More than half of the CO2 emitted by human activities each year are taken up by natural carbon sinks, on land and in the ocean. However, recent studies suggest that anthropogenic emissions may be outpacing the ocean’s ability to take up CO2.
Le Quéré et al. (2009) constructed a global CO2 budget for 1959 – 2008 using a wide range of geophysical and economic data, and modeled carbon uptake by CO2 sinks. Between 1959 and 2008, a little less than half of each year’s CO2 emissions remained in the atmosphere on average; the rest was absorbed by carbon sinks. Over this period, the percentage of CO2 emissions that remained in the atmosphere each year appears to have increased, from about 40% to 45%, and models suggest that this trend was caused by a decrease in the uptake of CO2 by the carbon sinks in response to climate change and variability.
The oceans are the largest sink for atmospheric CO2. Many climate models predict a slowdown of CO2 uptake by the oceans, but a study by Khatiwala et al. (2009) that reconstructs the accumulation of industrial carbon in the oceans from 1765 to 2008 is the first time scientists have actually measured it. While the oceans absorbed a record 2.3 billion tons of CO2 produced by fossil fuels last year, the proportion of CO2 absorbed may have decreased by 10% since 2000.
This slow down is likely due to natural chemical and physical limits on the ocean’s ability to absorb carbon. About 40% of the carbon enters the oceans through the frigid waters of the Southern Ocean, around Antarctica, because carbon dissolves more readily in cold dense seawater than in warmer waters. “Warm cola holds less fizz,” Jeffrey Park, an author of related study said. “The same thing happens in the ocean.” Warmer temperatures mean less dissolved CO2. Park (2009) found that the time that it takes for the atmospheric CO2 to adjust to changes in sea surface temperature (SST) has increased from 5 months to 15 months over the last twenty years. Weaker CO2 absorption could be caused by changes in ocean circulation or increases in SST.
In addition to rising SSTs, ocean acidification is reducing the ocean’s absorption capacity. “The more CO2 you put in, the more acidic the ocean becomes, reducing its ability to hold CO2,” said Khatiwala. Because of these limits, “the ocean is expected to become a less efficient sink of manmade carbon over time.” An interesting finding of Khatiwala’s study is that land areas may now be absorbing more CO2 than it is giving off. This may be because of reductions in deforestation as well as enhanced growth of plants due to increased CO2 levels.
Another cause of the reduction in the ocean’s CO2 absorption capacity may be a strengthening of the Antarctic westerlies — a product of the 40-year-old regional ozone hole. 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. The high carbon content of these surface waters may have reduced their ability to absorb CO2.
Baudains, S. (2009) Ocean science: Slowing sink? Nature Geoscience 2, 826 | doi:10.1038/ngeo716
Khatiwala, S., F. Primeau, & T. Hall (2009), Reconstruction of the history of anthropogenic CO2 concentrations in the ocean, Nature 462, 346-349 | doi:10.1038/nature08526
Le Quéré, C., M.R. Raupach, J.G. Canadell, G. Marland et al. (2009), Trends in the sources and sinks of carbon dioxide. Nature Geoscience 2, 831 – 836 | doi:10.1038/ngeo689
Park, J. (2009), A re-evaluation of the coherence between global-average atmospheric CO2 and temperatures at interannual time scales, Geophys. Res. Lett., 36, L22704 | doi:10.1029/2009GL040975