Even a slight slowdown of key Atlantic currents poses a 'stunning risk' to rainforests

Even a small slowdown to one of Earth's major ocean currents could nearly halve the rainfall over parts of the planet's rainforests, fueling droughts that could accelerate climate change, a new study warns.

The Atlantic Meridional Overturning Circulation (AMOC), which includes the Gulf Stream, plays a key stabilizing role in climates around the planet. Yet a number of studies indicate that the current is slowing, with some even suggesting its heading toward a disastrous collapse.

Now, a new study has analyzed 17,000-year-old climate records to connect the current's weakening with its effects on the planet's tropics. Published Wednesday (July 30) in the journal Nature, the research suggests that the possible impact presents "a stunning risk" that could send swathes of usually humid regions, in the Amazon rainforest and elsewhere, into drought.

"This is bad news, because we have these very important ecosystems in the Amazon," study lead author Pedro DiNezio, an atmospheric and ocean scientist at the University of Colorado Boulder, said in a statement. "The Amazon rainforest contains almost two years of global carbon emissions, making it a major carbon sink on Earth. Drought in this region could release vast amounts of carbon back into the atmosphere, forming a vicious loop that could make climate change worse."

The AMOC acts as a planetary conveyor belt, bringing nutrients, oxygen and heat north from tropical waters while moving colder water south — a balancing act that keeps both sides of the Atlantic 9 degrees Fahrenheit (5 degrees Celsius) warmer than it would otherwise be.

But research into Earth's climate history shows that the current has switched off in the past, and some studies have hinted that glacial meltwater released by climate change is causing the AMOC to slow. The worst-case scenarios predicted by some models suggest that the current may outright collapse sometime this century, leading to devastating and irreversible impacts felt across the globe.

Related: Atlantic ocean currents are weakening — and it could make the climate in some regions unrecognizable

These predictions remain controversial, yet the risks are large enough for scientists to have called for urgent investigation. The effects of a diminished AMOC would include plummeting temperatures in Europe and storms proliferating around the equator — but scientists have also pointed to other, less foreseeable, impacts in Earth's tropical regions.

To investigate these possible outcomes, the researchers behind the new study pooled data of ancient rainfall patterns preserved in cave formations and lake and ocean sediments. They then plugged them into climate models to simulate the shifts in the past and how they may change in the future.

These models predict that a weakening AMOC would cool the northern Atlantic, causing temperatures to drop in the tropical Atlantic and Caribbean. This change, accompanied by rising global temperatures due to climate change, would lead to a drop in precipitation over regions in the rainforest belt, with rainfall dropping by up to 40% over parts of the Amazon rainforest.

Yet despite this alarming prediction, the researchers stress that the situation isn't hopeless: Though the tropics may remain sensitive to small shifts in the AMOC's strength, they say it is unlikely to collapse completely.

The fate of the current, and how severely it slows, depends on tackling climate change now.

"We still have time, but we need to rapidly decarbonize the economy and make green technologies widely available to everyone in the world," DiNezio said. "The best way to get out of a hole is to stop digging."