资源环境科技发展态势分析平台

By Michael Le Page

Ian Joughin, University of Washi

Greenland lost more ice in 2019 than in any other year since measurements began. The ice sheet lost 15 per cent more ice than in 2012, the year the previous record was set.

“It is worrying, but not that much surprising,” says Ingo Sasgen at the Alfred Wegener Institute in Bremerhaven, Germany.

The finding is partially based on data from the GRACE satellites, which launched in 2002 and measured changes in Earth’s gravity. Among other things, they revealed just how much ice Greenland is losing as a result of melting and glacier flow into the ocean. The satellites were decommissioned in late 2017, but replacements launched in 2018, allowing measurements to resume.

These show that there was less ice loss than usual in 2017 and 2018, but a new record loss of around 530 gigatonnes followed in 2019.

The expectation is that ice loss will accelerate as warming continues, but the satellite record is too short to show such a trend. What is clear is that the weather in Greenland is becoming more variable and extreme.

In some years, such as 2019, there are more high-pressure systems over Greenland, drawing in warm air from North America that causes lots of melting. In others, such as 2017 and 2018, low-pressure systems are more prevalent, producing lots of snow. However, even in high-snow years, the ice sheet is still losing mass overall because of global warming, with many glaciers flowing faster and dumping more ice in the sea.

The suspicion is that this greater variability in the weather is linked to Arctic warming, but this isn’t certain. “At this time, it’s too early to nail it down,” says Sasgen.

The loss of all Greenland’s ice would add at least 6 metres to global sea level. It is thought this would happen gradually over millennia rather than there being a sudden collapse, as is expected to happen to parts of Antarctica’s ice sheet.

However, the process might already be unstoppable. Some studies, including one published last week, suggest we have triggered positive feedbacks that mean ice loss will continue even if the planet stopped warming today.

For instance, as the surface of the ice sheet falls, it will be exposed to the warmer air at lower altitudes, accelerating melting.

Sasgen thinks that we don’t know enough about these feedbacks to say for sure that we have passed the point of no return. “We know these tipping points are there, but can’t say when we have crossed them.”

There will be little sea level rise in the UK and other northern countries even if all Greenland’s ice melts. This is because ice sheets are so large that their gravity pulls water towards them, and this attraction is lost when they melt, causing the sea level around them to fall. But Antarctic melting will lead to bigger rises in these places than in most other regions.

Journal reference: Communications Earth & Environment, DOI: 10.1038/s43247-020-0010-1

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