While there have been signs the Atlantic Meridional Overturning Circulation has slowed, it could soon stop altogether. That would be very, very bad.

The collapse of one of the world’s most important ocean currents, a disaster that would wreak havoc on Earth's ecosystems, is generally considered unlikely. But in a newly issued open letter, leading climate scientists warn that the threat has been underestimated and call for immediate action.

The Atlantic Meridional Overturning Circulation (AMOC) brings warm water from the south up to the north Atlantic Ocean. Along with that water come carbon, nutrients, and other vital components essential for sustaining a healthy population of fish and other aquatic animals. Data has consistently shown the flow of warm water slowing. In their letter, which was addressed to the Nordic Council of Ministers, more than 40 prominent climate scientists from across Europe, China, the United States, Israel, and Australia, said the AMOC now could be just years away from total collapse.

The climate scientists quoted a 2023 Intergovernmental Panel on Climate Change (IPCC) report, which said there is a “medium confidence†the AMOC won’t abruptly collapse by 2100. If such a low likelihood event did happen, the report added, “it would very likely cause abrupt shifts in regional weather patterns and water cycle.â€

But recent research detected signs the IPCC may have understated the danger. Alarm over the AMOC’s condition has been growing in recent years. In 2023, an examination of ocean current data published in Nature Communications concluded AMOC could collapse “around mid-century under the current scenario of future emissions.†Another study, from April 2024, offered the faintest glimpses of optimism, suggesting that even if CO2 levels in the atmosphere hit twice the pre-Industrial Revolution level, the AMOC could conceivably recover. Unfortunately, that would take around 1,000 years, and that’s the best case scenario. If emissions continue to rise, it could take many more millennia before AMOC returns to normal.

“The passing of this tipping point is a serious possibility already in the next few decades,†the scientists wrote in their letter. “Despite significant research into the possibility and mechanisms of a collapse, the probability of such an occurrence remains highly uncertain.â€

While a collapse this century may not be assured, the scientists stressed that even the possibility must be taken incredibly seriously. They added that although the planet may have avoided a worst-case scenario in the 2000s, it doesn't mean humanity hasn’t set one in motion, which could unfold in the next century.

What would an AMOC collapse mean? In short, it could be catastrophic. In their letter, the researchers focused on the effects of the Nordic region. The region would face major cooling and "unprecedented extreme weather." The extent of the effects a possible collapse would have on the area's ecosystems, weather, and people requires further study. However, it would likely have a devastating impact on northwestern Europe's ability to grow food.

The letter was addressed to Nordic leaders because of the prominent role the Arctic plays in AMOC. Climate change is leading to changes in the Greenland Ice Sheet, ice in the Barents Sea, and other important natural systems which contribute to AMOC functioning as it does. However, if AMOC does collapse, the harm wouldn’t be contained to the five Nordic countries. In 2022, researchers determined there would be all sorts of secondary effects, including cooling in the Pacific Ocean, drier weather in the Northern Hemisphere, wetter conditions in the Southern Hemisphere, and shifts in trade winds.

Given the seriousness of the situation, the scientists urged action be taken. They called on the Council of Nordic Ministers to assess what a collapse would mean for their countries, and “take steps to minimize this risk as much as possible.â€

Those steps could possibly include upping the pressure on other global leaders to do their part to meet the aims of the 2016 Paris Agreement. The Trump administration pulled the United States out of the agreement, though the country rejoined in 2021. With Trump’s return to office possibly imminent, getting out and voting could be a great first step towards avoiding the collapse of one of the world’s most crucial ecological systems.