The pace of climate change matters more than overall heat in the race to save key Atlantic currents, study finds

Atlantic Ocean currents that regulate the global climate can adapt to rising temperatures, but the sheer pace of warming may cause the catastrophic collapse of these currents in the next few decades, a new study suggests.

Climate models show that when global temperatures rise slowly due to small year-over-year increases in atmospheric carbon dioxide (CO 2 ), evaporation and sea ice feedbacks maintain Atlantic currents' strength at temperatures exceeding 9 degrees Fahrenheit (5 degrees Celsius) of warming above preindustrial levels.

By contrast, a rapid rise in global temperatures driven by yearly greenhouse gas emissions equal to or higher than today's may trigger a tipping point at just 3.6 F (2 C) of warming, researchers found.

"Under slow warming, the entire ocean, from the surface down to its deepest layers, has time to gradually reorganise and adapt to the changing conditions," study co-author Henk Dijkstra , a professor of dynamical oceanography at Utrecht University in the Netherlands, said in a statement .

"Under faster warming, the ocean simply can't keep up." The study investigated how different rates of climate change could affect the stability of the Atlantic Meridional Overturning Circulation (AMOC), a giant network of ocean currents that includes the Gulf Stream and carries heat to the Northern Hemisphere.

An unstable AMOC could make the climate in some regions unrecognizable in the long term.

Specifically, slower currents could unleash freezing weather in parts of Europe, exacerbate sea level rise along the U.S.

East Coast, and trigger droughts around the equator, previous research has shown.

AMOC weakening originates in the North Atlantic Ocean, where surface waters sink less when temperatures rise. (Image credit: IPPC AR6 WGI, chapter 9) Previous research suggests that the AMOC is at its weakest in more than 1,000 years and might grind to a halt if global warming reaches a certain threshold, which scientists calculate could be around 7.2 F (4 C) above preindustrial levels .

However, the new study hints that there is more to the story than a single number.

The AMOC is vulnerable to global warming because the circulation relies on surface waters in the North Atlantic sinking to form bottom currents that propel the system.

Surface waters sink if they are denser — typically, colder and saltier — than the layers underneath.

But rising heat in the North Atlantic is warming surface waters, and Arctic meltwater is reducing their salt concentration, so these essential bottom currents are forming less reliably than they were before.

Dijkstra and his colleagues conducted model experiments known as CO 2 ramp simulations, in which it is possible to simulate increases in atmospheric CO 2 levels over time.