Oceans absorbing carbon dioxide more slowly, scientist finds

Jeffrey Park, professor of geology and geophysics and director of the Yale Institute for Biospheric Studies, used data collected from atmospheric observing stations in Hawaii, Alaska and Antarctica to study the relationship between fluctuations in global temperatures and the global abundance of atmospheric CO₂ on interannual (one to 10 years) time scales. A similar study from 20 years ago found a five-month lag between interannual temperature changes and the resulting changes in CO₂ levels. Park has now found that this lag has increased from five to at least 15 months.

"No one had updated the analysis from 20 years ago," Park said. "I expected to find some change in the lag time, but the shift was surprisingly large. This is a big change."

With a longer lag time, atmospheric CO₂ can no longer adjust fully to cyclical temperature fluctuations before the next cycle begins, suggesting that the oceans have lost some of their ability to absorb CO₂ from the atmosphere. Weaker CO₂ absorption could be caused by a change in ocean circulation or just an overall increase in the surface temperature. "Think of the oceans like soda," Park said. "Warm cola holds less fizz," Park said. "The same thing happens as the oceans warm up."

Increases in CO₂ levels have tended to precede increases in temperature over the past century, with the human influence on climate accumulating over many decades of burning fossil fuels and clearing forests. However, this relationship is reversed on interannual time scales, with multiyear temperature cycles leading multiyear cycles in CO₂ levels.

Park found particularly strong correlations between sea-surface temperatures and CO₂ levels in tropical ocean areas. Conversely, in places with a lot of trees and other biomass to soak up much of the atmospheric CO₂, there was little or no correlation between temperature and CO₂ on interannual time scales. In those places, such as the vast forests of North America and Eurasia, a large annual CO₂ cycle synchronizes with the seasonal growth and decay of plants.

"Researchers have used climate models that suggest the oceans have been absorbing less CO₂, but this is the first study to quantify the change directly using observations," Park said. "It strengthens the projection that the oceans will not absorb as much of our future CO₂ emissions, and that the pace of future climate change will quicken."