Earlier this year, we discovered that ocean circulation is at a 1,500-year low. Now, research published in the journal Nature suggests that it is not caused by global warming, as previously thought, but is instead part of a regular and decades-long cycle. That is not to say it won't have major implications on our climate in the coming years – or that this natural phenomenon won't be exacerbated by climate change.
“We have about one cycle of observations at depth, so we do not know if it’s periodic, but based on the surface phenomena we think it’s very likely that it’s periodic,” Ka-Kit Tung, a professor of applied mathematics at the University of Washington and co-author of the paper, said in a statement.
It all comes down to something called the Atlantic Meridional Overturning Circulation (AMOC), the large system of currents that sends the oceans' surface water northwards to the Atlantic Ocean. Here, the heavier (and saltier) water sinks to the bottom and retreats south, where it rises to the surface and flows north once again.
In April, a paper confirmed the AMOC had slowed by roughly 15 percent in the last few decades, putting it at its lowest rate in 1,500 years. At the time, the researchers postulated that this was the culmination of 150 years of greenhouse gas emissions and rising global temperatures – but a new study suggests otherwise.
Tung and co-author Xianyao Chen have revised this theory with their paper, published last week. After analyzing data from Argo floats, tidal records, ship-based temperature measurements, satellite images of sea-surface height, and high-tech tracking of the AMOC, they say the evidence points to it being part of a regular and periodic cycle of accelerating and decelerating currents, which occurs on a 60 to 70-year basis. This means we do not have to panic about an apocalyptic Day After Tomorrow-type scenario, where we are suddenly plunged into a new ice age.
When the AMOC is in a "fast phase", you have more warm and salty water from the tropics pouring into the North Atlantic. Gradually, this causes more glaciers to thaw so that the freshwater from the ice lightens the surface water, reducing the speed at which it sinks, slowing the current and introducing a "slow phase". At this point, the North Atlantic gets chillier and ice melt starts to slow. Over time, the amount of freshwater on the surface will drop and the saltier (and, therefore, heavier) water can sink, kicking off the whole process once again.
Tung and Chen say the AMOC is not collapsing – it's simply transitioning from one phase to another as part of a natural cycle. But (and it's a big but) that is not to say that the effects won't be exacerbated by climate change.
Between 1975 and 1998, the AMOC was experiencing a slow phase, which meant that there was a noticeable warming of surface temperatures. But from 2000 onwards, this warming plateaued. Tung and Chen believe that this is because the AMOC was in a fast phase and some of this excess heat sunk with the heavy water to the bottom of the ocean, where it is still stored. Now, the AMOC is speeding up – and that means we can expect surface warming to accelerate too.
“The good news is the indicators show that this slowdown of the Atlantic overturning circulation is ending, and so we shouldn’t be alarmed that this current will collapse any time soon,” Tung added.
“The bad news is that surface temperatures are likely to start rising more quickly in the coming decades.”