Changes in Earth's orbit are known to influence climate shifts from cold glacials to warm interglacials. How the vast West Antarctic ice sheet responds to these fluctuations is uncertain but, because its collapse could raise sea levels by about 5 metres, of great interest. Naish et al. have analysed the AND-1B ocean sediment core, extracted from beneath the Ross Ice Shelf as part of the ANDRILL drilling project, and find evidence that the ice sheet collapsed periodically during the early Pliocene (3-5 million years ago), when atmospheric CO2 levels were similar to, or slightly higher than today's. The pattern of collapse suggests an influence of approximately 40,000-year cycles in the tilt of Earth's rotational axis (obliquity). Also in this issue of Nature, in a numerical modelling study focused on the past 5 million years in Antarctica, David Pollard and Robert DeConto combine ice sheet (land-supported) and ice shelf (water-supported) modelling approaches to simulate the movement of the grounding line — the border between land and sea ice. Their results show that over the past 5 million years, the West Antarctic ice sheet transitioned between full, intermediate, and collapsed states in just a few thousand years. This means that the ice sheet is likely to disintegrate if ocean temperatures in the area rise by 5 C. The response of the vast West Antarctic Ice Sheet (WAIS) to climate shifts due to changes in Earth's orbit is uncertain, but there is potential for several metres of sea level change. Naish and co-authors extracted a sediment core from beneath the Ross Ice Shelf and found evidence that the WAIS periodically collapsed during the early Pliocene (3-5 million years ago); and the pattern of collapse suggests an influence of ∼40,000-year cycles in the tilt of Earth's rotational axis. Thirty years after oxygen isotope records from microfossils deposited in ocean sediments confirmed the hypothesis that variations in the Earth’s orbital geometry control the ice ages1, fundamental questions remain over the response of the Antarctic ice sheets to orbital cycles2. Furthermore, an understanding of the behaviour of the marine-based West Antarctic ice sheet (WAIS) during the ‘warmer-than-present’ early-Pliocene epoch (∼5–3 Myr ago) is needed to better constrain the possible range of ice-sheet behaviour in the context of future global warming3. Here we present a marine glacial record from the upper 600 m of the AND-1B sediment core recovered from beneath the northwest part of the Ross ice shelf by the ANDRILL programme and demonstrate well-dated, ∼40-kyr cyclic variations in ice-sheet extent linked to cycles in insolation influenced by changes in the Earth’s axial tilt (obliquity) during the Pliocene. Our data provide direct evidence for orbitally induced oscillations in the WAIS, which periodically collapsed, resulting in a switch from grounded ice, or ice shelves, to open waters in the Ross embayment when planetary temperatures were up to ∼3 °C warmer than today4 and atmospheric CO2 concentration was as high as ∼400 p.p.m.v. (refs 5, 6). The evidence is consistent with a new ice-sheet/ice-shelf model7 that simulates fluctuations in Antarctic ice volume of up to +7 m in equivalent sea level associated with the loss of the WAIS and up to +3 m in equivalent sea level from the East Antarctic ice sheet, in response to ocean-induced melting paced by obliquity. During interglacial times, diatomaceous sediments indicate high surface-water productivity, minimal summer sea ice and air temperatures above freezing, suggesting an additional influence of surface melt8 under conditions of elevated CO2.

Obliquity-paced Pliocene West Antarctic ice sheet oscillations