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The greenhouse-to-icehouse climate transition from the Eocene into the Oligocene is well documented by sea surface temperature records from the southwest Pacific and Antarctic margin, which show evidence of pronounced long-term cooling. However, identification of a driving mechanism depends on a better understanding of whether this cooling was also present in terrestrial settings. Here, we present a semi-continuous terrestrial temperature record spanning from the middle Eocene to the early Oligocene (~41–33 million years ago), using bacterial molecular fossils (biomarkers) preserved in a sequence of southeast Australian lignites. Our results show that mean annual temperatures in southeast Australia gradually declined from ~27 °C (±4.7 °C) during the middle Eocene to ~22–24 °C (±4.7 °C) during the late Eocene, followed by a ~2.4 °C-step cooling across the Eocene/Oligocene boundary. This trend is comparable to other temperature records in the Southern Hemisphere, suggesting a common driving mechanism, likely $$p{{\rm{CO}}_{2}}$$ . We corroborate these results with a suite of climate model simulations demonstrating that only simulations including a decline in $$p{{\rm{CO}}_{2}}$$ lead to a cooling in southeast Australia consistent with our proxy record. Our data form an important benchmark for testing climate model performance, sea–land interaction and climatic forcings at the onset of a major Antarctic glaciation. Terrestrial Southern Hemisphere cooling through the Eocene–Oligocene transition points to decreasing atmospheric CO2 dominantly driving global change, according to biomarker records from southeast Australian coals and palaeoclimate modelling.

Today, Northeastern India receives some of the highest annual rainfall totals globally. The major portion of annual precipitation in this region falls during the Indian Summer Monsoon season (June–September); however, this region also receives a significant amount of rainfall during the pre-monsoon season (March–May). Here, we quantitatively reconstruct the climate of the Upper Siwalik subgroup, eastern Himalayan region, based on fossil assemblages using the Coexistence Approach (CA). The age of the fossil assemblage is considered to be late Pliocene–early Pleistocene. Data reconstructed for the present-day and past pre-monsoon rainfall in Northeastern India indicate an increasing trend since the late Miocene–early Pliocene. During the late Pliocene–early Pleistocene (Upper Siwalik), the temperature seasonality between warm (27–28.1 °C) and cold months (22–23.6 °C) was less pronounced compared with present-day warm (27–27.7 °C) and cold (14.8–15.4 °C) months conditions at the fossil locality. The reconstructed rainfall data indicate a monsoonal type of climate having a strong seasonality in wet and dry seasons during the deposition of the Upper Siwalik sediments. Moreover, composition of the fossil floras and reconstructed palaeoclimate suggest a vegetation shift from dominantly wet evergreen to semi-evergreen at the fossil locality, coincident with an increase in length of the dry season. The comparison of reconstructed CA data and climate modelling data of a Gelasian time slice with that of previously reconstructed climate data by using Climate Leaf Analysis Multivariate Programme (CLAMP) analysis of the late Pliocene–early Pleistocene (Upper Siwalik) fossils of the same locality provides nearly the identical result. Furthermore, all the reconstructed data indicate a monsoonal type of climate during the deposition of the Upper Siwalik sediments.

Changes in the Greenland ice sheet (GIS) affect global sea level. Greenland stable water isotope (δ18O) records from ice cores offer information on past changes in the surface of the GIS. Here, we use the isotope-enabled Hadley Centre Coupled Model version 3 (HadCM3) climate model to simulate a set of last interglacial (LIG) idealised GIS surface elevation change scenarios focusing on GIS ice core sites. We investigate how δ18O depends on the magnitude and sign of GIS elevation change and evaluate how the response is altered by sea ice changes. We find that modifying GIS elevation induces changes in Northern Hemisphere atmospheric circulation, sea ice and precipitation patterns. These climate feedbacks lead to ice-core-averaged isotopic lapse rates of 0.49 ‰ (100 m)−1 for the lowered GIS states and 0.29 ‰ (100 m)−1 for the enlarged GIS states. This is lower than the spatially derived Greenland lapse rates of 0.62–0.72 ‰ (100 m)−1. These results thus suggest non-linearities in the isotope–elevation relationship and have consequences for the interpretation of past elevation and climate changes across Greenland. In particular, our results suggest that winter sea ice changes may significantly influence isotope–elevation gradients: winter sea ice effect can decrease (increase) modelled core-averaged isotopic lapse rate values by about −19 % (and +28 %) for the lowered (enlarged) GIS states, respectively. The largest influence of sea ice on δ18O changes is found in coastal regions like the Camp Century site.

A striking feature of the marine fossil record is the variable intensity of extinction during superficially similar climate transitions. Here we combine climate models and species trait simulations to explore the degree to which differing palaeogeographic boundary conditions and differing magnitudes of cooling and glaciation can explain the relative intensity of marine extinction during greenhouse–icehouse transitions in the Late Ordovician and the Cenozoic. Simulations modelled the response of virtual species to cooling climate using a spatially explicit cellular automaton algorithm. We find that palaeogeography alone may be a contributing factor, as identical changes in meridional sea surface temperature gradients caused greater extinction in Late Ordovician simulations than in Cenozoic simulations. Differences in extinction from palaeogeography are significant, but by themselves are insufficient to explain observed differences in extinction intensity. However, when simulations included inferred changes in continental flooding and interval-specific models of sea surface temperature, predicted differences in relative extinction intensity were more consistent with observations from the fossil record. Our results support the hypothesis that intense extinction in the Late Ordovician is partially attributable to exceptionally rapid and severe cooling compared to Cenozoic events. High extinction intensity during Late Ordovician but not Cenozoic transitions to glacial conditions can be attributed to both temperature and palaeogeography, according to analysis combining climate models and simulations of virtual species.

Stationary waves describe the persistent meanders in the west–east flow of the extratropical atmosphere. Here, changes in stationary waves caused by ice sheets over North America are examined and the underlying mechanisms are discussed. Three experiment sets are presented showing the stationary wave response to the albedo or topography of ice sheets, as well as the albedo and topography in combination, as the forcings evolve from 21 to 6 ka. It is found that although the wintertime stationary waves have the largest amplitude, changes due to an ice sheet are equally large in summer and winter. In summer, ice sheet albedo is the dominant cause of changes: topography alone gives an opposite response to realistic ice sheets including albedo and topography. In winter, over the Atlantic, stationary wave changes are due to the ice sheet topography; over the Pacific, they are due to the persistence of summertime changes, mediated by changes in the ocean circulation. It is found that the response of stationary waves over the last deglaciation echoes the above conclusions, with no evidence of abrupt shifts in atmospheric circulation. The response linearly weakens as the albedo and height decrease from 21 to 10 ka. As potential applications, the seasonal cycle over Greenland is shown to be sensitive primarily to changes in summer climate caused by the stationary waves; the annual mean circulation over the North Pacific is found to result from summertime, albedo-forced, stationary wave effects persisting throughout the year because of ocean dynamics.

The Arctic is warming faster than anywhere else of comparable size on Earth, impacting global climate feedbacks and the Arctic biota. However, a warm Arctic is not novel. The Late Cretaceous fossil record of the region enables a detailed reconstruction of polar environmental conditions, and a thriving extinct ecosystem, during a previous 'hothouse’ global climate. Using leaf form (physiognomy) and tree ring characteristics we reconstruct Cenomanian to Coniacian polar thermal and hydrological regimes over an average annual cycle at eight locations in NE Russia and northern Alaska. A new high spatial resolution (∼1 km) WorldClim2 calibration of the Climate Leaf Analysis Multivariate Program (CLAMP) yields results similar to, but often slightly warmer than, previous analyses, but also provides more detailed insights into the hydrological regime through the return of annual and seasonal vapour pressure deficit (VPD), potential evapotranspiration (PET) estimates and soil moisture, as well as new thermal overviews through measures of thermicity and growing degree days. The new results confirm the overall warmth of the region, particularly close to the Arctic Ocean, but reveal strong local differences that may be related to palaeoelevation in the Okhotsk–Chukotka Volcanogenic Belt in NE Russia. While rainfall estimates have large uncertainties due to year-round wet soils in most locations, new measures of VPD and PET show persistent high humidity, but with notably drier summers at all the Arctic sites.

Factorisation (also known as “factor separation”) is widely used in the analysis of numerical simulations. It allows changes in properties of a system to be attributed to changes in multiple variables associated with that system. There are many possible factorisation methods; here we discuss three previously proposed factorisations that have been applied in the field of climate modelling: the linear factorisation, the Stein and Alpert (1993) factorisation, and the Lunt et al. (2012) factorisation. We show that, when more than two variables are being considered, none of these three methods possess all four properties of “uniqueness”, “symmetry”, “completeness”, and “purity”. Here, we extend each of these factorisations so that they do possess these properties for any number of variables, resulting in three factorisations – the “linear-sum” factorisation, the “shared-interaction” factorisation, and the “scaled-residual” factorisation. We show that the linear-sum factorisation and the shared-interaction factorisation reduce to be identical in the case of four or fewer variables, and we conjecture that this holds for any number of variables. We present the results of the factorisations in the context of three past studies that used the previously proposed factorisations.

Disentangling the drivers of mean annual temperature change in Antarctica requires an understanding of seasonal temperature change. A high-resolution climate record capable of resolving summer and winter seasons could address long-standing questions about the role of orbitally-driven insolation in driving Antarctic mean-annual temperature change. Here, we present a continuous record of water isotope ratios from the West Antarctic Ice Sheet (WAIS) Divide ice core that reveals both summer and winter temperature change though the last 11,000 years. This novel record shows that summer temperatures increased through the early-to-mid Holocene, reached a plateau at 4 to 2 ka, and then decreased to the present. The observed changes are explained primarily by changes in maximum summer insolation. In the early to mid-Holocene, additional summer warming results from the retreat and thinning of the WAIS. The magnitude of summer temperature change constrains the lowering of the WAIS surface to less than 100 m since the early Holocene, consistent with geologic records. Importantly, annual mean temperatures cannot be fully explained by orbital forcing and ice sheet elevation change alone; in the early Holocene, large wintertime temperature excursions overwhelm the summer signal. These winter excursions indicate that regional heat transport anomalies, rather than local thermodynamics, can dominate the annual mean.

The climate evolution of the past few thousand years is essential for understanding the context in which civilisation arose and for understanding the natural background of anthropogenic influence. Proxy-inferred records show a complex picture of earlier warming and later cooling during the Holocene depending on region and reconstruction method. In contrast climate model simulations almost uniformly show warming throughout the past 10,000 years and for example also fail to reproduce a major advance of rainbelt over the Sahara. These discrepancies raise questions about the reliability of climate models on longer-time scales.We present a suite of four new transient Holocene simulations covering the last 8500 years using the HadCM3B-M21aD coupled general circulation. We use an optimised version of this model which is able to replicate the greening of the Sahara through changes to the atmospheric convection and vegetation schemes. We apply transient changes in Earth’s orbit, ice-sheets and sea-level and greenhouse gases, and optionally solar output, volcanic eruptions and anthropogenic land-use change. The simulations without land-use show a warming throughout the Holocene, albeit with significantly higher variability once volcanic eruptions are included. With the inclusion of land-use change temperature trends in Northern Hemisphere are reversed from around 4000 years before present.We explore the contribution of different forcings to the regional trends in the model ensemble and we compare the simulations against the Holocene reconstructions to evaluate the relative importance of each forcing. We also use the model ensemble to quantify the terrestrial coverage of proxy locations that is required to reliably infer global mean temperature variations.

Quaternary Caspian Sea level variations depended on geophysical processes (affecting the opening and closing of gateways and basin size/shape) and hydro-climatological processes (affecting water balance). Disentangling the drivers of past Caspian Sea level variation, as well as the mechanisms by which they impacted the Caspian Sea level variation, is much debated. In this study we examine the relative impacts of hydroclimatic change, ice-sheet accumulation and melt, and isostatic adjustment on Caspian Sea level change. We performed model analysis of ice-sheet and hydroclimate impacts on Caspian Sea level and compared these with newly collated published palaeo-Caspian sea level data for the last glacial cycle. We used palaeoclimate model simulations from a global coupled ocean-atmosphere-vegetation climate model, HadCM3, and ice-sheet data from the ICE-6G_C glacial isostatic adjustment model. Our results show that ice-sheet meltwater during the last glacial cycle played a vital role in Caspian Sea level variations, which is in agreement with hypotheses based on palaeo-Caspian Sea level information. The effect was directly linked to the reorganization and expansion of the Caspian Sea palaeo-drainage system resulting from topographic change. The combined contributions from meltwater and runoff from the expanded basin area were primary factors in the Caspian Sea transgression during the deglaciation period between 20 and 15 kyr BP. Their impact on the evolution of Caspian Sea level lasted until around 13 kyr BP. Millennial scale events (Heinrich events and the Younger Dryas) negatively impacted the surface water budget of the Caspian Sea but their influence on Caspian Sea level variation was short-lived and was outweighed by the massive combined meltwater and runoff contribution over the expanded basin. (c) 2022 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Orogenesis in northern Tibet determines the vegetation and plant diversity of eastern Asia by intensifying winter precipitation., The growth of the Tibetan Plateau throughout the past 66 million years has profoundly affected the Asian climate, but how this unparalleled orogenesis might have driven vegetation and plant diversity changes in eastern Asia is poorly understood. We approach this question by integrating modeling results and fossil data. We show that growth of north and northeastern Tibet affects vegetation and, crucially, plant diversity in eastern Asia by altering the monsoon system. This northern Tibetan orographic change induces a precipitation increase, especially in the dry (winter) season, resulting in a transition from deciduous broadleaf vegetation to evergreen broadleaf vegetation and plant diversity increases across southeastern Asia. Further quantifying the complexity of Tibetan orographic change is critical for understanding the finer details of Asian vegetation and plant diversity evolution.

The Tibetan Plateau was built through a succession of Gondwanan terranes colliding with Asia during the Mesozoic. These accretions produced a complex Paleogene topography of several predominantly east–west trending mountain ranges separated by deep valleys. Despite this piecemeal assembly and resultant complex relief, Tibet has traditionally been thought of as a coherent entity rising as one unit. This has led to the widely used phrase ‘the uplift of the Tibetan Plateau’, which is a false concept borne of simplistic modelling and confounds understanding the complex interactions between topography climate and biodiversity. Here, using the rich palaeontological record of the Tibetan region, we review what is known about the past topography of the Tibetan region using a combination of quantitative isotope and fossil palaeoaltimetric proxies, and present a new synthesis of the orography of Tibet throughout the Paleogene. We show why ‘the uplift of the Tibetan Plateau’ never occurred, and quantify a new pattern of topographic and landscape evolution that contributed to the development of today’s extraordinary Asian biodiversity.

Views differ on the uplift history of the SE Tibetan Plateau and causal geodynamic mechanisms, yet reliable age-constrained paleoaltimetry in this region could test growth models of the entire plateau. Here we apply carbonate clumped isotope thermometry to well-dated carbonate paleosols and marls in the Gonjo Basin, SE Tibet, to reveal the topographic evolution of the basin. The sedimentary ages of carbonates of the lower and upper Ranmugou Formation are constrained to 54-50 Ma and 44-40 Ma, respectively. The temperature derived from carbonate clumped isotope thermometry indicates the mean annual air temperature (MAAT) of the Gonjo Basin in the early Eocene was ∼24 °C, which is consistent with the warm climate indicated by palm fossils. The MAAT of the basin in the middle Eocene was ∼7 °C, 17 °C cooler than in the early Eocene. Carbonate clumped oxygen isotope thermometry-based paleoaltimetry shows the Gonjo Basin experienced a rapid uplift of 3.1 km, from ∼0.7 km in the early Eocene to ∼3.8 km in the middle Eocene. This rise explains the marked cooling. As a cause of this rapid rise, and the associated regional climate change transforming the landscape from desert to forest, we invoke crustal deformation and thickening induced by intracontinental subduction between the Lhasa and Qiangtang terranes that comprise the core of the Tibet.

Palsas and peat plateaus are climatically sensitive landforms in permafrost peatlands. Climate envelope models have previously related palsa/peat plateau distributions in Europe to modern climate, but similar bioclimatic modelling has not been attempted for North America. Recent climate change has rendered many palsas/peat plateaus in this region, and their valuable carbon stores, vulnerable. We fitted a binary logistic regression model to predict palsa/peat plateau presence for North America by relating the distribution of 352 extant landforms to gridded modern climate data. Our model accurately classified 85.3% of grid cells that contain observed palsas/peat plateaus and 77.1% of grid cells without observed palsas/peat plateaus. The model indicates that modern North American palsas/peat plateaus are supported by cold, dry climates with large seasonal temperature ranges and mild growing seasons. We used palaeoclimate simulations from a general circulation model to simulate Holocene distributions of palsas/peat plateaus at 500-year intervals. We constrained these outputs with timings of peat initiation, deglaciation, and postglacial drainage across the continent. Our palaeoclimate simulations indicate that this climate envelope remained stationary in western North America throughout the Holocene, but further east it migrated northwards during 11.5–6.0 ka BP. However, palsa extents in eastern North America were restricted from following this moving climate envelope by late deglaciation, drainage and peat initiation. We validated our Holocene simulations against available palaeoecological records and whilst they agree that permafrost peatlands aggraded earliest in western North America, our simulations contest previous suggestions that late permafrost aggradation in central Canada was climatically-driven.

Significance We present a quantitative test of end-Cretaceous extinction scenarios and how these would have affected dinosaur habitats. Combining climate and ecological modeling tools, we demonstrate a substantial detrimental effect on dinosaur habitats caused by an impact winter scenario triggered by the Chicxulub asteroid. We were not able to obtain such an extinction state with several modeling scenarios of Deccan volcanism. We further show that the concomitant prolonged eruption of the Deccan traps might have acted as an ameliorating agent, buffering the negative effects on climate and global ecosystems that the asteroid impact produced at the Cretaceous–Paleogene boundary., The Cretaceous/Paleogene mass extinction, 66 Ma, included the demise of non-avian dinosaurs. Intense debate has focused on the relative roles of Deccan volcanism and the Chicxulub asteroid impact as kill mechanisms for this event. Here, we combine fossil-occurrence data with paleoclimate and habitat suitability models to evaluate dinosaur habitability in the wake of various asteroid impact and Deccan volcanism scenarios. Asteroid impact models generate a prolonged cold winter that suppresses potential global dinosaur habitats. Conversely, long-term forcing from Deccan volcanism (carbon dioxide [CO2]-induced warming) leads to increased habitat suitability. Short-term (aerosol cooling) volcanism still allows equatorial habitability. These results support the asteroid impact as the main driver of the non-avian dinosaur extinction. By contrast, induced warming from volcanism mitigated the most extreme effects of asteroid impact, potentially reducing the extinction severity.

Accurately determining extratropical cyclone paths is key in determining regional impacts associated with precipitation and wind. It is known that the stratosphere plays an important role in atmospheric dynamics and can extend its influence down to the surface. Despite this, many attribution studies have not included a stratosphere in their experiments. We believe that not considering the stratosphere could affect the results of these experiments, so the role it has on North Atlantic storm tracks is analysed using an idealised, atmospheric only model named Isca. With the aim of identifying clear implications of including the stratosphere in storm track analysis in the North Atlantic basin, a large ensemble formed of 4 separate experiments is set up for the winter of 2013/2014. The four experiments are as follows; 1) no vertical layers in the stratosphere, 2) vertical levels extended to the upper stratosphere, 3) doubling of vertical levels throughout the atmosphere, and finally, 4) an increase of vertical levels at the tropopause. We expect that including the stratosphere, in addition to increasing vertical resolution, will help improve model representation of storm tracks and their intensities during the 2013/2014 winter. The results of this study hope to highlight how the inclusion of the stratosphere and increased vertical resolution can lead to the improvement in modelling storm track statistics, which in turn will help to make more reliable attribution statements in the future.

The Holocene ’greening’ and subsequent, possibly abrupt desertification of the Sahara is a fascinating example of natural environmental change. It was driven by a gradual decline in summer insolation but with land-atmosphere coupling by vegetation likely providing additional reinforcing feedbacks. However, the majority of general circulation models (GCMs) cannot produce enough precipitation to sustain a ’Green’ Sahara, and the transient evolution through the Holocene has therefore only been studied with a few models. We present a suite of transient simulations with the coupled atmosphere-ocean GCM HadCM3, the CMIP3 version of the Met Office’s Hadley Centre model. These simulations cover the Holocene from 10,000 years before present, and optionally include recently developed optimisations of the atmospheric convection and dynamic vegetation parameterisations. In the model run with both optimisations, HadCM3 shows a convincing ‘greening’ for the first time. This is followed by a series of abrupt oscillations in vegetation cover and hydrology, that culminates in an abrupt collapse at around 6000 years before present. We compare the behaviour in four model versions and make a detailed evaluation with available geological evidence. Our results show that the stability of climate models is determined by chosen parameter values and formulations. We conclude that novel methods of inferring suitable model state-space regions using both present day observations and palaeoclimate reconstructions are needed.

Ocean anoxic event 2 (OAE2) was a large perturbation in the Earth's ocean carbon system, occurring at approximately 93.5 Ma, and is characterised by widespread black shales deposition in sediment records. This record has been interpreted as evidence of large anoxia in the global ocean for a long period, resulting in large scale extinction of marine life. However, the exact causes of OAE2, and how it initially developed, are not fully understood. We modelled the period leading up to OAE2 using the HadCM3L global climate model with full ocean (HADOCC) and terrestrial carbon cycle (TRIFFID) modules. We compared our results to equivalent simulations using late Cretaceous (Maastrichtian) paleogeographies. This allowed us to analyse the effects of continental configuration on the development to the OAE. Our results show that restricted ocean circulation, caused by the paleobathymetry, is necessary for anoxic conditions to develop but is not sufficient alone. This suggests that continental configuration is highly important in determining the ability of the oceans to develop an OAE and may explain why they only occur during some times during Earth history.

International audience; North Pacific atmospheric and oceanic circulations are key missing pieces in our understanding of the reorganization of the global climate system since the Last Glacial Maximum. Here, using a basin-wide compilation of planktic foraminiferal δ 18 O, we show that the North Pacific subpolar gyre extended~3°further south during the Last Glacial Maximum, consistent with sea surface temperature and productivity proxy data. Climate models indicate that the expansion of the subpolar gyre was associated with a substantial gyre strengthening, and that these gyre circulation changes were driven by a southward shift of the midlatitude westerlies and increased wind stress from the polar easterlies. Using single-forcing model runs, we show that these atmospheric circulation changes are a nonlinear response to ice sheet topography/albedo and CO 2. Our reconstruction indicates that the gyre boundary (and thus westerly winds) began to migrate northward at~16.5 ka, driving changes in ocean heat transport, biogeochemistry, and North American hydroclimate. Plain language summary Despite the North Pacific's importance in the global climate system, changes in the circulation of this region since the last ice age are poorly understood. Today, the North Pacific Ocean has distinct properties north and south of~40°N: To the south, the warm surface waters form a circulation cell that moves clockwise (the subtropical gyre); to the north, the cold surface waters form a circulation cell that moves anticlockwise (the subpolar gyre). This difference in surface ocean circulation north and south of~40°N is determined by the wind patterns. Here, using a compilation of oxygen isotopes measured in the carbonate shells of fossil plankton from sediment cores across the basin, which tracks changes in the spatial pattern of temperature, we reconstruct how the position of the boundary between the gyres changed since the last ice age. Our results show that the boundary between the gyres was shifted southward by~3°during the last ice age; this indicates that the westerly winds were also shifted southward at this time. Using numerical simulations of the climate, we find that this ice age shift in the westerly winds is primarily due to the presence of a large ice sheet over North America.

The often-used phrase ‘the uplift of the Tibetan Plateau’ implies a flat-surfaced Tibet rose as a coherent entity, and that uplift was driven entirely by the collision and northward movement of India. Here, we argue that these are misconceptions derived in large part from simplistic geodynamic and climate modeling, as well as proxy misinterpretation. The growth of Tibet was a complex process involving mostly Mesozoic collisions of several Gondwanan terranes with Asia, thickening the crust and generating complex relief before the arrival of India. In this review, Earth system modeling, paleoaltimetry proxies and fossil finds contribute to a new synthetic view of the topographic evolution of Tibet. A notable feature overlooked in previous models of plateau formation was the persistence through much of the Cenozoic of a wide east–west orientated deep central valley, and the formation of a plateau occurred only in the late Neogene through compression and internal sedimentation.

Large uncertainties exist in Holocene climate estimates, especially for the early Holocene when large-scale reorganization occurred in the climate system. To improve our understanding of these uncertainties, we compare four Holocene simulations performed with the LOVECLIM, CCSM3, HadCM3 and FAMOUS climate models. The simulations are generally consistent for the large-scale Northern Hemisphere extratropics, while the multi-simulation consistencies are heterogeneous on the sub-continental scale. Consistently simulated temperature trends are found in Greenland, northern Canada, north-eastern and north-western Europe, and central-west Siberia. These Holocene temperatures show a pattern of an early Holocene warming, mid-Holocene warmth and gradual decrease towards the pre-industrial in winter, and the extent of early Holocene warming varies spatially, with 9 °C warming in northern Canada compared with 3 °C warming in central-west Siberia. In contrast, mismatched temperatures are detected: in Alaska, the warm early Holocene winter in LOVECLIM primarily results from strongly enhanced southerly winds induced by the ice sheets; in eastern Siberia, the intense early-Holocene summer warmth anomaly in CCSM3 is caused by large negative albedo anomalies due to overestimated snow cover at 0 ka; in the Arctic, cool winter conditons in FAMOUS can be attributed to extensive sea ice coverage probably due to simplified sea ice representations. Thus, the Holocene temperature trends in these regions remain inconclusive.

Much of the global annual mean temperature change over Quaternary glacial cycles can be attributed to slow ice sheet and greenhouse gas feedbacks, but analysis of the short-term response to orbital forcings has the potential to reveal key relationships in the climate system. In particular, obliquity and precession both produce highly seasonal temperature responses at high latitudes. Here, idealized single-forcing model experiments are used to quantify Earth’s response to obliquity, precession, CO2, and ice sheets, and a linear reconstruction methodology is used to compare these responses to long proxy records around the globe. This comparison reveals mismatches between the annual mean response to obliquity and precession in models versus the signals within Antarctic ice cores. Weighting the model-based reconstruction toward austral winter or spring reduces these discrepancies, providing evidence for a seasonal bias in ice cores., Periodic changes in the tilt of the Earth’s axis alter the distribution of incoming solar radiation. Here, the authors show that the temperature response to this forcing seemingly differs in models and Antarctic ice cores, with a better agreement reached if ice cores are recording a seasonally weighted signal.

The Laurentide ice sheet, which covered Canada during glacial periods, had a major influence on atmospheric circulation and surface climate, but its role in climate during the early Holocene (9–7 ka), when it was thinner and confined around Hudson Bay, is unclear. It has been suggested that the demise of the ice sheet played a role in the 8.2 ka event (an abrupt 1–3 °C Northern Hemisphere cooling lasting ~ 160 years) through the influence of changing topography on atmospheric circulation. To test this hypothesis, and to investigate the broader implications of changing ice sheet topography for climate, we analyse a set of equilibrium climate simulations with ice sheet topographies taken at 500 year intervals from 9.5 to 8.0 ka. Between 9.5 and 8.0 ka, our simulations show a 2 °C cooling south of Iceland and a 1 °C warming between 40° and 50°N in the North Atlantic. These surface temperature changes are associated with a weakening of the subtropical and subpolar gyres caused by a decreasing wind stress curl over the mid-North Atlantic as the ice sheet lowers. The climate response is strongest during the period of peak ice volume change (9.5–8.5 ka), but becomes negligible after 8.5 ka. The climatic effects of the Laurentide ice sheet lowering during the Holocene are restricted to the North Atlantic sector. Thus, topographic forcing is unlikely to have played a major role in the 8.2 ka event and had only a small effect on Holocene climate change compared to the effects of changes in greenhouse gases, insolation and ice sheet meltwater.

Heterogeneous Holocene climate evolutions in the Northern Hemisphere high latitudes are primarily determined by orbital-scale insolation variations and melting ice sheets. Previous inter-model comparisons have revealed that multi-simulation consistencies vary spatially. We, therefore, compared multiple model results with proxy-based reconstructions in Fennoscandia, Greenland, north Canada, Alaska and Siberia. Our model-data comparisons reveal that data and models generally agree in Fennoscandia, Greenland and Canada, with the early-Holocene warming and subsequent gradual decrease to 0 ka BP (hereinafter referred as ka). In Fennoscandia, simulations and pollen data suggest a 2 °C warming by 8 ka, but this is less expressed in chironomid data. In Canada, a strong early-Holocene warming is suggested by both the simulations and pollen results. In Greenland, the magnitude of early-Holocene warming ranges from 6 °C in simulations to 8 °C in δ18O-based temperatures. Simulated and reconstructed temperatures are mismatched in Alaska. Pollen data suggest strong early-Holocene warming, while the simulations indicate constant Holocene cooling, and chironomid data show a stable trend. Meanwhile, a high frequency of Alaskan peatland initiation before 9 ka can reflect a either high temperature, high soil moisture or large seasonality. In high-latitude Siberia, although simulations and proxy data depict high Holocene temperatures, these signals are noisy owing to a large spread in the simulations and between pollen and chironomid results. On the whole, the Holocene climate evolutions in most regions (Fennoscandia, Greenland and Canada) are well established and understood, but important questions regarding the Holocene temperature trend and mechanisms remain for Alaska and Siberia.

The hydroclimate over Asia has undergone important changes over the Holocene with spatially asynchronous trends. Proxy-based evidence shows that North Asia was markedly drier than today during the early Holocene, whereas East Asia, influenced by the monsoon system, was substantially wetter. Yet, the causes behind this contrast are only partly understood due to a lack of overview of the most important factors. Here we explore a combination of climate proxies and multiple climate-model simulations to show that the strong contrast between the dry North Asia and wet (mid-latitude) East Asia is explained by a complex interplay between the effects of remnant ice sheets and orbital forcing. In North Asia, the climate was dry due a weakening of the westerlies and reduced atmospheric humidity, linked to the ice sheets in North America and Fennoscandia. In East Asia, contrarily, the orbitally-forced enhancement of the summer monsoons caused the early Holocene climate to be much wetter than during the presentday. These results indicate that the sensitivity of the hydroclimate in Asia to climate-forcings is spatially different, with important implications for the interpretation of past and future climate changes in this region. (C) 2019 Elsevier Ltd. All rights reserved.

The monsoon has been ever present for ~56 Ma with the evolution controlled by paleogeography and is insensitive to changing CO2., The East Asian monsoon plays an integral role in human society, yet its geological history and controlling processes are poorly understood. Using a general circulation model and geological data, we explore the drivers controlling the evolution of the monsoon system over the past 150 million years. In contrast to previous work, we find that the monsoon is controlled primarily by changes in paleogeography, with little influence from atmospheric CO2. We associate increased precipitation since the Late Cretaceous with the gradual uplift of the Himalayan-Tibetan region, transitioning from an ITCZ-dominated monsoon to a sea breeze–dominated monsoon. The rising region acted as a mechanical barrier to cold and dry continental air advecting into the region, leading to increasing influence of moist air from the Indian Ocean/South China Sea. We show that, apart from a dry period in the middle Cretaceous, a monsoon system has existed in East Asia since at least the Early Cretaceous.

The youngest palm fossil record in Tibet suggests that there was no high Tibetan Plateau until the Neogene., The Late Paleogene surface height and paleoenvironment for the core area of the Qinghai-Tibetan Plateau (QTP) remain critically unresolved. Here, we report the discovery of the youngest well-preserved fossil palm leaves from Tibet. They were recovered from the Late Paleogene (Chattian), ca. 25.5 ± 0.5 million years, paleolake sediments within the Lunpola Basin (32.033°N, 89.767°E), central QTP at a present elevation of 4655 m. The anatomy of palms renders them intrinsically susceptible to freezing, imposing upper bounds on their latitudinal and altitudinal distribution. Combined with model-determined paleoterrestrial lapse rates, this shows that a high plateau cannot have existed in the core of Tibet in the Paleogene. Instead, a deep paleovalley, whose floor was 4 km) high mountain systems, formed a topographically highly varied landscape. This finding challenges prevailing views on tectonic processes, monsoon dynamics, and the evolution of Asian biodiversity.

The Earth system changed dramatically across the Eocene‐Oligocene Transition (EOT) on a variety of spatial and temporal scales. Understanding the many complex and interacting factors affecting the Earth's atmosphere and oceans at the EOT requires the combination of both data and modeling approaches and an understanding of the uncertainty in both of these elements. Here uncertainty in the Earth system response to various imposed forcings typical of changes at the EOT is assessed. By using an ensemble of simulations from the fully coupled general circulation model, HadCM3L, the uncertainty due to differences in the boundary conditions and insufficient model spin‐up is quantified. The surface temperature response in high‐latitude ocean regions, particularly where deep water formation occurs, is found to be highly sensitive to differences in boundary conditions (i.e., have the greatest magnitude of uncertainty), while low‐latitude oceans are the most insensitive to differences in boundary conditions (i.e., have the lowest magnitude of uncertainty). The length of spin‐up (or how far the model is from equilibrium) can have a significant effect on the response to some forcings and on the magnitude of uncertainty due to differences in boundary conditions. These findings are important to consider for future modeling work and for interpreting previous published simulations.

Quantifying how land surface height, such as that of the Tibetan region, has changed with time is crucial for understanding a range of Earth processes, including atmospheric dynamics, biotic evolution and tectonics. Elevation reconstructions are highly uncertain and controversial, in part because of assumptions used in their calculation. The largest uncertainties are in the choice of unconstrained thermal lapse rates. Thermal lapse rates are defined as a change in surface temperature with altitude and have long been used to estimate paleoelevation. If we know both the lapse rate and the temperature at two sites at different elevations, then in theory we can calculate their height difference. There are different types of lapse rates (Dry, Saturated, Environmental and Terrestrial), yet which is the most useful for paleoaltimetry is unknown. Previous paleoelevation studies have often used observed modern-day global annual mean free air or terrestrial thermal lapse rates to measure elevation change, with the assumption that observed modern-day lapse rates are similar to those of the past. Here, using the HadCM3L paleoclimate model we demonstrate that Eocene global mean free air and terrestrial thermal lapse rates are not only different from the modern, but also show little predictive skill in reproducing prescribed model topography. Free-air lapse rates are largely insensitive to increased pCO2 (showing only a decrease of ∼0.1-0.5 °C/km), whereas lapse rates at Earth's surface, the most applicable for fossil-based paleoaltimetry, differed significantly locally and globally in the past compared to the Pre-industrial. This suggests that modern terrestrial lapse rate expressions are inappropriate for tracking altitude changes through geologic time. Moist processes and resultant moisture content of airmasses play a critical role in much of this uncertainty. The use of a wet-bulb temperature-derived lapse rate reduces this uncertainty significantly improving the predictive skill. Local terrestrial thermal lapse rates can be useful in paleoaltimetry, but only through climate model mediation where uncertainties can be reduced and quantified. Critically, paleoclimate models offer the opportunity to provide mean sea-level surface temperature to derive an elevation estimate where proxy-based values may not be available.

Elucidating the source(s) of Meltwater Pulse 1a, the largest rapid sea level rise caused by ice melt (14-18 m in less than 340 years, 14,600 years ago), is important for understanding mechanisms of rapid ice melt and the links with abrupt climate change. Here, we quantify how much and by what mechanisms the North American ice sheet could have contributed to Meltwater Pulse 1a, by driving an ice sheet model with two transient climate simulations of the last 21,000 years. Ice sheet perturbed physics ensembles were run to account for model uncertainties, constraining ice extent and volume with reconstructions of 21,000 years ago to present. We determine that the North American ice sheet produced 3-4 m global mean sea level rise in 340 years due to the abrupt Bolling warming, but this response is amplified to 5-6 m when it triggers the ice sheet saddle collapse.

A range of proxy observations have recently provided constraints on how Earth's hydrological cycle responded to early Eocene climatic changes. However, comparisons of proxy data to general circulation model (GCM) simulated hydrology are limited and inter-model variability remains poorly characterised. In this work, we undertake an intercomparison of GCM-derived precipitation and P − E distributions within the extended EoMIP ensemble (Eocene Modelling Intercomparison Project; Lunt et al., 2012), which includes previously published early Eocene simulations performed using five GCMs differing in boundary conditions, model structure, and precipitation-relevant parameterisation schemes. We show that an intensified hydrological cycle, manifested in enhanced global precipitation and evaporation rates, is simulated for all Eocene simulations relative to the preindustrial conditions. This is primarily due to elevated atmospheric paleo-CO2, resulting in elevated temperatures, although the effects of differences in paleogeography and ice sheets are also important in some models. For a given CO2 level, globally averaged precipitation rates vary widely between models, largely arising from different simulated surface air temperatures. Models with a similar global sensitivity of precipitation rate to temperature (dP∕dT) display different regional precipitation responses for a given temperature change. Regions that are particularly sensitive to model choice include the South Pacific, tropical Africa, and the Peri-Tethys, which may represent targets for future proxy acquisition. A comparison of early and middle Eocene leaf-fossil-derived precipitation estimates with the GCM output illustrates that GCMs generally underestimate precipitation rates at high latitudes, although a possible seasonal bias of the proxies cannot be excluded. Models which warm these regions, either via elevated CO2 or by varying poorly constrained model parameter values, are most successful in simulating a match with geologic data. Further data from low-latitude regions and better constraints on early Eocene CO2 are now required to discriminate between these model simulations given the large error bars on paleoprecipitation estimates. Given the clear differences between simulated precipitation distributions within the ensemble, our results suggest that paleohydrological data offer an independent means by which to evaluate model skill for warm climates.

Last Interglacial (LIG), stable water isotope values (δ18O) measured in Greenland deep ice cores are at least 2.5‰ higher compared to the present day. Previous isotopic climate simulations of the LIG do not capture the observed Greenland δ18O increases. Here, we use the isotope-enabled HadCM3 (UK Met Office coupled atmosphere-ocean general circulation model) to investigate whether a retreat of Northern Hemisphere sea ice was responsible for this model-data disagreement. Our results highlight the potential significance of sea ice changes on the LIG Greenland isotopic maximum. Sea ice loss in combination with increased sea surface temperatures, over the Arctic, affect δ18O: water vapour enriched in heavy isotopes and a shorter distillation path may both increase δ18O values over Greenland. We show, for the first time, that simulations of the response to Arctic sea ice reduction are capable of producing the likely magnitude of LIG δ18O increases at NEEM, NGRIP, GIPS2 and Camp Century ice core sites. However, we may underestimate δ18O changes at the Renland, DYE3 and GRIP ice core locations. Accounting for possible ice sheet changes is likely to be required to produce a better fit to the LIG ice core δ18O values.

We use the Glimmer ice sheet model to simulate periodic surges over the Laurentide Ice Sheet during the Last Glacial Maximum. In contrast to previous studies we use the depth of water at the base of the ice sheet as the switch for these surges. We find that the surges are supported within the model and are quite robust across a very wide range of parameter choices, in contrast to many previous studies where surges only occur for rather specific cases. The robustness of the surges is likely due to the use of water as the switch mechanism for sliding. The statistics of the binge–purge cycles resemble observed Heinrich events. The events have a period of between 10 and 15 thousand years and can produce fluxes of ice from the mouth of Hudson Strait of 0.05 Sv – a maximum flux of 0.06 Sv is possible. The events produce an ice volume of 2.50 × 106 km3, with a range of 4.30 × 106–1.90 × 106 km3 possible. We undertake a suite of sensitivity tests varying the sliding parameter, the water drainage scheme, the sliding versus water depth parameterisation and the resolution, all of which support the ice sheet surges. This suggests that internally triggered ice sheet surges were a robust feature of the Laurentide Ice Sheet and are a possible explanation for the observed Heinrich events.

The Oligocene represents a transitional time period from a warm climate to a cooler climate that is more representative of the modern day; yet, a general view of continental climate pattern and forcings are still lacking. Different proxies and models show striking disparities, especially in mid-high latitudes, requiring validation of Oligocene climate reconstruction in order to understand the large-scale processes that drive the observed climate changes. Here, we compiled 149 macrofossil floras in the mid-high latitudes of Eurasia, then quantitatively reconstructed the Oligocene climate using Coexistence Approach (CA) and combined previously published paleoclimate data. During the Oligocene, Eurasian mid-high latitudes were mainly dominated by a humid subtropical climate. Mean annual temperature (MAT) ranged between 5.4 °C and 25.5 °C with mean annual precipitation (MAP) ranging from 338 to 2453 mm. Three regions (Europe, central Eurasia and eastern Asia) indicate different climatic regimes, with a generally warmer and wetter climate in Europe and a colder and drier climate in central Eurasia when compared to eastern Asia. No significant reorganization of climate was observed between the Early and Late Oligocene. The climate anomalies between the Oligocene and present indicate that geographic changes (e.g. retreat of the Paratethys Sea) played an important role in shaping the climate pattern of Eurasia. By comparing the fossil data to a range of different HadCM3L model simulations of the Oligocene with differing boundary conditions (e.g. CO2 and topography), we demonstrate similar large-scale climate spatial patterns between models and fossil data, however, models simulated much higher temperature seasonality (lower simulated winter temperatures and higher simulated summer temperatures) in Eurasia. Mean annual temperature analysis indicates that simulations with 560 and 1120 ppmv CO2 matched better with fossil data when compared to other simulations, depending on the topography. These results provide some constraints that should be considered for future paleoclimate modelling.

The Pliocene (c. 5.3 - 1.8 Myr BP) was the last epoch of geological time in which global temperatures were generally higher than modern. It is important if we are to understand the dynamics of warm climates. This is particuarly true of the interaction of climate and cryosphere, where the Pliocene may represent the first epoch in which ice sheets, at least on Antarctica, were a permanent feature. In this paper, we review the available evidence for the state of ice sheets during the Pliocene as well as previous attempts to model them. We then present new models and sensitivity studies of the mid-Pliocene East Antarctic Ice Sheet (EAIS) and consider the implications for the debate on ice-sheet stability during the Pliocene. These new reconstructions suggest that the mid-Pliocene EAIS was significantly smaller than modern, but the modelled average mid-Pliocene climate is not sufficient to cause the widespread deglaciation suggested by Sirius Group diatom evidence.

The cause of a rapid change in Atlantic Ocean circulation and northern cooling atthe onset of Heinrich Stadial 1 ~18.5 ka is unclear. Previous studies have simulated the event using ice sheet and/or iceberg meltwater forcing, but these idealized freshwater fluxes have been unrealistically large. Here we use a different approach, driving a high‐resolution drainage network model with a recent time‐resolved global paleo‐ice sheet reconstruction to generate a realistic meltwater forcing. We input this flux to the Hadley Centre Coupled Model version 3 (HadCM3) climate model without adjusting the timing oramplitude and find that an acceleration in northern ice sheet melting (up to~7.5 m/kyr global mean sea level rise equivalent) triggers a 20% reduction in the Atlantic Meridional Overturning Circulation. The simulated pattern of ocean circulation and climate change matches an array of paleoclimate and ocean circulation reconstructions for the onset of Heinrich Stadial 1, in terms of both rates and magnitude of change. This is achieved with a meltwater flux that matches constraints on sea level changes and ice sheet evolution around 19–18ka. Since the rates of melting are similar to those projected for Greenland by 2200, constraining the melt rates and magnitude of climate change during Heinrich Stadial 1 would provide an important test of climate model sensitivity to future ice sheet melt.Plain Language Summary Atlantic Ocean circulation plays a key role in redistributing heat around Earth's surface, and thus has an important influence on our climate. Because of this, sudden shifts in Atlantic Ocean circulation can drive rapid climate changes. One such example is at the onset of “Heinrich Stadial 1”, 18.5 thousand years ago, when geological records show that Atlantic circulation weakened and the Northern Hemisphere cooled while the Southern Hemisphere warmed. At the time, huge ice sheets (several kilometers thick) covered much of North America and northern Europe. Climate model results suggest that the freshwater produced by these melting ice sheets is responsible for weakening the Atlantic Ocean circulation and triggering the abrupt climate changes captured in the geological records. This result helps to elucidate the complex interaction between ice sheets, ocean circulation, and climate, and how these interactions can lead to sudden shifts in climates of the past and, potentially, the future. Indeed, the rate of melting we adopt in the present model is comparable to the melting projected for the Greenland Ice Sheet by 2200.

The uplift history of south-eastern Tibet is crucial to understanding processes driving the tectonic evolution of the Tibetan Plateau and surrounding areas. Underpinning existing palaeoaltimetric studies has been regional mapping based in large part on biostratigraphy that assumes a Neogene modernization of the highly diverse, but threatened, Asian biota. Here, with new radiometric dating and newly collected plant-fossil archives, we quantify the surface height of part of the south-eastern margin of Tibet in the latest Eocene (∼34 Ma) to be ∼3 km and rising, possibly attaining its present elevation (3.9 km) in the early Oligocene. We also find that the Eocene–Oligocene transition in south-eastern Tibet witnessed leaf-size diminution and a floral composition change from sub-tropical/warm temperate to cool temperate, likely reflective of both uplift and secular climate change, and that, by the latest Eocene, floral modernization on Tibet had already taken place, implying modernization was deeply rooted in the Palaeogene.

Understanding what drove Northern Hemisphere ice sheet melt during the last deglaciation (21–7 ka) can help constrain how sensitive contemporary ice sheets are to greenhouse gas (GHGs) changes. The roles of orbital forcing and GHGs in the deglaciation have previously been modelled, but not yet quantified. Here, for the first time we calculate the relative effect of these forcings on the North American deglaciation by driving a dynamical ice sheet model (GLIMMER-CISM) with a set of un-accelerated transient deglacial simulations with a full primitive equation based ocean atmosphere general circulation model (FAMOUS). We find that by 9 ka, orbital forcing has caused 50% of the deglaciation, GHG 30% and the interaction between the two 20%. Orbital forcing starts affecting the ice volume at 19 ka, 2,000 years before CO2 starts increasing in our experiments, a delay which partly controls their relative effect.

We compare multi-ice core data with δ18O model output for the early last interglacial Antarctic sea ice minimum. The spatial pattern of δ18O across Antarctica is sensitive to the spatial pattern of sea ice retreat. Local sea ice retreat increases the proportion of winter precipitation, depleting δ18O at ice core sites. However, retreat also enriches δ18O because of the reduced source-to-site distance for atmospheric vapor. The joint overall effect is for δ18O to increase as sea ice is reduced. Our data-model comparison indicates a winter sea ice retreat of 67, 59, and 43% relative to preindustrial in the Atlantic, Indian, and Pacific sectors of the Southern Ocean. A compilation of Southern Ocean sea ice proxy data provides weak support for this reconstruction. However, most published marine core sites are located too far north of the 128,000 years B.P. sea ice edge, preventing independent corroboration for this sea ice reconstruction. Plain Language Summary The Antarctic isotope and temperature maximum, which occurred approximately 128,000 years before present (B.P.) during the warmer than present last interglacial period, is associated with a major retreat of Antarctic sea ice. Understanding the details of this major sea ice retreat is crucial in order to understand the sensitivity of the Southern Hemisphere sea ice system and to evaluate the performance of climate model simulations in response to future warming. This work uses a multi-ice and ocean core data-model evaluation to assess the magnitude and spatial pattern of this sea ice retreat. Our results suggest that sea ice retreat was greatest in the Atlantic and Indian sectors of the Southern Ocean and less in the Pacific sector. These results may have had serious implications for the stability of marine terminating glaciers around the Antarctic Ice Sheet and their contribution to the last interglacial sea level rise. These results also support a hypothesized slowdown in northward ocean heat transport during the early last interglacial.

Climate plays a significant role in determining the style of depositional processes at different latitudes, which in turn influences the location of hydrocarbon systems. Results of climate modelling may therefore provide important information for predicting the presence or absence of suitable hydrocarbon plays. The critical step is to validate the model results against proxy data where they are available, to determine whether the models provide realistic results. Paleoclimate proxy data are most often derived from more accessible low to mid latitude regions and are biased towards warm climate states. However, General Circulation Models (GCMs) have traditionally been biased to colder temperatures, in particular at high latitudes, struggling to maintain the high latitude regions warm enough to sustain forests that were present during greenhouse periods, such as the mid-Cretaceous (~110-90 Ma), without exaggerated warming of the equatorial regions. To improve this approach the HadCM3L coupled atmosphere-ocean GCM, a state-of-the-art model for the long simulations required to reach an equilibrium climate, has been run for each Stage of the Cretaceous using new paleogeographic basemaps. Here, we compare the results for the Aptian (118.5 Ma) and Albian (105.8 Ma) with paleoclimate proxy data from the high northern latitudes in order to determine if the model produces viable results for this region. Paleoclimate analysis of fossil wood from conifer forests from Svalbard of Aptian-Albian age suggests that they grew in moist cool upland areas adjacent to warmer temperate lowland regions, probably with rivers and/or swamps present. Studies of conifers from the Canadian Arctic islands indicate that they grew under slightly cooler conditions than on Svalbard, similar to northern Canada today. The HadCM3L GCM results for Svalbard show that the dominant biome was evergreen taiga/montane forest with lowland temperate vegetation present during the Albian Stage possibly with an element of deciduous taiga/montane forest in the Aptian (both cold boreal forest with short hot summers according to the Köppen-Geiger classification). The modelled Mean Annual Temperature (MAT) was ~-3.7oC at the sample sites with summer temperatures rising to a mean of ~18oC during the Albian. Mean Annual Precipitation (MAP) was ~571 mm. In the Canadian Arctic the model results indicate that the biomes were more mixed than on Svalbard. The Aptian biome was dominantly deciduous taiga/montane forest with temperate vegetation in low laying areas. The Albian landscape was dominated by evergreen taiga/montane forest with some elements of deciduous taiga. Both Stages were classified as cold boreal forest with short hot summers under the Köppen-Geiger classification scheme. MAT was modelled to be ~-6.5oC at the sample sites with summer temperatures reaching a mean of ~13oC and MAP was ~406 mm. These results suggest that the HadCM3L GCM, coupled with updated paleogeographic maps, can produce a good match to the climate proxy data in these difficult-to-model high latitude areas.

The Paleocene-Eocene Thermal Maximum (PETM) hyperthermal, ~56 million years ago (Ma), is the most dramatic example of abrupt Cenozoic global warming. Characterised by surface temperature increases of between 5 – 9 °C and a rapid onset of less than 20 kyr, the PETM has been widely studied as a natural analogue for future anthropogenic-induced warming. The PETM provides a case study of the impacts of global warming on the Earth system, including both hydrological and associated biogeochemical feedbacks, and proxy data from the PETM can provide constraints on changes in warm climate hydrology simulated by general circulation models (GCMs). In this paper, we provide a critical review of biological and geochemical signatures interpreted as direct or indirect indicators of hydrological change at the PETM, explore the importance of adopting multi-proxy approaches, and present a preliminary model-data comparison. Hydrological records complement those of temperature and indicate that the climatic response at the PETM was complex, with significant regional and temporal variability. This is further illustrated by the biogeochemical consequences of inferred changes in hydrology and, in fact, changes in precipitation and the biogeochemical consequences are often conflated in geochemical signatures. There is also strong evidence in many regions for changes in the episodic and/or intra-annual distribution of precipitation which has not widely been considered when comparing proxy data to GCM output. Crucially, GCM simulations indicate that an intensification of the hydrological cycle at the PETM is associated with regions of both increased and decreased precipitation – evaporation (P – E), although existing proxy data are largely from the regions where GCMs predict an increase in PETM precipitation. We propose that comparison of hydrological proxies to GCM output can be an important test of model skill, but this will be enhanced by further data from regions of model-simulated aridity and simulation of extreme precipitation events.

The changes in the global energy budget in response to imposing an ice sheet’s topography, albedo, or topography and albedo combined are examined. The albedo of the ice sheet (here called a “White Plain”) causes an outgoing top of the atmosphere radiation anomaly over the ice sheet that is balanced by an incoming anomaly in the Southern Hemisphere. This causes a northward transport of heat across the equator that is carried equally by the ocean and atmosphere. The topography of the ice sheet (“Green Mountain”) causes an incoming radiation anomaly over the ice sheet that is balanced predominantly by an outgoing anomaly to the south of the ice sheet, with a smaller outgoing flux in the Southern Hemisphere. The heat is transported across the equator by the atmosphere alone. The combined topography and albedo of the ice sheet (“White Mountain”) cause an outgoing radiation anomaly over the ice sheet that is balanced equally by an incoming flux in the Southern Hemisphere and to the south of the ice sheet. Heat is transported across the equator by the ocean alone. With varying ice sheet geometry generally linear relationships between the various energy fluxes and the varying height and area of the ice sheet are found. In both the White Plain and White Mountain cases the ocean is always a significant carrier of heat across the equator, and in the White Mountain case it is preeminent.

Late Miocene tectonic changes in Mediterranean–Atlantic connectivity and climatic changes caused Mediterranean salinity to fluctuate dramatically, including a ten-fold increase and near-freshening. Recent proxy- and model-based evidence suggests that at times during this Messinian Salinity Crisis (MSC, 5.96–5.33 Ma), highly saline and highly fresh Mediterranean water flowed into the North Atlantic Ocean, whilst at others, no Mediterranean Outflow Water (MOW) reached the Atlantic. By running extreme, sensitivity-type experiments with a fully coupled ocean–atmosphere general circulation model, we investigate the potential of these various MSC MOW scenarios to impact global-scale climate. The simulations suggest that although the effect remains relatively small, MOW had a greater influence on North Atlantic Ocean circulation and climate than it does today. We also find that depending on the presence, strength and salinity of MOW, the MSC could have been capable of cooling mid–high northern latitudes by a few degrees, with the greatest cooling taking place in the Labrador, Greenland–Iceland–Norwegian and Barents seas. With hypersaline MOW, a component of North Atlantic Deep Water formation shifts to the Mediterranean, strengthening the Atlantic Meridional Overturning Circulation (AMOC) south of 35° N by 1.5–6 Sv. With hyposaline MOW, AMOC completely shuts down, inducing a bipolar climate anomaly with strong cooling in the north (mainly −1 to −3 °C, but up to −8 °C) and weaker warming in the south (up to +0.5 to +2.7 °C). These simulations identify key target regions and climate variables for future proxy reconstructions to provide the best and most robust test cases for (a) assessing Messinian model performance, (b) evaluating Mediterranean–Atlantic connectivity during the MSC and (c) establishing whether or not the MSC could ever have affected global-scale climate.

A frequently cited atmospheric CO2 threshold for the onset of Antarctic glaciation of ~780 ppmv is based on the study of DeConto and Pollard (2003) using an ice sheet model and the GENESIS climate model. Proxy records suggest that atmospheric CO2 concentrations passed through this threshold across the Eocene–Oligocene transition ~34 Ma. However, atmospheric CO2 concentrations may have been close to this threshold earlier than this transition, which is used by some to suggest the possibility of Antarctic ice sheets during the Eocene. Here we investigate the climate model dependency of the threshold for Antarctic glaciation by performing offline ice sheet model simulations using the climate from 7 different climate models with Eocene boundary conditions (HadCM3L, CCSM3, CESM1.0, GENESIS, FAMOUS, ECHAM5 and GISS_ER). These climate simulations are sourced from a number of independent studies, and as such the boundary conditions, which are poorly constrained during the Eocene, are not identical between simulations. The results of this study suggest that the atmospheric CO2 threshold for Antarctic glaciation is highly dependent on the climate model used and the climate model configuration. A large discrepancy between the climate model and ice sheet model grids for some simulations leads to a strong sensitivity to the lapse rate parameter.

Warmer than present Antarctic and Southern Ocean temperatures during the last interglacial, approximately 128,000 years ago, have been attributed to changes in north-south ocean heat transport, causing opposing hemispheric temperature anomalies. We investigate the magnitude of Antarctic warming and Antarctic ice core isotopic enrichment in response to Northern Hemisphere meltwater input during the early last interglacial. A 1,600 year HadCM3 simulation driven by 0.25 Sv of meltwater input reproduces 50-60 % of the peak Southern Ocean summer sea surface temperature anomaly, sea ice retreat and ice core isotope enrichment. We also find a robust increase in the proportion of cold season precipitation during the last interglacial, leading to lower isotopic values at the Antarctic ice core sites. These results suggest that a HadCM3 simulation including 0.25 Sv for 3,000-4,000 years would reconcile the last interglacial observations, providing a potential solution for the last interglacial 'missing heat' problem.

Investigation of Holocene climate variability remains challenging. This is largely due to chronological uncertainties and complexities associated with proxies and their relationship with climatic drivers. Pertinent questions still exist regarding the Holocene climate in Ireland, particularly in the early Holocene. We present a mean July air temperature reconstruction based on fossil chironomidae (non-biting midge flies), along with an assessment of chironomid functional traits from five guilds (based on their feeding habits) from Lough Nakeeroge, a small glacial lake in western Ireland. These records span the early to late Holocene (c. 10,000–1500 cal yr BP). The chironomid record is supplemented with pollen data to determine landscape vegetation dynamics, and compared to climate model simulations of the same period. As reliable models are essential for robust analysis of long-term climate change, we critically assess the value of the chironomid transfer function and explore the use of chironomid functional traits to infer past climate variability. While this study demonstrates the complexities of chironomid-based temperature reconstruction in Irish lakes, it endeavours to i) disentangle a complicated Holocene climate history through the exploration of other long-term Holocene records from the island; and ii) improve our understanding of environmental responses to climate variability in Ireland. The findings of this study suggest that the interpretation of chironomid-based temperature transfer functions can be challenging. However, our results demonstrate the influence of climate on the functioning of lake ecosystems over the Holocene, with the promising performance of the collector-filterer feeding guild as a palaeothermometer.

Since 2010, the INTIMATE (INTegration of Ice-core, MArine and TErrestrial records) network has been operating as a COST Action (designated ES0907). This paper outlines the accomplishments of the INTIMATE COST Action in the context of how the INTIMATE ideas have evolved during the network's twenty-year life span, and highlights a number of challenges that can guide further work. In the second part of the paper, the contributions that comprise this INTIMATE special issue are introduced., This study is a contribution to the INTIMATE project, supported by INQUA (INQUA Focus Group & project 1210F) and funded as the EU COST Action ES0907. We gratefully acknowledge Chris Turney for his dedicated efforts in obtaining the COST funding. We thank the countless individuals who have organised meetings, workshops, and training schools, who have hosted research exchange visits, and have otherwise contributed to the fruitful and engaging collaborative atmosphere within the network. We thank the Action's administrative and scientific officers at the COST office in Brussels, and Karin Norris and Louise Thers Nielsen at the EU office at University of Copenhagen for their essential contributions to the efficient management of the project. We thank Prof. Colin Murray-Wallace, Editor-in-Chief of Quaternary Science Reviews, and Journal Manager Debbie Barrett and the staff at Elsevier for their assistance with the editing and production of this special issue. Finally, we gratefully acknowledge the huge effort provided by the colleagues who agreed to review the papers of the issue.