Transient AMOC weakening during overshoot warming masks committed Greenland ice sheet loss

The Greenland ice sheet is a vast, ancient river of frozen water, holding enough ice to raise global sea levels by more than seven meters if it were to melt completely. For decades, scientists have understood that as the planet warms, this massive sheet begins to lose mass, primarily through surface melting and the calving of icebergs into the ocean. A central goal of international climate policy is to limit global warming to well below two degrees Celsius above pre-industrial levels, with a preference for 1.5 degrees, to prevent catastrophic changes. However, current emissions suggest the world may temporarily exceed these targets before human efforts to remove carbon dioxide from the atmosphere bring temperatures back down. This scenario, known as an "overshoot," raises a difficult question: if the planet gets too hot for a while and then cools, does the ice sheet recover, or is the damage already done? The answer depends not just on the average temperature of the globe, but on how the oceans and atmosphere move heat around the planet, particularly in the North Atlantic.

A team of researchers from the Bjerknes Centre for Climate Research in Norway has investigated this question using sophisticated computer simulations that link global climate models with detailed models of the Greenland ice sheet. Their work focuses on a specific mechanism: the Atlantic Meridional Overturning Circulation, or AMOC. This is a massive system of ocean currents that acts like a conveyor belt, transporting warm water from the tropics toward the North Atlantic and bringing cold water back south. The researchers found that during a period of overshoot warming, this ocean current weakens significantly. This weakening has a surprising, temporary effect: it cools the air over southern Greenland, which slows down the melting of the ice sheet in that region. For a time, this creates an illusion of stability, masking the fact that the ice sheet is still vulnerable to the underlying global warming.

The study reveals that this cooling is only a pause, not a cure. As the world eventually removes carbon dioxide and global temperatures begin to fall toward the target limits, the ocean currents slowly recover their strength. Once the AMOC returns to normal, it resumes pumping warm water northward. This triggers a sudden and rapid acceleration of melting in southwest Greenland, even though the global average temperature might now be within the safe limits set by international agreements. The researchers observed that in their simulations, once the ocean circulation recovered, the ice sheet in southwest Greenland became extremely sensitive to even tiny amounts of additional warming. A difference of just a few tenths of a degree in the final stabilized temperature was enough to shift the region from a state of slow melting to a state of rapid, committed loss.

This delayed reaction suggests that the ice sheet does not respond immediately to changes in global temperature. Instead, its fate is tied to the slower, lagging response of the ocean. The researchers found that during the overshoot phase, the weakening of the ocean current acted like a shield, temporarily protecting the ice from the full force of global warming. However, once that shield was removed as the currents recovered, the ice sheet was exposed to the accumulated heat of the past centuries. The simulations showed that for pathways where the world overshoots the temperature targets and then relies on carbon removal to cool down, the ice sheet in southwest Greenland may not stabilize when the world thinks it has. Instead, the destabilization is merely postponed, emerging centuries later when the ocean circulation recovers.

The study also highlights that different parts of Greenland react in different ways. While the southwest, which is dominated by surface melting, was temporarily protected by the cooling ocean currents, other regions like the southeast did not benefit in the same way. In the southeast, the weakening of the currents reduced the amount of moisture reaching the ice, leading to less snowfall. This lack of snow offset the cooling benefits, meaning that while the southwest paused its retreat, the southeast continued to lose mass. This asynchronous behavior means that looking at the total amount of ice lost is not enough to understand the full picture; the specific location and the timing of the ocean changes are critical.

When the researchers extended their simulations forward by ten thousand years to see what happens in the long term, the consequences of this delayed melting became clear. In scenarios where the final global temperature settled at 1.7 degrees Celsius, the ice sheet experienced a slow, multi-meter rise in sea levels over millennia. However, in scenarios where the temperature settled at 2 degrees Celsius, the outcome was drastically different. The ice sheet in southwest Greenland began a widespread retreat that led to the loss of almost all the ice in central Greenland, contributing between 3.6 and 4.7 meters to global sea levels. This dramatic shift occurred for a very small increase in the final temperature, suggesting that the threshold for long-term instability in this region lies very close to the current targets of 1.5 to 2 degrees.

The findings challenge a simplified view of climate overshoot pathways, where returning global temperatures to safe levels is assumed to stabilize the ice sheet. The research suggests that if the ocean circulation weakens significantly during the overshoot, it can hide the true extent of the damage being done. The ice sheet may appear to be holding steady while the ocean currents are weak, but once those currents recover, the ice sheet can enter a state of irreversible retreat. This means that the resilience of the Greenland ice sheet depends not only on the peak temperature reached but also on the transient changes in ocean circulation and regional climate feedbacks that occur along the way.

The researchers emphasize that their results are based on simulations using a specific climate model that shows a strong weakening of the ocean currents. Other models that predict a weaker decline in these currents might show different timing or intensity for the ice sheet's response. Nevertheless, the study underscores a critical point: the interaction between the ocean and the ice sheet creates a complex delay in the system's response. What looks like a safe, stabilized climate on a human timescale could be concealing a slow-motion collapse that only becomes visible centuries later. The study concludes that understanding the future of the Greenland ice sheet requires a deep understanding of how the ocean currents will evolve, as these currents act as the hidden switch that can trigger or delay the melting of the ice.