A warming world could slow the circulation of the Atlantic Ocean, potentially triggering African droughts and more rapid sea level rise around Europe. If it happens, it won’t be the first time the Atlantic has been disrupted during a warm period.
Water in the Atlantic is constantly on the move. In the icy north, cool and dense surface water sinks and flows south, forming the North Atlantic Deep Water. The NADW then encourages warm surface water in the south to flow north, creating the Gulf Stream.
In theory, this “conveyor belt” could weaken as a result of climate change. A hugely exaggerated version of this proposal was the premise for the film The Day After Tomorrow. But until now the evidence from warmer periods in Earth’s past suggested that temperature rises would not affect the circulation. A new study indicates otherwise.
Eirik Vinje Galaasen at the University of Bergen, Norway, and his colleagues looked at deep-sea sediments from a site off the southern tip of Greenland. Sediment builds up so rapidly there that 3.5 centimetres are deposited each century, meaning that important but short-lived climate shifts show up clearly.
The team focused on sediments from the last interglacial, a warm period between 130,000 and 115,000 years ago, before the last ice age. The ratio of carbon isotopes in fossilised microbes from this time showed several sudden shifts, each indicating an abrupt change in environmental conditions and probably in the NADW.
Some members of the team had seen a similar isotope ratio shift before, in marine sediments from 8200 years ago. In 2007 they showed that the shift occurred when a vast North American lake burst, sending 100,000 cubic kilometres of fresh water into the North Atlantic, and briefly reducing formation of the NADW. Galaasen says NADW reductions were common in the last interglacial, when the North Atlantic was warmer.
According to Galaasen, the NADW reductions have been missed because sediment builds up slowly at most deep-sea sites, making such brief events hard to spot. At most sites the shifts would be recorded in just a few millimetres of sediment, rather than a few centimetres as in the Greenland site.
“The deep Atlantic is not as stable as previously thought,” says Galaasen. “Perhaps especially so when the North Atlantic is warmer and fresher, which may be the case again in the near future.”
If the NADW conveyor did slow, we would certainly feel it. For starters, sea levels would rise faster around Europe, because the change in ocean currents would see more water ending up there.
What’s more, north Africa might suffer severe droughts, says Galaasen, because the altered ocean currents could affect atmospheric circulation in the subtropics. Climate records from the past 57,000 years support such a link (Paleoceanography, doi.org/d92bk9).
Less NADW could also mean that the planet would warm more than expected. “NADW formation is an important process by which anthropogenic carbon dioxide enters the deep ocean, thus helping to slow down the rise in atmospheric CO2 levels,” says David Thornalley at University College London in the UK. An NADW reduction could reduce the size of this carbon sink, leading to even more warming.
However, it is far too early to say whether reductions in NADW are likely in the short term, says Carl Wunsch at the Massachusetts Institute of Technology. We still know little about how ocean circulation varies over millennia. “This is the sort of paper that really needs to be evaluated over the coming years.”
The relevance to today’s climate will need further investigation, agrees Thomas Stocker at the University of Bern in Switzerland. The study suggests NADW reductions are particularly likely after an influx of fresh water from melting ice, and it is by no means clear that will happen. “Today much less terrestrial ice surrounds the North Atlantic than at the early stages of the past interglacial,” he says. The NADW weakening would only happen if a lot of fresh water came off the Greenland ice sheet. Stocker says that is a wild card.