Gulf Stream at risk of collapse as Earth warms, study warns
According to new research, the ocean current responsible for the temperate climate of Western Europe could be at risk of collapsing due to global warming.
Scientists at the German Institute for Climate Impact Research in Potsdam have found that the Atlantic Meridional Reversal Circulation (AMOC) may have reached a point of “almost complete loss of stability” over the past century.
AMOC is a system of ocean currents that acts as a conveyor belt carrying warm surface water from the tropics to the North Atlantic where it cools and sinks to the shallower depths of the ocean.
This cooler water gradually moves southward several kilometers deep, before warmer ocean temperatures bring it to the surface and the process begins again.
The Gulf Stream, the current of warm water flowing from the tip of Florida across the Atlantic to Europe, is part of AMOC and makes Western Europe much warmer than it would be otherwise.
Research has shown that AMOC has “two distinct modes of operation” – strong and weak – and if it were to switch from its current strong mode to weak, it could have huge ramifications for the climate.
Lead author Dr Niklas Boers said this could trigger “a cascade of additional transitions” in other key components of the global climate system, such as the Antarctic ice caps, tropical monsoon systems and the Amazon forest.
The research found early warning signals in eight independently measured datasets, derived from observational data of sea surface temperature and salinity from across the Atlantic Ocean basin.
He found that over the past century, AMOC may have evolved from “relatively stable conditions to a point near a critical transition”.
Climate modeling indicates that the switching of AMOC from its strong mode to its weak mode can be triggered by the addition of large amounts of fresh water to the North Atlantic, significantly reducing salinity there.
When the AMOC is in its strong mode, salinity levels increase as hot water flows north due to evaporation and makes the water more dense, causing it to flow to lower levels. of the ocean.
But as the Earth warms, huge volumes of fresh water are entering the ocean due to melting ice caps, with the Greenland ice cap having a particular impact on AMOC, decreasing salinity and disturbing this sinking process.
The report says the data indicates that the decline in AMOC’s strength over the past decade was not simply a normal fluctuation linked to climate variability.
He also said it did not appear to be a simple linear change in response to rising temperatures, comparable to other indicators of global warming.
Instead, he said the results “suggest that this decline may be associated with an almost complete loss of AMOC stability over the past century.”
He added: “AMOC could be near a critical transition to its weak circulation mode.”
The paper states that in addition to decreasing salinity, thermal expansion of ocean waters due to rising temperatures can also decrease the strength of AMOC.
Research has indicated that AMOC may enter a phase known as the “critical slowdown” in which Earth’s natural systems enter before their dynamics fundamentally switch to an alternating, stable state.
“Indications of a critical slowdown can provide key information for predicting future abrupt climate transitions,” the document said.
But he warned that while there is evidence that AMOC is weakening, it was difficult to predict when it might reach its critical transition point.
Grahame Madge, the Met Office’s climate spokesperson, told the PA news agency that its scientists did not believe AMOC was in danger of collapsing before the turn of the century.
“It is certainly the case that if it were to collapse it would be an extremely high impact event, but it is an event which at the moment is of relatively low probability,” he said.
“Other impacts of climate change are going to be much more apparent before we have the consequences of the AMOC collapse.”
Dr David Smeed, from the UK’s National Oceanography Center, said if man-made global warming were to cause AMOC to tip over, it could be a very long time before it returned to its strong mode.
He said: “If he returned to his (slow) state, it could take a very long time to go back – it could take ages.
“The idea of the two states is that when it’s in one state it’s hard to switch to the other, but when that changes it’s very hard to go back.”
Dr Stuart Umbo, from the Department of Geography and Environmental Sciences at Northumbria University, said the changes to AMOC have had a huge impact over the past millennia.
“There is good evidence to suggest that AMOC has changed rapidly throughout Earth’s recent history,” he said.
“Slowing or stopping AMOC is often associated with a cooling of several degrees Celsius in the northern hemisphere.”
Dr Umbo continued: “It may not seem like much, but the peak of the last ice age, around 20,000 years ago, was thought to be only six degrees cooler than it is today.
“A similar cooling would have an unimaginable impact on our modern world. “
The article, Observation-based early warning signals for an AMOC collapse, is published in the journal Nature Climate Change.