A German scientist has echoed the warnings of the film The Day After Tomorrow, finding that a major oceanic circulation system is becoming more unstable – with concerning implications for the climate.
A study published in Nature Climate Change observes that the Atlantic Meridional Overturning Circulation (AMOC) – a massive ocean current system that circulates through the Atlantic – may have been losing stability over the past century, due to the influx of melted freshwater into the ocean.
This is concerning because the AMOC is responsible for the Gulf Stream, a swift current that brings warm water masses from tropical regions to the northern hemisphere. Because it redistributes heat, this circulation system is not only responsible for creating mild temperatures across Europe but also influencing weather systems across the world.
“The Atlantic Meridional Overturning really is one of our planet’s key circulation systems,” says Niklas Boers, the study’s author from the Potsdam Institute for Climate Impact Research, Free University Berlin and Exeter University.
If it collapses, it could have impacts such as significantly cooling Europe and affecting tropical monsoon systems.
“We already know from some computer simulations and from data from Earth’s past, so-called paleoclimate proxy records, that the AMOC can exhibit – in addition to the currently attained strong mode – an alternative, substantially weaker mode of operation,” Boers says.
Other previous research has shown the AMOC is currently weaker than it has been in 1,000 years, but it’s unknown whether this means the system is unstable – and therefore close to a potentially irreversible tipping point to switch to a weaker regime.
Now, by studying sea-surface temperature and salinity patterns in the Atlantic, Boers found that the recent weakening of the AMOC is linked to a loss of stability.
“Most evidence suggests that the recent AMOC weakening is caused directly by the warming of the northern Atlantic Ocean,” he says.
But direct warming is unlikely to lead to this sudden transition; instead, it will more likely be driven by the inflow of a substantial amount of meltwater from ice sheets. Not only does meltwater cause sea level rise, it also influences ocean currents – freshwater is lighter than seawater and so influences the way the water sinks and rises from great depths.
“We’ve known for several decades that climate change and the melting of the Greenland ice sheets could push the system into a new regime,” explains Matthew England, a physical oceanographer and climate scientist from the University of New South Wales.
England, who was not involved in this study, confirms that this is important work.
“It identifies early warning signs of a collapse of the AMOC, and unfortunately the evidence is that we are close to a point of critical transition in the system. This shows how much we are perturbing Earth’s climate system with ongoing emissions of greenhouse gases.”
Boers says that his results are surprising: “I wouldn’t have expected that the excessive amounts of freshwater added in the course of the last century would already produce such a response in the overturning circulation.
“We urgently need to reconcile our models with the presented observational evidence to assess how far from or how close to its critical threshold the AMOC really is.”
England notes that Southern Hemisphere may also be affected: “There are similar overturning circulations around the Antarctic continent, and once again, with land-ice melt those overturning circulations could shut down under global warming. In fact, there is already evidence that this may be starting to happen.”
This is, he warns, yet another “wake-up call that we need to address ongoing greenhouse gas emissions”.