The ocean current responsible for western Europe’s temperate climate could be at risk of collapse due to global warming, according to new research.
Scientists at Germany’s Potsdam Institute for Climate Impact Research studied a system of currents in the Atlantic ocean. It’s called the Atlantic Meridional Overturning Circulation (AMOC). And it could have reached a point of “almost complete loss of stability” over the last century.
The Gulf Stream, the current of warm water flowing from the tip of Florida across the Atlantic towards Europe, is part of the AMOC. And it makes western Europe significantly warmer than it would otherwise be.
— Stefan Rahmstorf 🌏 (@rahmstorf) August 6, 2021
The AMOC acts like a conveyor belt in the Atlantic ocean. It carries warm surface water from the tropics to the North Atlantic. The water then cools and sinks to the lower depths of the ocean. This colder water gradually moves southward several kilometres deep. Then warmer ocean temperatures eventually pull it to the surface and the process begins again.
Research has found the AMOC has “two distinct modes of operation” – strong and weak. And if it were to flip from its current strong mode to weak, it could have huge ramifications for the climate.
Lead author Dr Niklas Boers said it could trigger “a cascade of further transitions” in other key components of the global climate system. These include the Antarctic ice sheets, tropical monsoon systems and the Amazon rainforest.
Climate modelling indicates the AMOC flipping from strong to weak mode can be triggered by adding large amounts of freshwater to the North Atlantic. As the Earth warms, huge volumes of fresh water are entering the ocean from melting ice caps, dramatically reducing salinity.
When the AMOC is in its strong mode, salinity levels increase. This happens as warm water flows north due to evaporation and makes the water denser. The higher density, in turn, causes the water to sink to the lowest levels of the ocean. But the Greenland ice sheet is having a particular impact on the AMOC. It’s decreasing salinity and disrupting this sinking process.
Moreover, rising temperatures cause thermal expansion of ocean waters. The paper said this, along with decreasing salinity, can also reduce AMOC strength.
‘Almost complete loss of stability’
The research found early warning signals in eight independently-measured data sets. It found that in the course of the last century, the AMOC may have evolved from “relatively stable conditions to a point close to a critical transition”.
The data indicates the decline in the AMOC’s strength during the last decade wasn’t just a normal fluctuation linked to climate variability.
It also said it did not appear to be a straightforward linear change in response to increasing temperatures, comparable to other indicators of global warming.
Instead, it said the findings “suggest that this decline may be associated with an almost complete loss of stability of the AMOC over the course of the last century”.
The AMOC could be close to a critical transition to its weak circulation mode.
The research said the AMOC could be entering a phase known as “critical slowing down”. The Earth’s natural systems enter this phase before their dynamic fundamentally flips to an alternative, stable state.
The paper said:
Indications of critical slowing down can provide key information for predicting future abrupt climate transitions
But it warned that although there’s evidence the AMOC was weakening, it was difficult to predict when it might reach its critical transition point.
‘Extremely high impact’
Grahame Madge, the Met Office’s climate spokesperson, spoke to the PA news agency about the report. He said Met Office scientists did not believe the AMOC was at risk of collapse before the end of the century. And he added:
It’s certainly the case that if it were (to collapse) it would be an extremely high impact event, but it is one that at the moment is of relatively low probability…
Other impacts of climate change are going to be a lot more apparent before we get the consequences of AMOC collapse.
The paper, Observation-based early-warning signals for a collapse of the AMOC, is published in the journal Nature Climate Change.
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