New German research has found “an almost complete loss of stability over the last century” in the series of currents that researchers call the Atlantic meridional overturning circulation (AMOC).
The currents are already at their slowest point in at least 1,600 years, but the new analysis shows they may be nearing a shutdown.
The study by Dr Niklas Boers, of the Potsdam Institute for Climate Impact Research, published in the journal Nature Climate Change, indicates that AMOC may have been losing stability over the course of the last century and that the process has accelerated.
AMOC transports warm water from the tropics northward at the ocean surface and cold water southward driving the Gulf Stream.
Climate scientist Dr Brian Kelleher, of DCU’s School of Chemical Sciences, said the Gulf Stream is the principal reason why Ireland has such mild winters given its relative high latitude.
Without the Gulf Stream, he said, Ireland would have winters similar to Toronto where, despite being at a lower latitute, temperatures are below zero for much of the winter.
“The breakdown is going to cause huge changes to water all over the world. It wouldn’t be a situation on its own. It would affect ocean currents all over the world, so we can’t say with certainty what will happen,” he said.
Dr Kelleher stressed that it was not clear if the changes in AMOC are a result of human activity or natural changes in the atmosphere.
“The more we know about the past, the more we know about the present and the future,” he said.
“The only safe thing to do is to stop pumping CO2 in the atmosphere. It’s another bit of evidence that it looks like we are causing things to change rather than it being a natural phenomenon.”
Prof Conor Murphy of Maynooth University said the paper seems to confirm evidence of scientists based at its Irish Climate Analysis and Research Unit (ICARUS), which have indicated a slowdown in ocean currents.
“There is a large amount of uncertainty in trying to preempt these tipping points in the climate system like that, but it is not what you want to be reading,” he said.
Intense winter storms
“What happens in the Atlantic Ocean is important for our climate. A slowdown in AMOC would see more intense winter storms, colder harsher conditions in winter and potentially drought conditions in summer. We have to go back to paleoclimate studies to see something similar.”
Maynooth University oceanographer Dr Gerald McCarthy said there have been times in the past when AMOC was off, as in the last glacial period, compared to now when it is on bringing mild temperatures to northern latitudes.
He said there was some good data in the study but its conclusions were “too sweeping”.
Dr Boers did not forecast when the AMOC system might collapse. It would be within decades or centuries, he stated, but it was best to prepare now for that eventuality by cutting greenhouse gas emissions.