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One of Earth’s most fundamental climate shifts – the greenhouse-icehouse transition 34 Ma ago – initiated Antarctic ice-sheet build-up, influencing global climate until today. However, the extent of the ice sheet during the Early Oligocene Glacial Maximum (~33.7–33.2 Ma) that immediately followed this transition, a critical knowledge gap for assessing feedbacks between permanently glaciated areas and early Cenozoic global climate reorganization, is uncertain. Here, we present shallow-marine drilling data constraining earliest Oligocene environmental conditions on West Antarctica’s Pacific margin – a key region for understanding Antarctic ice sheet-evolution. These data indicate a cool-temperate environment, with mild ocean and air temperatures preventing West Antarctic Ice Sheet formation. Climate-ice sheet modeling corroborates a highly asymmetric Antarctic ice sheet, thereby revealing its differential regional response to past and future climatic change.

One of Earth’s most fundamental climate shifts – the greenhouse-icehouse transition 34 Ma ago – initiated Antarctic ice-sheet build-up, influencing global climate until today. However, the extent of the ice sheet during the Early Oligocene Glacial Maximum (~33.7–33.2 Ma) that immediately followed this transition, a critical knowledge gap for assessing feedbacks between permanently glaciated areas and early Cenozoic global climate reorganization, is uncertain. Here, we present shallow-marine drilling data constraining earliest Oligocene environmental conditions on West Antarctica’s Pacific margin – a key region for understanding Antarctic ice sheet-evolution. These data indicate a cool-temperate environment, with mild ocean and air temperatures preventing West Antarctic Ice Sheet formation. Climate-ice sheet modeling corroborates a highly asymmetric Antarctic ice sheet, thereby revealing its differential regional response to past and future climatic change.

One of Earth’s most fundamental climate shifts, the greenhouse-icehouse transition 34 million years ago, initiated Antarctic ice sheet buildup, influencing global climate until today. However, the extent of the ice sheet during the Early Oligocene Glacial Maximum (~33.7 to 33.2 million years ago) that immediately followed this transition—a critical knowledge gap for assessing feedbacks between permanently glaciated areas and early Cenozoic global climate reorganization—is uncertain. In this work, we present shallow-marine drilling data constraining earliest Oligocene environmental conditions on West Antarctica’s Pacific margin—a key region for understanding Antarctic ice sheet evolution. These data indicate a cool-temperate environment with mild ocean and air temperatures that prevented West Antarctic Ice Sheet formation. Climate–ice sheet modeling corroborates a highly asymmetric Antarctic ice sheet, thereby revealing its differential regional response to past and future climatic change. Editor’s summary Earth’s climate underwent a dramatic transition around 34 million years ago, when the Antarctic Ice Sheet first began to form, but the regional evolution of that ice sheet remains poorly defined. Klages et al . present data from marine sediments near West Antarctica showing that conditions there during the beginning of the Oligocene were mild and unfavorable to the growth of a permanent ice sheet. Model results based on those data suggest that the ice sheet in West Antarctica did not begin to form until 7 or 8 million years after the process began in East Antarctica.

Extensive ice coverage largely prevents investigations of Antarctica’s unglaciated past. Knowledge about environmental and tectonic development before large-scale glaciation, however, is important for understanding the transition into the modern icehouse world. We report geochronological and sedimentological data from a drill core from the Amundsen Sea shelf, providing insights into tectonic and topographic conditions during the Eocene (~44 to 34 million years ago), shortly before major ice sheet buildup. Our findings reveal the Eocene as a transition period from >40 million years of relative tectonic quiescence toward reactivation of the West Antarctic Rift System, coinciding with incipient volcanism, rise of the Transantarctic Mountains, and renewed sedimentation under temperate climate conditions. The recovered sediments were deposited in a coastal-estuarine swamp environment at the outlet of a >1500-km-long transcontinental river system, draining from the rising Transantarctic Mountains into the Amundsen Sea. Much of West Antarctica hence lied above sea level, but low topographic relief combined with low elevation inhibited widespread ice sheet formation.

Back to topThe early Eocene (56 to 48 million years ago) is inferred to have been the most recent time that Earth's atmospheric CO2 concentrations exceeded 1000 ppm. Global mean temperatures were also substantially warmer than those of the present day. As such, the study of early Eocene climate provides insight into how a super-warm Earth system behaves and offers an opportunity to evaluate climate models under conditions of high greenhouse gas forcing. The Deep Time Model Intercomparison Project (DeepMIP) is a systematic model–model and model–data intercomparison of three early Paleogene time slices: latest Paleocene, Paleocene–Eocene thermal maximum (PETM) and early Eocene climatic optimum (EECO). A previous article outlined the model experimental design for climate model simulations. In this article, we outline the methodologies to be used for the compilation and analysis of climate proxy data, primarily proxies for temperature and CO2. This paper establishes the protocols for a concerted and coordinated effort to compile the climate proxy records across a wide geographic range. The resulting climate “atlas” will be used to constrain and evaluate climate models for the three selected time intervals and provide insights into the mechanisms that control these warm climate states. We provide version 0.1 of this database, in anticipation that this will be expanded in subsequent publications.

Observations and model experiments highlight the importance of ocean heat in forcing icesheet retreat during the present and geological past, but past ocean temperature data arevirtually missing in ice sheet proximal locations. Here we document paleoceanographicconditions and the (in)stability of the Wilkes Land subglacial basin (East Antarctica) duringthe mid-Miocene (~17–13.4 million years ago) by studying sediment cores from offshoreAdélie Coast. Inland retreat of the ice sheet, temperate vegetation, and warm oligotrophicwaters characterise the mid-Miocene Climatic Optimum (MCO; 17–14.8 Ma). After the MCO,expansion of a marine-based ice sheet occurs, but remains sensitive to melting upon episodicwarm water incursions. Our results suggest that the mid-Miocene latitudinal temperaturegradient across the Southern Ocean never resembled that of the present day. We demonstratethat a strong coupling of oceanic climate and Antarctic continental conditions existedand that the East Antarctic subglacial basins were highly sensitive to ocean warming.

Antarctic ice sheet and Southern Ocean paleoceanographic configurations during the late Oligocene are not well resolved. They are however important to understand the influence of high-latitude Southern Hemisphere feedbacks on global climate under CO2 scenarios (between 400 and 750ppm) projected by the IPCC for this century, assuming unabated CO2 emissions. Sediments recovered by the Integrated Ocean Drilling Program (IODP) at Site U1356, offshore of the Wilkes Land margin in East Antarctica, provide an opportunity to study ice sheet and paleoceanographic configurations during the late Oligocene (26–25Ma). Our study, based on a combination of sediment facies analysis, magnetic susceptibility, density, and X-ray fluorescence geochemical data, shows that glacial and interglacial sediments are continuously reworked by bottom currents, with maximum velocities occurring during the interglacial periods. Glacial sediments record poorly ventilated, low-oxygenation bottom water conditions, interpreted as resulting from a northward shift of westerly winds and surface oceanic fronts. Interglacial sediments record more oxygenated and ventilated bottom water conditions and strong current velocities, which suggests enhanced mixing of the water masses as a result of a southward shift of the polar front. Intervals with preserved carbonated nannofossils within some of the interglacial facies are interpreted as forming under warmer paleoclimatic conditions when less corrosive warmer northern component water (e.g., North Atlantic sourced deep water) had a greater influence on the site. Spectral analysis on the late Oligocene sediment interval shows that the glacial–interglacial cyclicity and related displacements of the Southern Ocean frontal systems between 26 and 25Ma were forced mainly by obliquity. The paucity of iceberg-rafted debris (IRD) throughout the studied interval contrasts with earlier Oligocene and post-Miocene Climate Optimum sections from Site U1356 and with late Oligoc

Past warm periods provide an opportunity to evaluate climate models under extreme forcing scenarios, in particular high ( > 800 ppmv) atmospheric CO2 concentrations. Although a post hoc intercomparison of Eocene ( ∼ 50 Ma) climate model simulations and geological data has been carried out previously, models of past high-CO2 periods have never been evaluated in a consistent framework. Here, we present an experimental design for climate model simulations of three warm periods within the early Eocene and the latest Paleocene (the EECO, PETM, and pre-PETM). Together with the CMIP6 pre-industrial control and abrupt 4 × CO2 simulations, and additional sensitivity studies, these form the first phase of DeepMIP – the Deep-time Model Intercomparison Project, itself a group within the wider Paleoclimate Modelling Intercomparison Project (PMIP). The experimental design specifies and provides guidance on boundary conditions associated with palaeogeography, greenhouse gases, astronomical configuration, solar constant, land surface processes, and aerosols. Initial conditions, simulation length, and output variables are also specified. Finally, we explain how the geological data sets, which will be used to evaluate the simulations, will be developed.

The West Antarctic Ice Sheet (WAIS) is largely marine-based and, thus, highly sensitive to climatic and oceanographic changes. It probably had a very dynamic history over the last several million years. A complete collapse of the WAIS would result in a global sea-level rise of 3.3-4.3 m, yet, the world’s scientific community is not able to predict its future behavior. Moreover, knowledge about the past dynamics of the WAIS is poor, in particular during geological times with climatic conditions similar to those expected for the near and distant future. Reconstructions and quantifications of partial or complete WAIS collapses in the past are urgently needed for constraining and testing ice sheet models that aim to predict future WAIS behavior and the potential contribution of the WAIS to global sea-level rise. Large uncertainties exist regarding the chronology, extent, rates, and spatial and temporal variability of past advances and retreats of the WAIS across the continental shelves. These uncertainties mainly result from the fundamental lack of data from drill cores recovered proximal to the WAIS. The continental shelf and rise of the Amundsen Sea are prime targets for drilling, because the records are expected to yield archives of pure WAIS dynamics unaffected by other ice sheets, and the WAIS sector draining into the Amundsen Sea Embayment (ASE) currently experiences the largest ice loss in Antarctica. A series of drill sites are planned for the ASE shelf where seismic data reveal oceanward dipping sedimentary sequences that span the time from the pre-glacial depositional phase to the youngest glacial periods. Our strategy is to drill transects from the oldest sequences close to the bedrock-basin boundary at the middle-inner shelf transition to the youngest sequences on the outer shelf in both the western and the eastern ASE. These transects will provide a detailed history of the glacial cycles in the Amundsen Sea region and allow comparison to the WAIS history kn

Comparing simulations of key warm periods in Earth history with contemporaneous geological proxy data is a useful approach for evaluating the ability of climate models to simulate warm, high-CO2 climates that are unprecedented in the more recent past. Here we use a global data set of confidence-assessed, proxy-based temperature estimates and biome reconstructions to assess the ability of eight models to simulate warm terrestrial climates of the Pliocene epoch. The Late Pliocene, 3.6–2.6 million years ago, is an accessible geological interval to understand climate processes of a warmer world. We show that model-predicted surface air temperatures reveal a substantial cold bias in the Northern Hemisphere. Particularly strong data–model mismatches in mean annual temperatures (up to 18 °C) exist in northern Russia. Our model sensitivity tests identify insufficient temporal constraints hampering the accurate configuration of model boundary conditions as an important factor impacting on data–model discrepancies. We conclude that to allow a more robust evaluation of the ability of present climate models to predict warm climates, future Pliocene data–model comparison studies should focus on orbitally defined time slices.

Climate and environments of the mid-Pliocene warm period (3.264 to 3.025 Ma) have been extensively studied. Whilst numerical models have shed light on the nature of climate at the time, uncertainties in their predictions have not been systematically examined. The Pliocene Model Intercomparison Project quantifies uncertainties in model outputs through a coordinated multi-model and multi-model/data intercomparison. Whilst commonalities in model outputs for the Pliocene are clearly evident, we show substantial variation in the sensitivity of models to the implementation of Pliocene boundary conditions. Models appear able to reproduce many regional changes in temperature reconstructed from geological proxies. However, data/model comparison highlights that models potentially underestimate polar amplification. To assert this conclusion with greater confidence, limitations in the time-averaged proxy data currently available must be addressed. Furthermore, sensitivity tests exploring the known unknowns in modelling Pliocene climate specifically relevant to the high latitudes are essential (e.g. palaeogeography, gateways, orbital forcing and trace gasses). Estimates of longer-term sensitivity to CO2 (also known as Earth System Sensitivity; ESS), support previous work suggesting that ESS is greater than Climate Sensitivity (CS), and suggest that the ratio of ESS to CS is between 1 and 2, with a "best" estimate of 1.5.

The West Antarctic Ice Sheet (WAIS), which is grounded below present-day sea level and, thus, is highly sensitive to climatic changes, is likely to have had a very dynamic history over the last several million years. WAIS collapse would result in a global sealevel rise of 3-5 m over present levels yet little is known of how it has behaved in the past. Large uncertainties exist regarding the chronology, extent, rates, and spatial and temporal variability of past advances and retreats of the WAIS across the continental shelves, particularly in the Amundsen Sea sector, where no drilling has taken place despite the intense interest in the current and potential behavior of the glaciers feeding into the over-deepened Pine Island Bay. These uncertainties are mainly due to the paucity of data from drill core from this sector. We propose a series of drill cores for the Amundsen Sea Embayment shelf, which is unambiguously fed by the WAIS, where seismic data reveal seaward-dipping sedimentary sequences that are estimated to span the time from the pre-glacial conditions, Miocene or older, to the most recent glacial cycle. Our drilling strategy is to target a transect that includes the oldest sedimentary sequences deposited onto bedrock in the south to the youngest sequences in the north of the western and eastern Amundsen Sea Embayment continental shelf. This transect will yield a detailed history of the glacial cycles in the Pine Island-Amundsen Sea region and allow correlations to the glacial history known from the Ross Sea and to global climate records. In addition, deep-water sites on the continental rise of the embayment are selected for recovering continuous records of glacially transported sediments and the details of climatic and oceanographic changes throughout glacial-interglacial cycles. Because so little is known about the stratigraphy on the Amundsen Sea shelf, new drill core will allow a wide range of questions to be addressed, including: (1) What is the history o

The mid-Piacenzian climate represents the most geologically recent interval of long-term average warmth relative to the last million years, and shares similarities with the climate projected for the end of the 21st century. As such, it represents a natural experiment from which we can gain insight into potential climate change impacts, enabling more informed policy decisions for mitigation and adaptation. Here, we present the first systematic comparison of Pliocene sea surface temperature (SST) between an ensemble of eight climate model simulations produced as part of PlioMIP (Pliocene Model Intercomparison Project) with the PRISM (Pliocene Research, Interpretation and Synoptic Mapping) Project mean annual SST field. Our results highlight key regional and dynamic situations where there is discord between the palaeoenvironmental reconstruction and the climate model simulations. These differences have led to improved strategies for both experimental design and temporal refinement of the palaeoenvironmental reconstruction.

Climate and environments of the mid-Pliocene warm period (3.264 to 3.025 Ma) have been extensively studied. Whilst numerical models have shed light on the nature of climate at the time, uncertainties in their predictions have not been systematically examined. The Pliocene Model Intercomparison Project quantifies uncertainties in model outputs through a coordinated multi-model and multi-model/data intercomparison. Whilst commonalities in model outputs for the Pliocene are clearly evident, we show substantial variation in the sensitivity of models to the implementation of Pliocene boundary conditions. Models appear able to reproduce many regional changes in temperature reconstructed from geological proxies. However, data/model comparison highlights that models potentially underestimate polar amplification. To assert this conclusion with greater confidence, limitations in the time-averaged proxy data currently available must be addressed. Furthermore, sensitivity tests exploring the known unknowns in modelling Pliocene climate specifically relevant to the high latitudes are essential (e.g. palaeogeography, gateways, orbital forcing and trace gasses). Estimates of longer-term sensitivity to CO2 (also known as Earth System Sensitivity; ESS), support previous work suggesting that ESS is greater than Climate Sensitivity (CS), and suggest that the ratio of ESS to CS is between 1 and 2, with a "best" estimate of 1.5.

The West Antarctic Ice Sheet (WAIS), which is grounded below present sea level and, thus, is highly sensitive to climatic changes, is likely to have had a very dynamic history over the last several million years. Its collapse would result in a global sea-level rise of 3-5 m over present levels yet the world’s scientific community is not able to predict how it might behave in the future, nor is much known of how it has behaved in the past. The reconstruction and quantification of partial or complete WAIS collapses in the geological past are needed in order to provide necessary constraints for ice sheet models predicting future WAIS behaviour and its potential contributions to global sea-level rise. Large uncertainties exist regarding the chronology, extent, rates, and spatial and temporal variability of past advances and retreats of the WAIS across the continental shelves. These uncertainties are mainly due to the fundamental lack of data from drill core. A series of drill sites are proposed for the Amundsen Sea Embayment shelf where seismic data reveal oceanward dipping sedimentary sequences that span the time from the pre-glacial depositional phase to the youngest glacial periods. Our drilling strategy is to target a transect from the oldest sequences close to the bedrock-basin boundary in the south to the youngest sequences in the north of the western and eastern Amundsen Sea Embayment continental shelf. This transect will yield a detailed history of the glacial cycles in the Pine Island-Amundsen Sea region and allow correlations to the WAIS history known from the Ross Sea. In addition, deep-water sites on the continental rise of the embayment are selected for recovering continuous records of glacially transported sediments and the details of climatic and oceanographic changes throughout glacial-interglacial cycles.

Bayon et al. (Reports, 9 March 2012, p. 1219) interpreted unusually high aluminum-potassium ratio values in an Atlantic sediment core as indicating anthropogenic deforestation around 2500 years before the present (B.P.). We argue that there is no terrestrial evidence for forest destruction by humans and that the third millennium B.P. rainforest crisis can be clearly attributed mostly to climatic change.

In 2008 the temporal focus of the Palaeoclimate Modelling Intercomparison Project was expanded to include a model intercomparison for the mid-Pliocene warm period (3.29–2.97 million years ago). This project is referred to as PlioMIP (Pliocene Model Intercomparison Project). Two experiments have been agreed upon and comprise phase 1 of PlioMIP. The first (Experiment 1) will be performed with atmosphere-only climate models. The second (Experiment 2) will utilise fully coupled ocean-atmosphere climate models. The aim of this paper is to provide a detailed model intercomparison project description which documents the experimental design in a more detailed way than has previously been done in the literature. Specifically, this paper describes the experimental design and boundary conditions that will be utilised for Experiment 1 of PlioMIP.

The Pliocene Research, Interpretation and Synoptic Mapping (PRISM) paleoenvironmental reconstruction is an internally consistent and comprehensive global synthesis of a past interval of relatively warm and stable climate. It is regularly used in model studies that aim to better understand Pliocene climate, to improve model performance in future climate scenarios, and to distinguish model-dependent climate effects. The PRISM reconstruction is constantly evolving in order to incorporate additional geographic sites and environmental parameters, and is continuously refined by independent research findings. The new PRISM three dimensional (3D) reconstruction differs from previous PRISM reconstructions in that it includes a subsurface ocean temperature reconstruction, integrates geochemical sea surface temperature proxies to supplement the faunal-based temperature estimates, and uses numerical models for the first time to augment fossil data. Here we describe the components of PRISM3D and describe new findings specific to the new reconstruction. Highlights of the new PRISM3D reconstruction include removal of Hudson Bay and the Great Lakes and creation of open waterways in locations where the current bedrock elevation is less than 25m above modern sea level, due to the removal of the West Antarctic Ice Sheet and the reduction of the East Antarctic Ice Sheet. The mid-Piacenzian oceans were characterized by a reduced east-west temperature gradient in the equatorial Pacific, but PRISM3D data do not imply permanent El Niño conditions. The reduced equator-to-pole temperature gradient that characterized previous PRISM reconstructions is supported by significant displacement of vegetation belts toward the poles, is extended into the Arctic Ocean, and is confirmed by multiple proxies in PRISM3D. Arctic warmth coupled with increased dryness suggests the formation of warm and salty paleo North Atlantic Deep Water (NADW) and a more vigorous thermohaline circulation system that may ha