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Climate change could weaken the AMOC. Credit: IPCC
For several years, climate scientists have shown that the Atlantic Meridional Overturning Circulation (AMOC) could come to a halt if the world warms too much. New research from Utrecht University shows that this picture is incomplete: The pace of warming also determines whether the AMOC stays stable.
The study was published in the journal Nature Climate Change.
AMOC is the system of ocean currents that transports warm water from the tropics northward. It plays a major role in redistributing heat across the planet and helps keep the climate in Western Europe relatively mild.
Scientists have long suspected that this "heat engine" of the Atlantic Ocean could reach a tipping point. This means that the system would shift from its present-day strong state to a much weaker state within decades. Such a tipping event could be triggered by external influences, such as an increasing amount of meltwater from the polar regions or global warming itself.
Until now, the AMOC was thought to tip and collapse around +4°C of warming. Researchers at the Institute for Marine and Atmospheric Research Utrecht now show that there's more to the story. "Our results show there is not necessarily a fixed temperature beyond which the AMOC inevitably collapses," says lead author René van Westen. "The stability of the circulation depends on how fast the climate is changing."
AMOC strength for a slow (+0.5 ppm yr⁻¹, black) and fast (+2.5 ppm yr⁻¹, blue) increase in atmospheric CO2. The yellow star marks the onset of the AMOC collapse under fast warming, at around +2°C in this model. Credit: Courtesy of the researchers
Slow versus fast warming
To investigate how the rate of climate change affects the strength of the AMOC, van Westen and his colleagues ran a climate model twice. In their simulations, they used a gradually increasing amount of atmospheric CO 2 , but at different speeds. In one simulation, CO 2 concentrations rose slowly (0.5 ppm per year); in the other, much faster (2.5 ppm per year), comparable to today's rate.
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