For the Earth’s polar ice sheets on Greenland and Antarctica, which together hold enough ice to raise sea level by 65 meters, risks of non-reversible melting increase as temperature and rates of warming rise. The Earth’s climate record makes clear that warming above even 1°C over pre-industrial levels has resulted in very different coastlines in Earth’s past, due to extensive melting of the West Antarctic Ice Sheet (WAIS), Greenland and by 1.5°C, possibly parts of East Antarctica. While some of these changes occurred very slowly in the past, over thousands of years, there have also been periods where extremely rapid sea-level rise (around 4 meters per century) has occurred, due to rapid ice sheet collapse. Termed “melt-water pulses,” the last took place around 14,000 years ago, when global sea levels rose between 12–18 meters in just 350 years.

The observed human-induced global temperature increase over the past few decades is much faster than anything documented in Earth’s past. CO2 increases in the last 50 years are 200 times greater than during the end of the last Ice Age. This means that future rates of ice sheet loss and sea-level rise (SLR) could increase even further beyond the acceleration that has been observed over the past few decades, and could potentially be more rapid than at any other time in the past 130,000 years. Better understanding of ice sheet behavior, especially interactions between the ice and the warming oceans that surround them, informs us that ice sheet collapse and rapid sea-level rise cannot be ruled out, especially if peak warming were to exceed 3°C. This is especially the case for the West Antarctic Ice Sheet (WAIS): some studies show the threshold for WAIS collapse may already have passed at around 0.8°C above pre-industrial although the WAIS could hold stable for some centuries unless further warming occurs. Even if ice sheet loss is inevitable once triggered, this can be slowed to take place over longer timescales if temperatures remain close to 1.5°, with an aim to return below that level as soon as possible. This would give coastal communities greater time to adapt to rising sea levels.

There is strong consensus that the risk of extensive melting from the ice sheets increases as both the peak in global temperatures and the rate of warming rise. The massive Greenland and Antarctic ice sheets consist of compressed snow that fell, in the oldest sections, over a million years ago. In equilibrium, calving of icebergs and outflow of melt water into the ocean are balanced by mass gain via snowfall. Observations now confirm that this equilibrium has been lost on Greenland, the WAIS, and the Antarctic Peninsula; and potentially portions of the ten-times-larger East Antarctic Ice Sheet.

“Once ice sheet melt accelerates due to higher temperatures, it cannot be stopped or reversed for many thousands of years, even once temperatures stabilize.” — State of the Cryosphere 2022: Growing Losses, Global Impacts

All changes in the total mass of ‘land ice’ bound within the Earth’s ice sheets have direct consequences for global sea level. During Ice Age periods, when the ice sheets expanded significantly, sea level was around 130 meters lower than it is today. During periods of warming, when the ice sheets lost mass, sea level rose accordingly, with occasional meltwater pulses (noted above) probably as a result of catastrophic collapse of the Laurentide Ice Sheet over Canada.

Antarctic ice shelves also play an important role in ice-loss processes, as they hold back, or “buttress,” the ice sheets upstream. Loss of this buttressing effect through ice shelf thinning and break-up can accelerate the rate of ice flow from the land into the sea. From 1997 to 2021, Ant- arctic ice shelves experienced a net loss of 36,701 ± 1,465 km2, equal to the size of the country Guinea-Bissau31. Antarctic ice shelf thinning has also accelerated over recent decades, driven by intrusion of warmer ocean currents. Reduced ice-shelf buttressing driven by such warm ocean currents accounts for a significant portion of Antarctic mass loss33 and could drive increasingly significant sea- level rise in the future.

Ice sheets in Greenland and (parts of) Antarctica have certain thresholds where irreversible melt becomes inevitable and, in the case of the WAIS, potentially rapid. In Earth’s past, several of these thresholds have occurred somewhere between 1 and 2 degrees of warming: about 1°C for the WAIS and Antarctic Peninsula (containing about 5 meters SLR); and between 1.5°C and 2°C for Greenland (approximately 7 meters SLR). (It should be noted that changes around past thresholds were driven by slow increases in atmospheric greenhouse gases but were paced by slow changes in Earth’s orbit – unlike today’s rapid, human-caused rates of change.) Parts of East Antarctica, especially the massive Wilkes and Aurora Basins (~4 meters of potential SLR), may also have a threshold around or just beyond 2°C.

Because of the existence of these thresholds, when temperatures reached 2°C above pre-industrial in the Earth’s past, sea levels peaked at around 12–20 meters higher than present-day levels. During the height of the Pliocene 3 million years ago, when CO2 levels were comparable to today and temperatures stabilized at 2–3°C higher than pre-industrial, sea levels may have peaked at around 20 meters higher than today’s. Such extensive sea-level rise would be catastrophic for today’s coastal communities – yet we are currently on track for even higher temperature peaks than those that drove these past sea-level rises.

The rate of future sea-level rise, and associated risks to security and development, now largely depends on human decisions on future emissions of greenhouse gases.

For more information, please access the State of the Cryosphere 2022 Report, co-authored by more than 60 leading cryosphere scientists and IPCC authors and released at COP27 in Sharm El Sheikh, Egypt during November 2022.

State of the Cryosphere 2022: Growing Losses, Global Impacts