Kilometres thick and spanning continents, ice sheets hold such vast amounts of freshwater that, if melted even slightly, they can raise global sea levels. Ice sheets can grow rapidly through self-sustaining processes, like increasing thickness that cools their surface that in turn promotes ice accumulation, or through their sheer weight bowing underlying crust that provides more room to grow vertically. Reinforcing feedbacks like these make ice sheets resistant to fully melting away, though retreat can still happen quickly when climatic and geographic circumstances align. In this issue of Nature Geoscience, several studies challenge assumptions about what can, and cannot, cause ice sheets to grow and decay.

East coast of the Greenland Ice Sheet. Credit: Haltner Thomas / Prisma by Dukas Presseagentur GmbH / Alamy

The ice sheet that currently covers Antarctica first emerged about 34 million years ago. Large fluctuations in seafloor calcite oxygen isotope records followed throughout the Oligocene, with one explanation being that this reflected the waxing and waning of the early ice sheet1. However, Flavia Boscolo-Galazzo and colleagues show in an Article that this variability was almost entirely the result of changing deep ocean temperature. The volume of the Antarctic Ice Sheet therefore changed little even as periodic changes in Earth’s orbit caused the global climate to warm and cool. This stability is linked to the ice sheet not reaching the coast, despite being a substantial proportion of its modern size, which insulated it from ocean-related melting.

When the Antarctic Ice Sheet has reached the coast, or even beyond it, oceanic processes haven’t always affected different sectors of it in the same way. In an Article by Molly O. Patterson and colleagues, they evaluate how the East and West Antarctic ice sheets responded to an interval of relatively warm global temperatures during the mid-Pliocene, around 3.3 to 3 million years ago, when carbon dioxide concentrations were similar to today. They find that the West Antarctic Ice Sheet was particularly vulnerable to upwelling warm deep water causing ocean-driven melting of the base of marine-terminating ice shelves. Differences in topography and ocean circulation meant that the marine-terminating sectors of the East Antarctic Ice Sheet were far less sensitive to similar oceanic warming, though more gradual atmospheric warming still led to widespread surface melting. These nuanced changes hint at how the current Antarctic ice sheets will respond to ongoing warming.

Mid-latitude ice sheets stand apart from the polar ice sheets in terms of the drivers of when and how they waxed and waned. Typically, orbital forcings mean that when high latitude ice sheets expand in the Northern Hemisphere, they contract in the Southern Hemisphere, leading to a ‘bipolar seesaw.’ An Article by Samuel Toucanne and colleagues shows the seesaw disappeared during Heinrich Stadials, short intervals of intense cooling that occurred during the last glacial period. The authors find that glaciers from a large ice field in New Zealand retreated within about a thousand years of mid-latitude ice sheets in North America and Eurasia. This geologically rapid interhemispheric connection is suggested to be mediated by icebergs bringing fresh water into the North Atlantic, which slowed down the Atlantic overturning circulation, allowing heat to build up in the Southern Ocean. The synchroneity shows just how quickly events in one region can propagate through the broader climate system.

The Greenland Ice Sheet also has a propensity to respond quickly and strongly to changing climatic conditions. An Article by Caleb Walcott-George and colleagues reports that the currently five-hundred-metre-thick, land-based Prudhoe Dome, which protrudes from the larger ice sheet in northwest Greenland, entirely melted away about seven thousand years ago. Only a few degrees of regional atmospheric warming, particularly in the summer months, was enough to melt this volume of ice. This shows how susceptible the edge of Greenland’s ice sheet may be to warming that is likely to exceed these levels by the end of the next century.

These studies highlight how ice sheet behaviour can change as environmental conditions evolve. They also inform thinking about the future of ice sheets, though the rapid pace and spatial patterns of ongoing oceanic and atmospheric warming mean unexpected interactions may lay in store outside these lessons from Earth history.