In a new study scientists say they have found Greenland is losing 30 million metric tons of ice an hour, 20% more than had been calculated earlier. Published in Nature, the study—Ubiquitous acceleration in Greenland Ice Sheet calving from 1985 to 2022—adds to the concern of some scientists that this deluge of fresh water could mean the ocean currents of the Atlantic meridional overturning circulation—the AMOC—could be closer to collapse than they previously thought.
The study’s lead researcher, Chad Greene, a climatologist at NASA’s Jet Propulsion Laboratory, said, “The changes around Greenland are tremendous and they’re happening everywhere—almost every glacier has retreated over the past few decades. It makes sense that if you dump fresh water onto the north Atlantic Ocean, then you certainly get a weakening of the AMOC, though I don’t have an intuition for how much weakening.”
The AMOC is an ocean conveyor belt carrying nutrients, carbon, and heat from the south to the north, where the water cools and sinks and continues its circular flow. A collapse would mean severe disruptions across the planet, from changes in weather patterns to food security. Not only western Europe and parts of North America would be affected, but even the Sahel in Africa could also face disruption of the crucial annual monsoon.
Evidence for weakening of the AMOC has been growing ever since the system began being seriously examined in 2004. Another study in Nature published in July—Warning of a forthcoming collapse of the Atlantic meridional overturning circulation—also concluded weakening has been occurring. Those researchers speculated that collapse could begin as soon as 2025, and is ever more likely to happen as the 21st century unfolds.
Lead author Peter Ditlevsen, a professor of physics and climate science at the Niels Bohr Institute in Copenhagen, told Live Science last summer, "I don't consider myself very alarmist. In some sense it's not fruitful. So my result annoys me, in some sense. Because it [the window for possible collapse] is so close and so significant that we have to take immediate action now."
Damien Carrington at The Guardian reports on the latest study:
The Amoc was already known to be at its weakest in 1,600 years and in 2021 researchers spotted warning signs of a tipping point. A recent study suggested the collapse could happen as soon as 2025 in the worst-case scenario. A significant part of the Greenland ice sheet itself is also thought by scientists to be close to a tipping point of irreversible melting, with ice equivalent to 1-2 metres of sea level rise probably already expected. [...]
The scientists said: “There is some concern that any small source of freshwater may serve as a ‘tipping point’ that could trigger a full-scale collapse of the Amoc, disrupting global weather patterns, ecosystems and global food security. Yet freshwater from the glacier retreat of Greenland is not included in oceanographic models at present.” The influx of less dense freshwater into the sea slows the usual process of heavier salty water sinking in the polar region and driving the Amoc.
Not every scientist agrees. Andrew Shepherd, at the University of Northumbria, told The Guardian, “Although there was a step-change in glacier retreat at the turn of the century, it’s reassuring to see that the pace of ice loss has been steady since then and is still well below the levels needed to disturb the AMOC.”
Other scientists are also skeptical. Assessments by the Intergovernmental Panel on Climate Change, based on the Climate Model Intercomparison Project model simulations, indicate that a full collapse is unlikely within the 21st century.
Here’s the study abstract:
Nearly every glacier in Greenland has thinned or retreated over the past few decades1–4, leading to glacier acceleration, increased rates of sea-level rise and climate impacts around the globe5–9. To understand how calving-front retreat has affected the ice-mass balance of Greenland, we combine 236,328 manually derived and AI-derived observations of glacier terminus positions collected from 1985 to 2022 and generate a 120-m-resolution mask defining the ice-sheet extent every month for nearly four decades. Here we show that, since 1985, the Greenland Ice Sheet (GrIS) has lost 5,091 ± 72 km2 of area, corresponding to 1,034 ± 120 Gt of ice lost to retreat. Our results indicate that, by neglecting calving-front retreat, current consensus estimates of ice-sheet mass balance4,9 have underestimated recent mass loss from Greenland by as much as 20%. The mass loss we report has had minimal direct impact on global sea level but is sufficient to affect ocean circulation and the distribution of heat energy around the globe10–12. On seasonal timescales, Greenland loses 193 ± 25 km2 (63 ± 6 Gt) of ice to retreat each year from a maximum extent in May to a minimum between September and October. We find that multidecadal retreat is highly correlated with the magnitude of seasonal advance and retreat of each glacier, meaning that terminusposition variability on seasonal timescales can serve as an indicator of glacier sensitivity to longer-term climate change.