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Efforts to improve sea level forecasting on a warming planet have focused on determining the temperature, sea level and extent of polar ice sheets during Earth’s past interglacial warm periods1–3. About 400,000 years ago, during the interglacial period known as Marine Isotopic Stage 11 (MIS11), the global temperature was 1 to 2 degrees Celsius greater2 and sea level was 6 to 13 metres higher1,3. Sea level estimates in excess of about 10 metres, however, have been discounted because these require a contribution from the East Antarctic Ice Sheet3, which has been argued to have remained stable for millions of years before and includes MIS114,5. Here we show how the evolution of 234U enrichment within the subglacial waters of East Antarctica recorded the ice sheet’s response to MIS11 warming. Within the Wilkes Basin, subglacial chemical precipitates of opal and calcite record accumulation of 234U (the product of rock–water contact within an isolated subglacial reservoir) up to 20 times higher than that found in marine waters. The timescales of 234U enrichment place the inception of this reservoir at MIS11. Informed by the 234U cycling observed in the Laurentide Ice Sheet, where 234U accumulated during periods of ice stability6 and was flushed to global oceans in response to deglaciation7, we interpret our East Antarctic dataset to represent ice loss within the Wilkes Basin at MIS11. The 234U accumulation within the Wilkes Basin is also observed in the McMurdo Dry Valleys brines8–10, indicating11 that the brine originated beneath the adjacent East Antarctic Ice Sheet. The marine origin of brine salts10 and bacteria12 implies that MIS11 ice loss was coupled with marine flooding. Collectively, these data indicate that during one of the warmest Pleistocene interglacials, the ice sheet margin at the Wilkes Basin retreated to near the precipitate location, about 700 kilometres inland from the current position of the ice margin, which—assuming current ice volumes—would have contributed about 3 to 4 metres13 to global sea levels. Uranium isotopes in subglacial precipitates from the Wilkes Basin of the East Antarctic Ice Sheet reveal ice retreat during a warm Pleistocene interglacial period about 400,000 years ago.

Past episodes of greenhouse warming provide insight into the coupling of climate and the carbon cycle and thus may help to predict the consequences of unabated carbon emissions in the future.

An abrupt climate warming of 5 to 10 degrees C during the Palaeocene/Eocene boundary thermal maximum (PETM) 55 Myr ago is linked to the catastrophic release of approximately 1,050-2,100 Gt of carbon from sea-floor methane hydrate reservoirs. Although atmospheric methane, and the carbon dioxide derived from its oxidation, probably contributed to PETM warming, neither the magnitude nor the timing of the climate change is consistent with direct greenhouse forcing by the carbon derived from methane hydrate. Here we demonstrate significant differences between marine and terrestrial carbon isotope records spanning the PETM. We use models of key carbon cycle processes to identify the cause of these differences. Our results provide evidence for a previously unrecognized discrete shift in the state of the climate system during the PETM, characterized by large increases in mid-latitude tropospheric humidity and enhanced cycling of carbon through terrestrial ecosystems. A more humid atmosphere helps to explain PETM temperatures, but the ultimate mechanisms underlying the shift remain unknown.

The late Oligocene and early Miocene periods, some 21 to 27 million years ago, have generally been viewed as times of moderate global warmth and ice-free conditions. Yet several lines of evidence suggest that this interval was punctuated by at least one, and possibly several, episodes of high-latitude cooling and continental glaciation1,2,3. Here, we present stable-isotope and per cent coarse-fraction data from an equatorial, western Atlantic deep-sea-sediment core that provide high-resolution records of the climate variability across the Oligocene/Miocene transition (22.5–25.7 million years ago). A strong 40-kyr periodicity in the oxygen isotope record is consistent with a high-latitude orbital (obliquity) control on ice-volume and temperature. Orbital influences are also apparent at precession and eccentricity frequencies, including a series of ∼400-kyr oscillations that culminate in distinct maxima at the Oligocene/Miocene boundary, about 23.7 million years ago. Covariance between the carbon and oxygen isotope records suggests that the oceanic carbon cycle may have contributed to global cooling during the ∼400-kyr cycles, particularly at the Oligocene/Miocene boundary.

CHANGES in the isotope content of the large marine carbon reservoir can force shifts in that of the smaller carbon pools in the atmosphere and on land. The carbon isotope compositions of marine carbonate sediments from the late Palaeocene vary considerably, exhibiting a sudden decrease close to the Palaeocene/Eocene boundary which coincides with deep-sea benthic extinctions1 and with changes in ocean circulation. Here we report that these fluctuations in the marine carbon isotope record are closely tracked by the terrestrial records provided by palaeosol carbonates and mammalian tooth enamel. In using palaeosol carbonates to reconstruct the CO2 content of the ancient atmosphere2, isotope shifts of this sort will have to be taken into account. The sharp decrease in 13C/12C ratios in the late Palaeocene provides a datum for precise correlation of marine and continental records, and suggests that abrupt climate warming at this time may have played an important role in the evolution of land mammals.

Tektite-like glasses preserved at the Cretaceous/Tertiary boundary at Beloc in Haiti provide clear evidence of an impact event. The glass composition suggests that the impact occurred on a continental shelf region, generating a silica-rich glass with chemical composition that reflects the melting of continental crustal rocks, and a calcium-rich glass produced by the fusion of marl sediments. These findings indicate that catastrophic release to the atmosphere of 1015 moles of C02 from vaporized marl occurred during the impact.

The Palaeocene/Eocene thermal maximum — a period of intense global warming about 55 million years ago — was associated with a massive release of isotopically distinctive greenhouse gases into the ocean-atmosphere system. It remains unclear, however, whether this input caused or resulted from the global warming and environmental change that characterize the event. Sluijs et al. use high-resolution records of environmental change across the Palaeocene/Eocene boundary from two sediment sections in New Jersey to shed light on this question. They find that the onset of environmental change and surface-ocean warming preceded the input of greenhouse gases by several thousand years at this location. This sequence of events is consistent with the proposal that deep-ocean warming caused the dissociation of submarine gas hydrates, releasing massive amounts of the greenhouse gas methane. But the cause of the early warming remains uncertain. Exceptionally high resolution records of environmental change across the Palaeocene/Eocene boundary from two sediment sections in New Jersey find that the onset of environmental change and surface–ocean warming preceded the input of greenhouse gases by several thousand years. This sequence is consistent with the proposal that warming of the deep ocean caused the dissociation of submarine gas hydrates, which released massive amounts of methane. The start of the Palaeocene/Eocene thermal maximum—a period of exceptional global warming about 55 million years ago—is marked by a prominent negative carbon isotope excursion that reflects a massive input of 13C-depleted (‘light’) carbon to the ocean–atmosphere system1. It is often assumed2 that this carbon injection initiated the rapid increase in global surface temperatures and environmental change that characterize the climate perturbation3,4,5,6,7, but the exact sequence of events remains uncertain. Here we present chemical and biotic records of environmental change across the Palaeocene/Eocene boundary from two sediment sections in New Jersey that have high sediment accumulation rates. We show that the onsets of environmental change (as recorded by the abundant occurrence (‘acme’) of the dinoflagellate cyst Apectodinium) and of surface-ocean warming (as evidenced by the palaeothermometer TEX86) preceded the light carbon injection by several thousand years. The onset of the Apectodinium acme also precedes the carbon isotope excursion in sections from the southwest Pacific Ocean8 and the North Sea, indicating that the early onset of environmental change was not confined to the New Jersey shelf. The lag of ∼3,000 years between the onset of warming in New Jersey shelf waters and the carbon isotope excursion is consistent with the hypothesis that bottom water warming caused the injection of 13C-depleted carbon by triggering the dissociation of submarine methane hydrates1,9,10, but the cause of the early warming remains uncertain.

Reconstruction from oxygen-isotope data of global sea-surface temperatures (SSTs) that prevailed during periods when greenhouse-gas levels were higher than at present (that is, in the early Eocene and Cretaceous epochs) indicates that temperatures were significantly warmer than present in mid- to high latitudes, but similar to or slightly cooler in the tropics. However, Pearson and colleagues1 claim, on the basis of oxygen-isotope data from planktonic foraminifers recovered from clay beds in Tanzania and other localities, that the tropics were warm during the Eocene, attributing the discrepancy between these and previous tropical SST estimates to the effects of diagenetic overprinting of the latter. Here we note some potential flaws in their interpretation of the new oxygen-isotope data.

RECONSTRUCTIONSof early Eocene climate depict a world in which the polar environments support mammals and reptiles, deciduous forests, warm oceans and rare frost conditions 1–5. At the same time, tropical sea surface temperatures are interpreted to have been the same as or slightly cooler than present values6. The question of how to warm polar regions of Earth without noticeably warming the tropics remains unresolved; increased amounts of greenhouse gases would be expected to warm all latitudes equally7. Oceanic heat transport has been postulated as a mechanism for heating high latitudes8–10, but it is difficult to explain the dynamics that would achieve this7,11. Here we consider estimates of Eocene wetland areas and suggest that the flux of methane, an important greenhouse gas, may have been substantially greater during the Eocene than at present. Elevated methane concentrations would have enhanced early Eocene global warming, and also might specifically have prevented severe winter cooling of polar regions because of the potential of atmospheric methane to promote the formation of optically thick, polar stratospheric ice clouds12–14.

The normal, biologically productive ocean is characterized by a gradient of the 13C/12C ratio from surface to deep waters. Here we present stable isotope data from planktonic and benthic micro-fossils across the Cretaceous/Tertiary boundary in the North pacific, which reveal a rapid and complete breakdown in this biologically mediated gradient. The fluxes of barium (a proxy for organic carbon) and CaCO3 also decrease significantly at the time of the major marine plankton extinctions. The implied substantial reduction in oceanic primary productivity persisted for ∼0.5 Myr before the carbon isotope gradient was gradually re-established. In addition, the stable isotope and preservational data indicate that environmental change, including cooling, began at least 200 kyr before the Cretaceous/Tertiary boundary, and a peak warming of ∼3 °C occurred 600 kyr after the boundary event.