For Greenland, 2021 was not a quiet year. For the first time in its history, rain fell on its summit. In August it recorded one of the most intense ice melt events in its recent history. 2021 was also the third record year for melting in the last decade.

This year the ice sheet lost more ice than it gained, for the twenty-fifth consecutive year. Between September 2020 and August 2021, Greenland lost a total of 166 billion tonnes of ice. This figure is in line with that of recent decades. But what is unusual is the way in which Greenland came to lose all this ice.

Greenland is becoming increasingly unstable

According to observations made by researchers at the Lamont-Doherty Earth Observatory of Columbia University, the island experienced anomalous swings, alternating between periods of intense ice melt and unusual snowfall (some of it caused by a hurricane). While snowfall was abundant, the losses due to melting (and iceberg calving) were the highest on record, at least since satellite monitoring began in 1986.

"This shift from very large accumulation to intense melting is the signal of a highly unstable system that is trying to find a way to reach a new equilibrium," Marco Tedesco, a researcher at Lamont, explained to the Washington Post. According to the researcher, the instability is accelerating the disappearance of ice in Greenland.

Greenland is losing 12 billion tonnes of ice per day

Scientists calculate the total mass losses and gains of Greenland by taking several factors into account. First, they examine the net accumulation of snow on the surface of the ice sheet, a phenomenon known as the "mass balance" — that is, from when the first snowflakes fall (generally in September) through to the end of the warm season, when the ice melts (the following August).

Winter snowfall was good. Then at the end of June, almost unexpectedly, a near-record amount of snow fell. Not only did the snow cover add mass to the ice sheet, it also helped reflect sunlight back into the atmosphere, delaying the onset of summer melting. Overall, the ice sheet managed to gain 396 billion tonnes of mass through snowfall from September 2020 to August 2021, a figure that falls within the average for recent decades, although still significantly lower than in the 1990s.

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But from July onwards, Greenland faced three notable ice melt events. One of these episodes triggered a dramatic mass loss, estimated at 8–12 billion tonnes per day. The last of the three episodes, which occurred in August, caused widespread melting as rain fell for the first time on the summit of the ice sheet.

Ice sheet melt has raised sea levels by 1.5 centimetres

It is not only the instability of atmospheric conditions that causes large portions of ice to break away, but also the warming of seawater (while a much smaller percentage of ice is lost due to the heat flux that seeps from the polar ice sheet into the ground). This year, scientists calculated that approximately 500 billion tonnes were lost due to iceberg calving, the highest figure in 35 years of satellite records.

In particular, the Jakobshavn Glacier in western Greenland, already known as the "fastest in the world", lost around 45 billion tonnes in the last year alone, 10 per cent of the total annual melt. This is a particularly alarming figure given that the Jakobshavn Glacier had recently begun to regrow. Thus, the high rate of ice loss significantly affected the mass balance of the ice sheet. It could have been worse, however, had the surface mass accumulation not been so high. In 2019, poor winter snowfall and a very warm summer resulted in a net ice loss of 329 billion tonnes.

In total, from September 1968 to August 2021, the Greenland ice sheet lost approximately 5,500 billion tonnes of ice, equivalent to 1.5 centimetres of average global sea level rise.