Scientists have discovered a key warning sign before a crucial Atlantic current collapses and plunges the northern hemisphere into climate chaos.
The Atlantic Meridional Circulation (AMOC) transports warm water northward from the Southern Hemisphere, where it releases heat and freezes. The freezing process concentrates the salt in the unfrozen portion of the ocean water; This extrasaline water sinks, travels back south, and regains heat again, restarting the conveyor belt. (The Gulf Stream is part of this belt).
This release of heat helps keep Europe, and to some extent North America, warmer than it would otherwise be. But sediment records from the past 100,000 years suggest that the AMOC has sometimes closed abruptly, causing major climate changes in just a few decades.
Scientists believe we could be veering towards this scenario once again (potentially as early as 2025) as a result of climate change. Until now, however, researchers had no way of knowing whether the current is on its way to one of these tipping points.
In a new study, published today (February 9) in the journal Scientific advances, scientists discovered that the flow of fresh water into the Atlantic Ocean at a latitude of 34 degrees south (the latitude where South Africa is located) may indicate a key warning signal for an imminent AMOC collapse. The team found that about 25 years before the AMOC collapses, this flow reaches a minimum).
Scientists do not have a long enough record of observations of freshwater flow at this location to predict how far the AMOC is from a tipping point at this time. However, they do know that this flow has been decreasing.
Related: A controversial study on climate change claims that we will exceed 2 °C before 2030
“We are approaching the tipping point, but we cannot deduce the distance to the tipping point,” says the first author of the study René M. van Westena postdoctoral researcher in marine and atmospheric sciences at Utrecht University told Live Science.
Because the rise and fall of the AMOC depends on the salinity of the water, this circulation is very sensitive to freshwater inputs, van Westen said. As the climate warms and precipitation patterns change, the patterns of freshwater flow into the ocean also change.
However, it is difficult to predict the results and finding the AMOC tipping point requires simulating a gradual increase in freshwater flow in the North Atlantic over more than 2,000 years, van Westen said. This is a long and computationally expensive process, but trying to take shortcuts by simulating large pulses of freshwater is not as realistic or accurate.
The researchers modeled this gradual increase in freshwater using state-of-the-art climate models. They found a long negative trend in freshwater flow at 34 degrees south (the southern border of the Atlantic Ocean), reaching a minimum about 25 years before the AMOC collapsed. The minimum is not tied to a specific salinity value, but is relative to previous patterns, so researchers are not sure how these conditions compare to today. The collapse of AMOC caused a complete lack of circulation and a loss of approximately 75% of south-to-north heat transport.
If the AMOC were to collapse in the near future, the consequences would be dire. Without the AMOC, the Northern Hemisphere would become colder and the Southern Hemisphere would warm, although to a lesser degree. The effects vary by region, but Europe would be hit hard, van Westen said, cooling by 9 to 18 degrees Fahrenheit (5 to 10 degrees Celsius) within a century. This is a huge change, even compared to the current level of climate change, which is already having an impact.
“On average, the global climate warms about 0.2 degrees C (0.36 F) per decade,” van Westen said.
The collapse of the AMOC would also cause changes in precipitation around the world. For example, wet and dry seasons in the Amazon rainforest would trade places, leading to significant ecological impacts, the researchers wrote in the paper.
“We know that under climate change this AMOC will gradually weaken and this (freshwater) parameter will become more negative, thus further destabilizing the AMOC,” says van Westen. The message, he added, is that the need to stop climate change is urgent: “We need to stop emitting as a global society.”