated and unique groups of herbivorous beetles—Ectemnorhinini weevils. Combining phylogenomic, phylogenetic, and phylogeographic approaches, we demonstrate that these weevils colonized the sub-Antarctic islands from Africa at least 50 Ma ago and repeatedly dispersed among them. As the climate cooled from the mid-Miocene, diversification of the beetles accelerated, resulting in two species-rich clades. One of these clades specialized to feed on cryptogams, typical of the polar habitats that came to prevail under Miocene conditions yet remarkable as a food source for any beetle. This clade’s most unusual representative is a marine weevil currently undergoing further speciation. The other clade retained the more common weevil habit of feeding on angiosperms, which likely survived glaciation in isolated refugia. Diversification of Ectemnorhinini weevils occurred in synchrony with many other Antarctic radiations, including penguins and notothenioid fishes, and coincided with major environmental changes. Our results thus indicate that geo-climatically driven diversification has progressed similarly for Antarctic marine and terrestrial organisms since the Miocene, potentially constituting a general biodiversity paradigm that should be sought broadly for the region’s taxa.

Antarctica’s isolation, cooling, and glacial–interglacial cycles over the Cenozoic have resulted in the remarkable diversification of a unique marine fauna (1, 2). The investigation of marine radiations in Antarctica has reshaped modern understanding of biodiversity processes, for example, by revealing a surprising inverse latitudinal gradient in diversification rates for fish and brittle stars (3–5). In contrast, Antarctica’s paleoclimatic legacy for terrestrial communities has long been considered one of widespread extinction due to glaciation. Evidence of terrestrial species surviving in Antarctic glacial refugia (6) and discoveries of substantial endemic diversity and biogeographic structuring in some groups (7, 8) is changing this narrative, indicating extended evolutionary histories on land. Yet, such evolutionary histories remain obscured by a lack of large-scale molecular phylogenetic work, with most Antarctic terrestrial research focused on small subsets of species or populations (9, 10). The few studies that have taken a multilocus phylogenetic approach have uncovered hidden terrestrial diversity and signals of long-term allopatric divergence (e.g., refs. 11 and 12), hinting that Cenozoic climatic processes may have driven terrestrial diversification in ways similar to that for marine life.The hypothesis that diversification has proceeded similarly in Antarctic marine and terrestrial groups has not been tested. While the extinction of a diverse continental Antarctic biota is well established (13), mounting evidence of significant and biogeographically structured Antarctic terrestrial diversity (8, 14, 15) with a long evolutionary history (6, 16) suggests the possibility of broadly similar diversification processes across marine and terrestrial Antarctic systems. If valid for some taxa, further tests should then be sought across a wider variety of organisms. Here, we therefore evaluate the terrestrial applicability of the paradigm emerging for Antarctic marine biodiversity—that a major cooling phase from the mid-Miocene climatic transition (14 Ma) onwards, and subsequent habitat restructuring, have led to significant and ongoing diversification for many taxa, including those with much older origins in the region (2, 4, 17). We do so by using one of the most well-known and speciose groups from the sub-Antarctic, the herbivorous Ectemnorhinini weevils (Coleoptera: Curculionidae) (18–20).Preliminary molecular studies indicate that the Ectemnorhinini, along with numerous other terrestrial taxa, have long histories in the sub-Antarctic, extending to the Miocene or earlier [e.g., beetles (21), midges (22), and springtails (11)]. This enables a comparison of their evolution throughout the same periods of environmental change that drove the diversification of Antarctic marine taxa. Moreover, the sub-Antarctic islands overlap spatially with the Southern Ocean, with climates that reflect oceanic conditions both past and present (23, 24). While in some respects quite different to the continental Antarctic, the islands are in other ways quite similar, providing a window into diversification processes that might be sought for continental groups, especially given their age and biogeographic structuring. Both regions share many higher taxa (e.g., ref. 25), a dynamic geo-climatic history (6, 26), a profound degree of isolation, and indications that climatic events likely structured their biota (6, 8, 27). The terrestrial habitat on the continent and its surrounding islands is fragmented by large expanses of ice or ocean, respectively, and has been further isolated by the Antarctic Circumpolar Current for at least 34 Ma (28, 29). Cyclic growth and contraction of ice sheets throughout the Plio–Pleistocene, though typically associated with the continent, has also had extensive impacts on the sub-Antarctic islands (26). The more intensively surveyed sub-Antarctic faunas thus provide an opportunity to investigate terrestrial diversification processes for the wider Antarctic while recognizing that for many groups on the continent, the main legacy of change has been extinction.To test the hypothesis that a major phase of cooling from the mid-Miocene onwards and subsequent habitat restructuring has led to the diversification of Antarctic terrestrial taxa, we integrate three tiers of molecular data to reveal a comprehensive evolutionary history for the Ectemnorhinini weevils. This additionally allows us to resolve the geographic, taxonomic, and temporal origins of the Ectemnorhinini and the role of dispersal and colonization in the development of the region’s biogeography. We first resolve the controversial origins of these weevils (19, 30) with a phylogenomic approach using anchored hybrid enrichment (AHE) for up to 515 genes across 12 representative species of Ectemnorhinini and a worldwide sample of 87 species of putative relatives and known outgroups, mostly from the beetle subfamily Entiminae (18, 30, 31). We then build on these outcomes by exploring the timing and patterns of taxonomic diversification, including divergence times and proposed dispersal events, using a multilocus phylogenetic dataset (three mitochondrial and two nuclear genes) for an extensive sample of Ectemnorhinini from each archipelago on which they are known to occur. Finally, we reveal contemporary limits to gene flow and examine the population structure of the littoral-dwelling ectemnorhinine weevil Palirhoeus eatoni using phylogeographic methods applied to a library of 5,859 genome-wide single-nucleotide polymorphisms (SNPs). This unusually widespread species is found on all four archipelagos of the Kerguelen Province known to host Ectemnorhinini: Crozet, Kerguelen, Prince Edward Islands (PEI), and Heard Island and McDonald Islands (HIMI).     

Abrupt change of Antarctic moisture origin at the end of Termination II

The deuterium excess of polar ice cores documents past changes in evaporation conditions and moisture origin. New data obtained from the European Project for Ice Coring in Antarctica Dome C East Antarctic ice core provide new insights on the sequence of events involved in Termination II, the transition between the penultimate glacial and interglacial periods. This termination is marked by a north–south seesaw behavior, with first a slow methane concentration rise associated with a strong Antarctic temperature warming and a slow deuterium excess rise. This first step is followed by an abrupt north Atlantic warming, an abrupt resumption of the East Asian summer monsoon, a sharp methane rise, and a CO₂ overshoot, which coincide within dating uncertainties with the end of Antarctic optimum. Here, we show that this second phase is marked by a very sharp Dome C centennial deuterium excess rise, revealing abrupt reorganization of atmospheric circulation in the southern Indian Ocean sector.     

East African megadroughts between 135 and 75 thousand years ago and bearing on early-modern human origins

The environmental backdrop to the evolution and spread of early Homo sapiens in East Africa is known mainly from isolated outcrops and distant marine sediment cores. Here we present results from new scientific drill cores from Lake Malawi, the first long and continuous, high-fidelity records of tropical climate change from the continent itself. Our record shows periods of severe aridity between 135 and 75 thousand years (kyr) ago, when the lake's water volume was reduced by at least 95%. Surprisingly, these intervals of pronounced tropical African aridity in the early late-Pleistocene were much more severe than the Last Glacial Maximum (LGM), the period previously recognized as one of the most arid of the Quaternary. From these cores and from records from Lakes Tanganyika (East Africa) and Bosumtwi (West Africa), we document a major rise in water levels and a shift to more humid conditions over much of tropical Africa after approximately 70 kyr ago. This transition to wetter, more stable conditions coincides with diminished orbital eccentricity, and a reduction in precession-dominated climatic extremes. The observed climate mode switch to decreased environmental variability is consistent with terrestrial and marine records from in and around tropical Africa, but our records provide evidence for dramatically wetter conditions after 70 kyr ago. Such climate change may have stimulated the expansion and migrations of early modern human populations.     

The current refugial rainforests of Sundaland are unrepresentative of their biogeographic past and highly vulnerable to disturbance

Understanding the historical dynamics of forest communities is a critical element for accurate prediction of their response to future change. Here, we examine evergreen rainforest distribution in the Sunda Shelf region at the last glacial maximum (LGM), using a spatially explicit model incorporating geographic, paleoclimatic, and geologic evidence. Results indicate that at the LGM, Sundaland rainforests covered a substantially larger area than currently present. Extrapolation of the model over the past million years demonstrates that the current “island archipelago” setting in Sundaland is extremely unusual given the majority of its history and the dramatic biogeographic transitions caused by global deglaciation were rapid and brief. Compared with dominant glacial conditions, lowland forests were probably reduced from approximately 1.3 to 0.8 × 10⁶ km² while upland forests were probably reduced by half, from approximately 2.0 to 1.0 × 10⁵ km². Coastal mangrove and swamp forests experienced the most dramatic change during deglaciations, going through a complete and major biogeographic relocation. The Sundaland forest dynamics of fragmentation and contraction and subsequent expansion, driven by glacial cycles, occur in the opposite phase as those in the northern hemisphere and equatorial Africa, indicating that Sundaland evergreen rainforest communities are currently in a refugial stage. Widespread human-mediated reduction and conversion of these forests in their refugial stage, when most species are passing through significant population bottlenecks, strongly emphasizes the urgency of conservation and management efforts. Further research into the natural process of fragmentation and contraction during deglaciation is necessary to understand the long-term effect of human activity on forest species.     

From the Cover: An adaptability limit to climate change due to heat stress

Despite the uncertainty in future climate-change impacts, it is often assumed that humans would be able to adapt to any possible warming. Here we argue that heat stress imposes a robust upper limit to such adaptation. Peak heat stress, quantified by the wet-bulb temperature T_W, is surprisingly similar across diverse climates today. T_W never exceeds 31 °C. Any exceedence of 35 °C for extended periods should induce hyperthermia in humans and other mammals, as dissipation of metabolic heat becomes impossible. While this never happens now, it would begin to occur with global-mean warming of about 7 °C, calling the habitability of some regions into question. With 11–12 °C warming, such regions would spread to encompass the majority of the human population as currently distributed. Eventual warmings of 12 °C are possible from fossil fuel burning. One implication is that recent estimates of the costs of unmitigated climate change are too low unless the range of possible warming can somehow be narrowed. Heat stress also may help explain trends in the mammalian fossil record.     

Galápagos hydroclimate of the Common Era from paired microalgal and mangrove biomarker 2H/1H values

Tropical maritime precipitation affects global atmospheric circulation, influencing storm tracks and the size and location of subtropical deserts. Paleoclimate evidence suggests centuries-long changes in rainfall in the tropical Pacific over the past 2,000 y, but these remain poorly characterized across most of the ocean where long, continuous proxy records capable of resolving decadal-to-centennial climate changes are still virtually nonexistent despite substantial efforts to develop them. Here we apply a new climate proxy based on paired hydrogen isotope ratios from microalgal and mangrove-derived sedimentary lipids in the Galápagos to reconstruct maritime precipitation changes during the Common Era. We show that increased rainfall during the Little Ice Age (LIA) (∼1400–1850 CE) was likely caused by a southward migration of the Intertropical Convergence Zone (ITCZ), and that this shift occurred later than previously recognized, coeval with dynamically linked precipitation changes in South America and the western tropical Pacific. Before the LIA, we show that drier conditions at the onset of the Medieval Warm Period (∼800–1300 CE) and wetter conditions ca. 2 ka were caused by changes in the El Niño/Southern Oscillation (ENSO). Collectively, the large natural variations in tropical rainfall we detect, each linked to a multicentury perturbation of either ENSO-like variability or the ITCZ, imply a high sensitivity of tropical Pacific rainfall to climate forcings.Tropical Pacific precipitation patterns have a profound impact on global climate, and changes are projected far outside the region of origin (1). Coherent understanding of these climate dynamics is therefore critical for understanding when and how the distribution and intensity of global precipitation patterns have changed in the past and will change in the future, with far-reaching implications for managing the demand for freshwater resources in major population and agricultural centers in the tropics and midlatitudes. Tropical Pacific precipitation is largely dominated by zonally asymmetric variability associated with El Niño/Southern Oscillation (ENSO) and the zonally symmetric annual north–south migration of the Intertropical Convergence Zone (ITCZ). The extensive geographic footprint and intensity of these phenomena suggests that capturing their evolution in the paleoclimate record and within Earth system models should be straightforward, but in practice these targets have proven elusive. Perennial problems persist in simulating realistic ITCZ and ENSO dynamics, even in the latest generation of state-of-the-art climate models (2, 3). Paleoclimate records should theoretically be able to help constrain some of these dynamics, but it is challenging to distinguish between ITCZ- and ENSO-driven changes in a record from a single location because rainfall alone is influenced by both phenomena (4–8). Networks of paleoclimate records can help to resolve these issues, but proxy archives that are within the core ITCZ and ENSO regions (i.e., at sea level in the tropical Pacific) and have both the temporal resolution and duration to record the decadal to centennial changes that are of greatest societal relevance have been difficult to obtain.The Galápagos archipelago in the eastern equatorial Pacific is in a key center of action for ENSO and is ideally located for testing hypotheses regarding changes in the southern extent of annual ITCZ migration (7, 9). Precipitation is highly variable on seasonal and interannual timescales and correlated with the Niño 1–4 indices (Fig. 1 and Fig. S1) as well as with the multivariate ENSO index, local sea surface temperature, and the isotopic composition of precipitation (4, 5, 10). El Niño events bring heavy rain to the Galápagos, and changes in their intensity or recurrence interval manifest in the local precipitation record, but with increased sensitivity to eastern Pacific, as opposed to central Pacific, El Niño events (Fig. S1) (4). The islands are at the modern southernmost extent of annual ITCZ migration, and changes in its maximum southerly range bring large increases in annual rainfall (Fig. 1).Open in a separate windowFig. 1.Precipitation data and base map for Isabela lakes. (A) Average wet- (February–April) and (B) dry-season (August–October) precipitation (1997–2008; NASA GPCP). Locations: Galápagos (star), Cariaco Basin (diamond), Lake Pumacocha (square), Makassar Straight (circle). (C) Niño1+2 precipitation anomaly shows influence of eastern Pacific El Niño and La Niña events on Galápagos precipitation. Anomalies in figure are the difference between years where Niño 1+2 > 1 °C and years where Niño 1+2 < 1 °C (dataset from A and B). Comparison between Niño 1+2 and Niño 3.4 is shown in Fig. S1. (D) Map shows locations of Isabela Island lakes within the Galápagos archipelago (Map data: Google, DigitalGlobe) and coring locations in each lake (Diablas, red; Verdes, green; Escondida, blue). Bnb, Bainbridge Crater; EJ, El Junco Lake; Is, Isabela lakes.Open in a separate windowFig. S1.Comparison between Niño 1+2 and Niño 3.4 precipitation anomalies. (A) Niño 1+2 precipitation anomaly as shown in Fig. 1C, or the difference between years where Niño 1+2 > 1 °C and years where Niño 1+2 < 1 °C (dataset from Fig. 1 A and B). (B) As in A, but for Niño 3.4 index. Differences highlight the enhanced sensitivity of the Galápagos to eastern Pacific El Niño events but also demonstrate that the region is sensitive to ENSO activity as defined by both indices.The δ²H value of tropical precipitation as it falls and is temporarily sequestered in lakes (δ²H_Water) reflects its transport history, making δ²H_Water values an excellent hydroclimate proxy (11). Photoautotrophic organisms use hydrogen from these waters to synthesize lipids, transforming δ²H_Water values into lipid δ²H values (δ²H_Lipid) that are preserved in sediments over geologic time. However, δ²H_Lipid values are offset from δ²H_Water values by isotopic fractionation that occurs during biosynthesis and that varies by organism, and in response to environmental conditions such as salinity (12). This complicates efforts to apply δ²H_Lipid values as paleoclimate proxies in coastal sediments where salinity varies over time and common biomarkers are synthesized by a wide variety of organisms. Mangrove trees and cyanobacteria living in these locations access common source water, but salinity has an opposing effect on ²H/¹H fractionation expressed in their lipids (13–15). In phytoplankton ²H/¹H fractionation decreases by 0.7–2.0‰ per unit increase in salinity (14, 15), whereas in mangroves it increases by 0.7–1.7‰ (13, 16). Combining these calibrations with measured algal and mangrove δ²H_Lipid values provides a method to simultaneously and quantitatively reconstruct salinity and δ²H_Water values. This approach circumvents shortcomings that have previously hindered even qualitative application of δ²H_Lipid values in these environments and opens the door for widespread application of this technique in the high-accumulation-rate saline coastal lakes that are common in the tropics. This newly developed paired biomarker approach was applied to sediments collected from three coastal saline ponds on Isabela Island in the Galápagos archipelago (Fig. 1) to reconstruct salinity and δ²H_Water values spanning the past 2,000 y of the Common Era.     

Paleo-ENSO influence on African environments and early modern humans

In this study, we synthesize terrestrial and marine proxy records, spanning the past 620 ky, to decipher pan-African climate variability and its drivers and potential linkages to hominin evolution. We find a tight correlation between moisture availability across Africa to El Niño Southern Ocean oscillation (ENSO) variability, a manifestation of the Walker Circulation, that was most likely driven by changes in Earth’s eccentricity. Our results demonstrate that low-latitude insolation was a prominent driver of pan-African climate change during the Middle to Late Pleistocene. We argue that these low-latitude climate processes governed the dispersion and evolution of vegetation as well as mammals in eastern and western Africa by increasing resource-rich and stable ecotonal settings thought to have been important to early modern humans.

The role of climatically driven environmental change in triggering key stages of hominin evolution over the last 6 My has long been recognized (1–3). More recently, environmental changes across Africa have been implicated in major shifts in the population structure of hominins over the last half of a million years—the key demographic context for the emergence of Homo sapiens (4–8). However, evaluating the impact of environmental changes and their possible effects on hominin evolution and demography is difficult, as high-resolution climate archives are temporally and spatially sparse (9–11). Further problems are introduced by the fact that available proxy studies usually detail climate variability of only one study site or region (3, 12, 13), which makes it difficult to study its consequences for evolutionary processes across large spatial scales. Here, we provide a pan-African view on climate change during the Middle to Late Pleistocene in order to construct a framework for understanding hominin evolution within this timeframe. To achieve this, we have combined 11 terrestrial lacustrine and marine sedimentary archives (Fig. 1 and SI Appendix, Tables S1 and S2; see details on site selection criteria in the SI Appendix) detailing wet–dry variability of eastern and western Africa during the last 620 ky—the time interval of the emergence of H. sapiens in Africa and its subsequent out-of-Africa dispersal (Fig. 1 and SI Appendix, Table S1).Open in a separate windowFig. 1.Suite of study sites and the global WC. (A) Location map of marine and terrestrial proxy records used for the reconstruction of African climate. Note that the black dots associated with marine sites (small gray dots) are referred to their respective hinterland region. SAH = Ocean Drilling Program (ODP) Site 659; LIB = ODP663; BOS = Lake Bosumtwi; CON = ODP Site 1075; NAM1 = GeoB1028-5; NAM2 = ODP Site 1082; LOM = MD96-2048; MAL = Lake Malawi; MAG = Lake Magadi; CHB = Paleolake Chew Bahir; and MED = ODP Site 967. The full list of references and coordinates for all sites is provided in SI Appendix, Table S1. Blue dots mark marine sites used for the reconstruction of the WC (SI Appendix, Table S2). (B) SST anomalies and resulting changes in tropical heating and convection (related to WC) under El Niño conditions (positive ENSO phase). (C) SST anomalies and resulting changes in tropical heating and convection (related to WC) under La Niña conditions (negative ENSO phase). For more details on the effect of El Niño/La Niña on African precipitation, see SI Appendix. Blue areas = cooling relative to normal conditions; red areas = warming relative to normal conditions; and black arrows indicate transport direction.Today, the climate of tropical Africa is governed by convection, with the seasonal migration of the tropical rain belt dictating the pattern of precipitation (9). Changes in seasonal positioning of the rain belt relate to insolation variability, with rainfall occurring in northern/southern Africa during boreal/austral summer (14). In addition, observational data suggest that the African climate is highly sensitive to changes in the Walker circulation (WC), which is manifested via the El Niño Southern Oscillation (ENSO) (15, 16; see SI Appendix for more details). ENSO originates from sea surface temperature (SST) anomalies in the equatorial Pacific Ocean, and these changes impact the atmospheric WC, which in turn alters the location and strength of tropical convection cells (Fig. 1) (17). Through this coupled ocean–atmosphere system, ENSO events are propagated around the globe by Kelvin and Rossby waves (16), eventually reaching the African continent (see SI Appendix for more details). Here, changes in ENSO state alter the east–west trending moisture gradient across Africa (18–20). This leads to opposing dry and humid conditions between eastern and western Africa so that, during La Niña, eastern Africa experiences drier conditions than western Africa and vice versa during El Niño events. For instance, during El Niño years, eastern Africa experiences positive precipitation anomalies of up to 60% (or 200 mm per year) relative to the yearly precipitation budget during non–El Niño years, while western Africa experiences a 20 to 40% precipitation reduction (21, 22). Besides these modern driving mechanisms, spatiotemporal precipitation changes in Africa on much longer time scales have also been attributed to changes in Atlantic meridional overturning circulation, global atmospheric CO₂ concentrations (pCO₂), and/or the waning and waxing of global ice sheets (23–26). However, the interplay of these various driving mechanisms on orbital time scales and their pan-African impact on precipitation remains ambiguous.     

Low clouds suppress Arctic air formation and amplify high-latitude continental winter warming

High-latitude continents have warmed much more rapidly in recent decades than the rest of the globe, especially in winter, and the maintenance of warm, frost-free conditions in continental interiors in winter has been a long-standing problem of past equable climates. We use an idealized single-column atmospheric model across a range of conditions to study the polar night process of air mass transformation from high-latitude maritime air, with a prescribed initial temperature profile, to much colder high-latitude continental air. We find that a low-cloud feedback—consisting of a robust increase in the duration of optically thick liquid clouds with warming of the initial state—slows radiative cooling of the surface and amplifies continental warming. This low-cloud feedback increases the continental surface air temperature by roughly two degrees for each degree increase of the initial maritime surface air temperature, effectively suppressing Arctic air formation. The time it takes for the surface air temperature to drop below freezing increases nonlinearly to ∼10 d for initial maritime surface air temperatures of 20 °C. These results, supplemented by an analysis of Coupled Model Intercomparison Project phase 5 climate model runs that shows large increases in cloud water path and surface cloud longwave forcing in warmer climates, suggest that the “lapse rate feedback” in simulations of anthropogenic climate change may be related to the influence of low clouds on the stratification of the lower troposphere. The results also indicate that optically thick stratus cloud decks could help to maintain frost-free winter continental interiors in equable climates.One of the persistent mysteries of the “equable climates” of the Eocene and Cretaceous,  ～  143–33 million years ago, is the warmth of midlatitude and high-latitude continental interiors during winter and, in particular, the frost-intolerant flora and fauna in parts of what is now Wyoming and southern Canada (1). Climate models can simulate warm conditions over the ocean, but they have difficulty simulating continental warmth away from the moderating effects of the ocean, especially if tropical warming is constrained to be ≲10 °C. Although recent work suggests a relaxation of such tropical constraints (2), model−data agreement has been found only for model CO₂ concentrations that seem unrealistically high, and the mechanisms that maintain high-latitude warmth over land remain poorly understood (2, 3). Previous proposed mechanisms to explain the overall reduction of the equator−pole temperature contrast in equable climates include polar stratospheric clouds (4, 5), dramatic expansion of the Hadley circulation (6), increased poleward ocean heat transport due to ocean mixing by stronger tropical cyclones (7, 8), and a convective cloud feedback (9). The convective cloud feedback has now appeared in multiple simulations of past and future climates at high CO₂ (10, 11) but is not effective at explaining warmth over land; the other possible mechanisms remain speculative at this point.Mechanisms that underlie high-latitude continental warmth in past equable climates are also potentially relevant to understanding current and future climate change. The Arctic and high-latitude land in North America and Asia have warmed much more rapidly than the global mean temperature in recent decades (12, 13). Furthermore, climate models predict a significant future amplification of winter warming over the Arctic, both land and ocean (14–16). Numerous mechanisms have been proposed to explain Arctic amplification (15), including ice albedo feedbacks (17), meridional structure in the Planck feedback (18), increased moist static energy transport by the atmosphere (19), a convective cloud feedback (9), and changes in the stability of the atmosphere, with stronger warming near the surface (20). Such surface-amplified warming—also referred to as a positive “lapse rate feedback”—leads to a smaller increase in outgoing longwave radiation than would occur for the same amount of warming spread over the depth of the troposphere, because much of the emission from the lower troposphere is absorbed before reaching the top of the atmosphere. Recent analysis across a set of climate models suggests that the lapse rate feedback is the strongest contributor to the enhanced high-latitude warming (16).Unfortunately, our understanding of why warming is surface-amplified at high latitudes is relatively poor. The high-latitude lapse rate feedback is merely a diagnostic measure, which identifies enhanced warming as a consequence of the a priori unknown vertical structure of that warming. The vertical temperature structure of the Arctic atmosphere may change due to many mechanisms, including changes in sea ice area, clouds, water vapor, or atmospheric heat transport (21). This differs from the simpler case of the tropics, where constraints imposed by moist convection allow for a direct prediction of the lapse rate and how it changes with climate (16).The goal of this paper is to suggest that both the high-latitude lapse rate feedback and winter continental warmth in equable climates may be tied to the process of very cold continental air mass formation during winter. It has been shown that lower-tropospheric mixed-phase clouds play a critical role in the formation of Arctic air (22). We show that Arctic air formation may be suppressed in warmer climates, leading to significant continental warming during winter. Specifically, we perform simulations with a single-column model of an air mass that begins over the ocean and is advected over high-latitude land. We find that increased lifetime of low-level liquid clouds with warming of the initial atmospheric state leads to a slower surface cooling rate and to a less stably stratified lower troposphere, consistent with the lapse rate feedback in climate models. We also analyze Coupled Model Intercomparison Project phase 5 (CMIP5) climate model results and show that aspects of Arctic amplification in such models are consistent with the proposed mechanism.     

From the Cover: Pluvials,droughts, the Mongol Empire,and modern Mongolia

Although many studies have associated the demise of complex societies with deteriorating climate, few have investigated the connection between an ameliorating environment, surplus resources, energy, and the rise of empires. The 13th-century Mongol Empire was the largest contiguous land empire in world history. Although drought has been proposed as one factor that spurred these conquests, no high-resolution moisture data are available during the rapid development of the Mongol Empire. Here we present a 1,112-y tree-ring reconstruction of warm-season water balance derived from Siberian pine (Pinus sibirica) trees in central Mongolia. Our reconstruction accounts for 56% of the variability in the regional water balance and is significantly correlated with steppe productivity across central Mongolia. In combination with a gridded temperature reconstruction, our results indicate that the regional climate during the conquests of Chinggis Khan’s (Genghis Khan’s) 13th-century Mongol Empire was warm and persistently wet. This period, characterized by 15 consecutive years of above-average moisture in central Mongolia and coinciding with the rise of Chinggis Khan, is unprecedented over the last 1,112 y. We propose that these climate conditions promoted high grassland productivity and favored the formation of Mongol political and military power. Tree-ring and meteorological data also suggest that the early 21st-century drought in central Mongolia was the hottest drought in the last 1,112 y, consistent with projections of warming over Inner Asia. Future warming may overwhelm increases in precipitation leading to similar heat droughts, with potentially severe consequences for modern Mongolia.Abrupt climate changes have immediate and long-lasting consequences for ecosystems and societies. Although studies have linked the demise of complex societies with deteriorating climate conditions (1–4), few, if any, have investigated the connection between climate, surplus resources, energy, and the rise of empires. The rapid expansion of the Mongols under Chinggis Khan (also known as Genghis Khan) from 1206 to 1227 CE resulted in the largest contiguous land empire in world history (Fig. 1, Inset). The Mongol conquests affected the history of civilizations from China to Russia, Persia to India, and even left a genetic fingerprint on the people of Eurasia (5). Although historians have proposed climate as a possible factor in Mongol history (6), few paleoenvironmental data of the necessary temporal resolution are available to evaluate the role of climate, grassland productivity, and energy in the rise of the 13th-century Mongol Empire.Open in a separate windowFig. 1.Tree-ring drought reconstruction site (green cross) and inferred temperature site (8) (white cross) are 50 km apart. Map of the Mongol Empire near its zenith (aqua) in 1260 CE (Inset). The ancient capital city of Karakorum (black triangle) and current capital of Mongolia, Ulaanbaatar (black star).Lake sediment data from central Mongolia suggest that the climate of the Mongol Empire may have been unusually wet (7), but the temporal resolution of these records is too coarse to capture conditions during the 2 decades of rapid growth of the Mongol Empire. Annual tree-ring records of past temperature from central Mongolia extending back to 558 CE document warm conditions during the 11th century, consistent with other Northern Hemisphere records, but also indicate a subsequent warm period during the 12th and 13th centuries (8, 9). Millennium-long reconstructions of past precipitation in western China, mostly located on the Tibetan Plateau and north central China (10–15), document drought during the early 1200s. However, periods of drought in central Mongolia are generally out of phase with drought on the Tibetan Plateau (2), and there is little reason to believe that moisture conditions on the Tibetan Plateau and north central China would be consistent with that of central Mongolia. Here we present the first, to our knowledge, annually resolved record of moisture balance covering the last millennium for the Asian steppe. This new record allows us to evaluate the hypothesis that drought drove the 13th-century Mongol expansion into Eurasia (6, 16).