Slowing sink?

Credit: © ISTOCKPHOTO/RUSTYCLOUD

So far, the oceans have protected us from the full force of anthropogenic climate change. The global ocean is the largest active carbon sink on Earth, and since the industrial revolution it has soaked up around one third of all anthropogenic carbon dioxide emissions. But how this fraction will change in the future is uncertain.

According to Samar Khatiwala and colleagues, the rate of increase in oceanic uptake of CO₂ may have started to decline some decades ago (Nature 462, 346–349; 2009). They reconstructed the history of anthropogenic CO₂ concentrations in the ocean between 1765 and 2008 by analysing the transport and storage of oceanic tracers, such as natural ¹⁴C and chlorofluorocarbons, and temperature and salinity. The researchers noted a sharp increase in the oceanic uptake rate since the 1950s, which coincided with an increase in the growth rate of atmospheric CO₂ levels. But from the 1990s onwards, uptake rates appear to have failed to keep pace with ever-rising atmospheric CO₂ concentrations.

A strengthening of the Antarctic westerlies — a product of the 40-year-old regional ozone hole — could be responsible for this slowdown. Specifically, an intense atmospheric circulation around the South Pole is linked to fast oceanic overturning and the movement of carbon-rich waters to the surface. A higher carbon content in surface waters may have reduced the ocean's ability to absorb CO₂.

The impact of changes in atmospheric circulation over a decadal timescale — a geological blink of an eye — on the vast and slow-moving oceans is hotly debated. However, given that the Southern Ocean acted as a conduit for over 40% of anthropogenic CO₂ entering the oceans in 2008, changes in Southern Ocean uptake rates could quite plausibly have noticeable global consequences.

Carbon dioxide emissions from the use of fossil fuels are likely to continue to increase, at least in the medium term. Unfortunately, a gradual saturation of the oceanic carbon sink may already have started to exacerbate the consequences for atmospheric CO₂ levels — and therefore for the Earth's climate.