by The Greenland ice cap is losing an average of 30m tonnes of ice per hour due to the climate crisis, a study has shown, which is 20% more than previously thought.
Some scientists are concerned that this additional source of fresh water pouring into the North Atlantic could mean the ocean currents known as the Atlantic meridional transverse circulation (Amoc) are closer to being triggered, with consequences serious for mankind.
Large ice loss from Greenland as a result of global warming has been recorded for many years. The techniques used so far, such as measuring the height of the ice sheet or its weight through gravity data, are good for determining the losses that occur in the ocean and raise the sea level.
However, they cannot account for the retreat of the glaciers that are already mostly below sea level in the narrow fjords around the island. In the study, scientists analyzed satellite photographs to determine the final position of many Greenland glaciers every month from 1985 to 2022. This showed a large and widespread shortening that amounted to a trillion tons of ice lost in total.
“The changes across Greenland are huge and they’re happening everywhere – almost every glacier has retreated in the last few decades,” said Dr Chad Greene, at NASA’s Jet Propulsion Laboratory in the US, who was led the research. “It makes sense that if you dump fresh water on the north Atlantic Ocean, the Amoc will certainly weaken, although I have no idea how much.”
The Amoc was already known to be at its weakest level for 1,600 years and in 2021 researchers saw warning signs of a tipping point. A recent study suggested that the collapse could occur as early as 2025 in the worst case scenario. Scientists also think that a significant portion of the Greenland ice sheet itself is close to a tipping point of irreversible melting, and that ice is likely to face 1-2 meters of sea level rise already.
The study, published in the journal Nature, used artificial intelligence techniques to map more than 235,000 glacier terminus locations over the 38-year period, at a resolution of 120 metres. This showed that the Greenland ice sheet has lost an area of about 5,000 square km of ice at its edge since 1985, which equates to a trillion tons of ice.
The latest update from a project that coordinates all other measurements of Greenland ice found that 221bn tonnes of ice have been lost each year since 2003. The new study adds another 43bn tonnes a year, bringing the total loss to about 30m tonnes per hour on average.
The scientists said: “There is some concern that any small source of fresh water could be a ‘tipping point’ that could trigger a complete collapse of the Amoc, affecting global weather patterns, ecosystems and global food security . But oceanographic models do not currently include freshwater from the retreat of the Greenland ice sheet.” A less dense influx of fresh water into the sea slows down the normal process of heavier salt water sinking in the polar region and driving the Amoc.
Professor Tim Lenton, at the University of Exeter, UK, who is not part of the study, said: “This additional freshwater input to the North Atlantic is a cause for concern, particularly in relation to the creation of deep water in the Labrador and Irminger Seas. within the subpole. gyre, as other evidence suggests, these are the regions most at risk of being stuck in an ‘off’ or collapse state.”
“That would be like a partial collapse of Amoc, but falling faster and affecting the UK, western Europe, parts of North America, and the Sahel region, where there could be disruption very much on the west African monsoon,” he said. “Whether this previously unaccounted source of freshwater is enough to make a difference depends on how close we are to that subpolar gynecological tipping point. Recent models suggest that it may already be close at the current level of global warming.”
However, Professor Andrew Shepherd, at the University of Northumbria, UK, said: “Although there was a dramatic change in glacier retreat at the turn of the century, it is encouraging to see that the pace of ice loss has been steady since then and still well below the levels needed to disrupt the Amoc.
The discovery of additional ice loss is also important for calculating the Earth’s energy imbalance, ie how much extra solar heat the Earth is capturing due to human-caused greenhouse gas emissions, Greene said. “It takes a lot of energy to melt 1tn tonnes of ice. So if we want very accurate energy balance models for the Earth, that has to be taken into account.”
The glaciers analyzed in the study were already mostly below sea level, so the lost ice was replaced by sea water and did not affect sea level directly. But Green said: “It certainly has an indirect effect, by allowing glaciers to speed up. These narrow fjords are the bottleneck, so if you start chipping away at the edges of the ice, it’s like removing a plug in the drain.”
Chad and colleagues also analyzed the size of the Antarctic ice shelves over time in a study published in 2022. They found that the total lost from the ice shelves since 1997 doubled to about 12tn tonnes when the size of the shrinking area on the shelves was accounted for and added to the thinning of the shelves.
