The waters around the Antarctic may be one of the last places on Earth to feel the effects of man-made climate change. According to researchers at the University of Washington (UW) and the Massachusetts Institute of Technology (MIT), ancient seawater upwelling from the depths explain why the sea surface has remained roughly the same temperature while most of the planet has experienced temperature rises.
Using a combination of observations from floating ocean current trackers and cutting-edge computer simulations, the new Nature Geoscience study shows that this centuries-old seawater hasn’t been to the surface since the dawn of the Industrial Revolution. Although the cooler waters around the Antarctic were previously blamed on ocean currents drawing sea surface heat down to the depths, it appears that cold water yet to experience the newly-warmed atmosphere is currently rising up to the surface.
“With rising carbon dioxide you would expect more warming at both poles, but we only see it at one of the poles, so something else must be going on,” the study’s lead author Kyle Armour, a UW assistant professor of oceanography and of atmospheric sciences, said in a statement. “We show that it's for really simple reasons, and ocean currents are the hero here.”
Observed warming over the past 50 years, as measured in degrees Celsius per decade. It’s clear that the Southern Ocean has warmed by only a fraction, and it appears ocean currents are to blame for this unusual refrigeration mechanism. Kyle Armour/UW
Seawater from the deepest depths of the world’s oceans upwell at different times, and they do so when they become less dense than the water above them. This can happen for many reasons, including a reduction in salt concentration or an influx of heat at depth, both of which make them more buoyant. On occasion, there can be a mechanical driver of seawater upwelling, such as persistent winds.
This is what’s happening in the Southern Ocean, where extremely powerful westerly winds keep pushing warming surface water northwards; this gives the deeper, older water “space” to upwell into. The novel aspect of the waters here is that they have to upwell from depths of several thousand meters, far beyond the depths that most other oceanic currents reach. This means that it takes them an incredibly long time to reach the surface and interact with the atmosphere.
According to the models run by the team, the water only just beginning to reach the surface off the coast of Antarctica last experienced the Earth’s atmosphere centuries ago in the North Atlantic, before any serious man-made climate change had the chance to significantly heat it up. In fact, their simulations show that the oceanic currents that have experienced the most warming appear to be gathering at the North Pole, which also partly explains why Arctic sea ice is disintegrating so rapidly.
“When we hear the term 'global warming,' we think of warming everywhere at the same rate,” Armour added. “We are moving away from this idea… and more toward the idea of regional patterns of warming, which are strongly shaped by ocean currents.”
The fact that Antarctic sea ice has been growing just as the Arctic’s has been disintegrating has baffled scientists for some time; irritatingly, this discrepancy is often cited by climate change deniers as proof that climatologists don’t know what they’re talking about. It was only a matter of time before several explanations emerged, and this new study represents one of two corroborating theories helping to explain why the sea ice around Antarctica has been unexpectedly growing.