Something strange is swirling in the waters around Antarctica. From the 1970s until 10 years ago, floating sea ice radiating from the continent continued to expand, even though climate change was already in full swing. Then in 2016, as rising global temperatures appeared to catch up with Antarctica, it suddenly shrunk dramatically, but it has yet to recover. More than just a local problem, the loss of sea ice has a major impact on Antarctica’s vast ice sheet, and its disappearance would cause sea levels to rise by 190 feet.
Now, scientists say they’ve identified what’s behind this rise and sudden fall, thanks to the help of deep-sea robots. It all depends on salt, wind, and agitation. “One of the key takeaways from this study is that the ocean plays a huge role in regulating how sea ice changes from year to year and from decade to decade,” said Earl Wilson, a polar oceanographer at Stanford University and lead author of a new paper describing the study.
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Now, forget about robots and think about swimming in a lake. When you dive, you will suddenly be hit with cold water. That’s because while the sun warms the surface, the depths remain cold. This also happens in oceans around the world, but obviously the colder water goes deeper.
The opposite happens in the waters around Antarctica. The ocean floor is so cold that air cools the surface, causing warm water to swirl beneath it. (The Argo robot was able to detect this in detail during its ascent and descent.) As warmer liquid moves away from the surface, more sea ice can form.
As sea ice expanded in the decades leading up to 2016, increased precipitation caused surface water to become fresher, as opposed to the saltier sea at the bottom, resulting in stratification. (The more salt a liquid has, the more dense it is.) This allows heat to be trapped deep down and accumulate.
Then, as the winds strengthened and changed, the atmosphere played yet another trick. This moved surface water away from the South Pole and stirred up its deep warmth. “What we witnessed was basically a very intense release of pent-up heat from the ocean floor that was related to sea ice loss,” Wilson said.
This explosion was likely caused, at least in part, by climate change. As the Earth warms, atmospheric temperature gradients develop, winds grow stronger and their patterns change. But scientists are still figuring out how much of this change is due to “natural variation,” or what would happen if humans hadn’t emitted so much carbon since the Industrial Revolution.
In any case, this system changed around 2016. In addition to conjuring up warm water, all that wind could have broken the ice by pushing against the blocks or creating waves. “Recent studies have shown that both atmospheric and ocean warming are likely to have contributed to the abrupt changes in Antarctic sea ice extent since 2016, and this paper helps further develop the point that deep ocean warming is playing an important role,” said Zachary Love, a climate scientist at the research group Climate Central who studies Antarctic ice but was not involved in the paper.
As sea ice declines, more ice is at risk elsewhere. The land-based Antarctic ice sheet is supported by floating ice shelves along the coast. These vital aid supplies are already in serious trouble as their bellies are eroded and weakened by warming oceans and severe underwater storms. If we also lose the surrounding sea ice, we lose a significant amount of buffering material as the floating chunks absorb wave energy. Additionally, a healthy amount of sea ice is so bright that it reflects the sun’s heat back into space, lowering local temperatures. Because ice shelves support ice sheets, their loss means an accelerated decline in the continent’s extraordinary amount of frozen water.
The Argo float provided valuable data, but scientists are eager to obtain even more measurements. “Overall, we need more international support to continue building an observation network across the Antarctic Polar region, both for ocean and atmospheric monitoring,” Love said. “This is of great importance given that rapid changes are beginning to be observed in this region of the world amid a warming climate, which could have significant impacts on global sea level rise.”
The big question now is whether we are witnessing a permanent state of low sea ice, or whether atmospheric and ocean conditions may recede enough to encourage growth for many years. The promise of this new study is that it will help researchers refine models to predict how much and how fast the ocean around Antarctica will change. Sea ice will probably decline rapidly over several years and then increase over several years. “But the long-term trend over several decades will be negative,” Wilson said. “That’s my guess, but I don’t know for sure.”
This story was originally published by Grist. Sign up for Grist’s weekly newsletter here.
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