Ancient ocean layers reflect changes in the strength of the Atlantic Circumpolar Current (ACC) during the last 5.3 million years. Comparisons with climatic circumstances reveal a convoluted picture, although, in recent years, the current has been strongest when the Earth is at its hottest. As a result, it’s not surprising that it’s happening again as humans exert their influence over global temperatures.
The ACC is by far the world’s largest water conveyor, moving 100 times more water than all of the world’s rivers combined. The latest discovery has a profound irony, as the ACC is assumed to be the primary reason the Earth is much cooler now than it was 35 million years ago.
Tasmania and Patagonia were closer to Antarctica at the time, and their presence slowed the development of a circulation that now rounds the southern continent indefinitely. The ACC’s presence prevents warm water from the tropics from reaching Antarctica’s coastlines and melting the ice during the summer, thereby warming the globe by lowering light reflection into space.
The ACC does not need to be as powerful as it is to play that role, thus after shaping the Earth’s climate, it is now being molded by it in turn. It has moved substantially faster in recent years, which has been linked to stronger winds in the roaring sixty, the ACC’s main latitudes.
The direct reason for this acceleration is believed to be a 40% increase in wind intensity across the Southern Ocean. What climatologists want to know is whether the greater winds are the result of human-caused global warming or something else. Recent data suggests that even Mars’ gravitational pull can influence the speed of the ACC, but this would only have an impact on much longer periods.
To answer the question, a team of scientists from a dozen countries used sediment cores from the deep ocean to explore the history of the ACC. These cores are difficult to gather; in addition to the issue of drilling into the seabed when the ocean above it is kilometers deep, the driving winds create some of the roughest waves on Earth. Nonetheless, climate scientists rose to the challenge and successfully removed five cores from the South Pacific Ocean, beneath the ACC, using the research vessel JOIDES Resolution. Two of them were taken near Point Nemo, which is the furthest from land on Earth.
When the ACC moves slowly, little particles dominate the sediment, but as it accelerates, it grows larger, providing a record of its speed across millions of years. Millions of years ago, the ACC strengthened as the planet cooled, but over the last 800,000 years, it has been strongest during warmer periods. For example, Ice Age speeds were half that of the Holocene average.
If these cores are to be believed, current speeds are far from the maximum that the ACC can achieve. Some previous interglacial eras were accompanied by speeds 80 percent faster than today, indicating that there is plenty of possibility for expansion.
Despite the ACC’s function in cooling the globe, there is such a thing as having too much of something beneficial. It is believed that the increased energy in a faster current can be transmitted to ice on the continent’s boundaries, hastening its melting.
“If you leave an ice cube out in the air, it takes quite a while to melt,” research co-author Dr Gisela Winckler said in a statement. “If you put it in contact with warm water, it goes rapidly.” As a result, the link between temperature and a stronger ACC may be a case of each magnifying the other, at least to some extent, rather than a simple instance of more heat driving a quicker current.
The slowdown of the Atlantic Meridional Overturning Circulation (AMOC) receives the most attention and is more likely to pose a threat to humans in the short term, but the ACC’s acceleration may be equally important in the long run.
