Slowing ocean currents could dry out the tropics

Some of the wettest regions in the tropics could dry out dramatically in the coming decades. New research suggests that climate change may slash annual rainfall by nearly half in places like the Amazon if a key Atlantic Ocean current slows down.

The scientists behind this work are from the University of Colorado Boulder. The study shows that even a modest weakening of the Atlantic Meridional Overturning Circulation (AMOC) could disrupt global precipitation patterns, endanger ecosystems, and put millions of people at risk.

“That’s a stunning risk we now understand much better,” said lead author Pedro DiNezio, associate professor in CU Boulder’s Department of Atmospheric and Oceanic Sciences. He noted that some parts of the Amazon rainforest could see up to a 40% drop in yearly rainfall.

How the ocean moves heat and moisture

The AMOC is a powerful system of currents that circulates warm, salty water from the tropics to the North Atlantic. This flow helps regulate global climate. It also keeps the tropical rain belt – a narrow zone near the equator where rainfall is heaviest – in place.

As polar ice melts and rainfall increases with global warming, the ocean’s surface water becomes less salty and less dense. This change could slow the AMOC.

While we’ve only been monitoring this current directly for about 20 years, early signs of change have already appeared.

“A few years ago, this monitoring system recorded signs of a decline in the AMOC, but it later rebounded. So we weren’t sure if it was just a fluke. The problem is, we haven’t been measuring the ocean long enough to detect meaningful long-term change,” DiNezio said.

Reconstructing the past to see the future

To better understand what could happen, DiNezio and his team looked to the past. About 17,000 years ago, the AMOC slowed down due to natural causes. That event left behind evidence in cave formations, lake beds, and ocean sediments that preserved past rainfall levels.

By comparing that ancient record with modern climate models, the researchers identified which simulations best matched historical patterns. Then they used those models to project what a future slowdown would look like.

The results were striking. As the AMOC weakens and cools the North Atlantic, that cooling spreads toward the tropical Atlantic and Caribbean. Combined with rising global temperatures, this leads to sharp drops in rainfall over Central America, the Amazon, and West Africa.

“This is bad news, because we have these very important ecosystems in the Amazon,” DiNezio said. The Amazon stores nearly two years’ worth of global carbon emissions.

“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 future of the tropics

The AMOC probably won’t grind to a halt entirely. But even a small slowdown could have major effects across the tropics.

How much and how fast it weakens depends on what we do next. The more greenhouse gases we emit, the more likely we are to trigger a dangerous shift.

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

The takeaway is clear: the behavior of ocean currents isn’t just a matter for climate scientists. It’s tied to the lives, lands, and livelihoods of people across the tropics – and beyond.

Why the tropics deserve more attention

For many, climate change brings to mind melting ice sheets or rising seas. But the tropics – home to over 40% of the world’s population – face threats that are just as urgent.

Shifts in rainfall aren’t only environmental concerns; they’re deeply human ones. They affect farming, food supplies, drinking water, and public health.

In regions already vulnerable to climate stress, losing seasonal rains could push communities past their limits.

The research highlights a growing truth: the future of the tropics will be shaped as much by global decisions as by local resilience. Ignoring this region is no longer an option.

The full study was published in the journal Nature.

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