Melting Ice Sheets Could Hasten Ocean Current’s Collapse.

The climate crisis could push an important ocean current past a critical tipping point sooner than expected, new research suggests.

The Atlantic Meridional Overturning Circulation (AMOC) helps move heat from the tropics to the Northern Hemisphere, and is one of the reasons why Europe has relatively mild winters, Science Alert explained. However, the current has begun slowing down in recent years, and scientists want to know what it would take for the current to stop. A study published in Proceedings of the National Academy of Sciences Monday calculates that this moment could come sooner than expected.

"It is worrying news. Because if this is true, it reduces our safe operating space," Johannes Lohmann of the Niels Bohr Institute at the University of Copenhagen and study co-author said in a press release.

Essentially, the new findings come down to timing. The AMOC is threatened by increasing freshwater into the North Atlantic as the Greenland ice sheet melts. A certain amount of freshwater will cause the current to stop. But Lohmann wanted to test what would happen if that water were added quickly instead of gradually.

"These tipping points have been shown previously in climate models, where meltwater is very slowly introduced into the ocean," Lohmann told Gizmodo. "In reality, increases in meltwater from Greenland are accelerating and cannot be considered slow."

So researchers used an ocean model called Veros to calculate when the current would stop if freshwater were gradually added, the press release explained. Researchers then added freshwater at increased rates and found that quickly added freshwater would stop currents before reaching the initial freshwater threshold.

This has serious implications for the AMOC current itself. If it were to completely halt, tropical monsoon patterns would shift, rainfall in the Northern Hemisphere would decrease and the North Atlantic would get stormier. It also raises concerns about other climate tipping points, such as polar ice sheets collapsing or the Amazon rainforest drying out. If the AMOC can stop before the previously calculated tipping point is reached, is the same true for other systems?

"The results show that the safe operating space of elements of the Earth system with respect to future emissions might be smaller than previously thought," the study authors wrote.

However, outside scientists urged caution. Dave Sutherland, an associate professor in Earth sciences at the University of Oregon, told Gizmodo that the findings were important and timely, but that the models did not take all details into account, such as the location of the meltwater entering the ocean from Greenland.

Lohmann agreed that more testing is needed, but also pressed the urgency of climate action.

"Due to the potentially increased risk of abrupt climate change in parts of the Earth system that we show in our research, it is important that policymakers keep pushing for ambitious short- and mid-term climate targets to slow down the pace of climate change, especially in vulnerable places like the Arctic," Lohmann told Gizmodo.

25 February 2021

EcoWatch