17th August 2020
Greenland ice sheet melting has passed point of no return
Based on 40 years of satellite data, scientists at Ohio State University conclude that melting of the Greenland ice sheet has now passed the point of no return – locking in several metres of global sea level rise.
Glaciers on Greenland – the world's largest island – have shrunk so much that even if global warming were to stop today, the ice sheet would continue shrinking. That is according to a new study published in the journal Communications Earth and Environment. It means that Greenland's glaciers have passed a tipping point where snowfall that replenishes the ice sheet each year cannot keep up with the ice that is flowing into the ocean from glaciers.
"We've been looking at these remote sensing observations to study how ice discharge and accumulation have varied," said Michalea King, lead author and a researcher at Ohio State University's Byrd Polar and Climate Research Center. "And what we've found is that the ice that's discharging into the ocean is far surpassing the snow that's accumulating on the surface of the ice sheet."
King and other researchers analysed monthly satellite data from more than 200 large glaciers draining into the ocean around Greenland. Their observations show how much ice breaks off into icebergs or melts from the glaciers into the ocean. They also show the amount of snowfall each year – the way in which these glaciers get replenished.
The researchers found that, throughout the 1980s and 90s, snow gained through accumulation and ice melted or calved from glaciers were mostly in balance, keeping the ice sheet intact. Through those decades, the ice sheets generally lost about 450 gigatons (450 billion tons) each year from flowing outlet glaciers, which was replaced with snowfall.
However, the amount of ice being discharged each year began to increase steadily from around the year 2000, with glaciers losing about 500 gigatons annually. Snowfall did not increase at the same time, resulting in a net loss. Since then, the rate of ice loss from glaciers has stayed at or above this new high level, meaning the ice sheet has continued losing ice more rapidly than is being replenished.
"We are measuring the 'pulse' of the ice sheet – how much ice glaciers drain at the edges of the ice sheet – which increases in the summer. And what we see is that it was relatively steady until a big increase in ice discharging to the ocean during a short five- to six-year period," explains King. "Glaciers have been sensitive to seasonal melt for as long as we've been able to observe it, with spikes in ice discharge in the summer. But starting in 2000, you start superimposing that seasonal melt on a higher baseline – so you're going to get even more losses."
Before 2000, the ice sheet would have had about the same chance to gain or lose mass each year. In the current climate, the ice sheet will gain mass in only one out of every 100 years.
Large glaciers across Greenland have retreated about 3 kilometres (1.9 miles) on average since 1985 – "that's a lot of distance," said King. The glaciers have shrunk back enough that many of them are sitting in deeper water, meaning more ice is in contact with water. Warm ocean water melts glacier ice, and also makes it difficult for the glaciers to grow back to their previous positions.
This means that even if humans were somehow miraculously able to stop climate change in its tracks, ice lost from glaciers draining ice to the ocean would likely still exceed ice gained from snow accumulation, and the ice sheet would continue to shrink for some time.
"Glacier retreat has knocked the dynamics of the whole ice sheet into a constant state of loss," said Ian Howat, co-author and a Professor of Earth Sciences at Ohio State. "Even if the climate were to stay the same or even get a little colder, the ice sheet would still be losing mass."
Shrinking glaciers in Greenland are a problem for the entire planet. Ice that melts or breaks off from Greenland ends up in the Atlantic Ocean and – eventually – all of the world's oceans. Greenland's ice sheet is huge, spanning over 660,000 square miles and containing 8% of Earth's fresh water. If the current warming trend continues, the island could lose almost 5% of its ice by 2100, adding up to 33 cm (13") of global sea level rise. All of the ice could potentially melt by the year 3000, contributing as much as 7.3 m (24 ft) – on top of additional melting from the Arctic, Antarctic and elsewhere.
The new findings are bleak, but King points out a silver lining: "It's always a positive thing to learn more about glacier environments, because we can only improve our predictions for how rapidly things will change in the future. And that can only help us with adaptation and mitigation strategies. The more we know, the better we can prepare."