The slowdown in deep ocean currents, caused by melting Antarctic ice, is coming sooner than expected: according to a new study, it is happening decades "ahead of schedule," threatening marine life and risking accelerating global warming.
Scientists have long warned that an acceleration of Antarctic ice melt and rising temperatures, driven by the emission of man-made greenhouse gases, is expected to have a significant effect on the global web of ocean currents that carry nutrients, oxygen and carbon.
An earlier study, using computer models, suggested that water circulation in the deepest parts of the oceans would slow by 40% by 2050 if emissions remained high.
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But the study published Thursday in Nature Climate Change, based largely on observational data collected by hundreds of scientists over decades, shows that this process has actually already slowed by 30 percent between the 1990s and 2010.
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Our data show that the impacts of climate change are ahead of schedule," said lead author Kathryn Gunn, from Australia's science agency CSIRO and Britain's University of Southampton. "In a way, the fact that this is happening is not surprising. But the timing is" more so, stressed the scientist.
The oceans, important climate regulators
The implications could be significant, with Antarctica's deep ocean acting as a key "pump" for the global network of ocean currents. "When ocean circulation slows, there is more carbon dioxide and heat left in the atmosphere, which accelerates global warming," Gunn explained.
The oceans are a crucial climate regulator, absorbing large amounts of the additional carbon humans have released into the atmosphere since the mid-1800s, as well as more than 90 percent of the increase in Earth's heat.
Sea surface temperatures have risen dramatically – hitting new records earlier this year – while warming is also melting ice sheets in polar regions, dumping huge amounts of fresh water into the ocean. This disrupts a vital function essential to marine life.
The Nature Climate Change study reveals that oxygen reaching the deep ocean has decreased. These oxygen losses can disrupt biodiversity, forcing "deep-sea animals to seek refuge in other regions or adapt their behavior."
But beyond disrupting wildlife, changes to these key ocean pumps should also reduce the amount of carbon the ocean can absorb and bring to the surface carbon that has been stored deep in the ocean for hundreds of thousands of years.