Southern seas slow their uptake of CO2

The rate at which oceans in the Southern Hemisphere soak up carbon dioxide from the atmosphere has slowed in recent decades, a phenomenon that could cause atmospheric concentrations of the planet-warming greenhouse gas to increase even faster than had been expected.

Today, the atmospheric concentration of carbon dioxide is rising by about 3 parts per million each year. As the concentration of carbon dioxide in the air goes up, the amount that dissolves into the ocean should also rise, says Corinne Le Quéré, an oceanographer at the University of East Anglia in Norwich, England.

Computer models suggest that between 1981 and 2004 the oceans south of 45°S should have absorbed, on average, an additional 18.7 million metric tons of carbon dioxide each year. Instead, Le Quéré notes, comparison of known carbon dioxide–emission rates with measurements of the gas’ atmospheric concentration suggest that carbon dioxide absorption in the southern oceans declined by 11.3 million metric tons with each passing year.

In recent decades, the expanding ozone hole over Antarctica and increasing global temperatures have conspired to boost wind speeds over the southern oceans, Le Quéré and her colleagues speculate in the June 22 Science. Stronger winds increase the upwelling of nutrient-rich waters, which are less effective than cleaner water at absorbing carbon dioxide from the atmosphere, the scientists propose.

For at least the next 25 years, carbon dioxide absorption in the southern oceans will continue to decline, the team’s analysis suggests. Greenhouse-gas buildup in the atmosphere may therefore affect global temperatures more quickly than scientists had previously estimated, says Le Quéré.

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