Published online 22 October 2007 | Nature | doi:10.1038/news.2007.186


Greenhouse-gas levels accelerating

Decline in oceans' ability to store carbon partly to blame, study suggests.

How much carbon is being absorbed in the oceans? Perhaps not as much as hoped.NASA

Levels of greenhouse gases are rising faster this decade than they did during the 1990s, according to a new analysis. The research blames the acceleration on a combination of factors, including economic growth, intensive fossil-fuel use and, perhaps most worringly, an apparent decline in the oceans' ability to suck up carbon dioxide from the atmosphere.

Before 2000, atmospheric carbon dioxide levels had been rising by about 1.5 parts per million (p.p.m.) per year. Since the turn of the century, that figure has jumped to 1.9 p.p.m. per year, says Josep Canadell of the Global Carbon Project in Canberra, Australia, who led the new survey.

According to their analysis, almost two-thirds of the change is due to increasing world economic activity. Roughly half of the rest is down to increased reliance on high-emission fossil fuels throughout much of the world's economy, with China a significant contributor, Canadell says.

The rest of the observed difference is explained by the worsening performance of the oceans in absorbing carbon dioxide, Canadell's team reports. "Fifty years ago, if we emitted a tonne of CO2, sinks [such as oceans] would remove 600 kilograms of it," he says. "Now it's 550 kilos and this amount is declining," he says.

It is unclear why the oceans' capacity is going down, but it is thought that the surface layers may be becoming saturated with greenhouse gas — partly because of increasing levels in the atmosphere and partly because of reduced ocean mixing that would bring unsaturated waters to the surface.

Troubled waters

Canadell and his colleagues studied available measurements of world carbon dioxide levels for 1959–2006. They looked in detail at trends in emission sinks and sources from 1959 to 2000, and then projected these forward to 2006. By comparing those projections with what actually happened, they identified the key changes that have led to the increase observed over the past few years. The results appear in Proceedings of the National Academy of Sciences1.

Canadell says that the models used by the Intergovernmental Panel on Climate Change (IPCC) did not predict such a rapid decline in ocean-sink performance. Only now are observational data highlighting the problem.

In another new study, Ute Schuster and Andrew Watson of the University of East Anglia in Norwich, UK, surveyed carbon dioxide uptake in the North Atlantic using data collected by instruments on board commercial ships. They report in the Journal of Geophysical Research2 that in some areas of the North Atlantic the ability of oceans to take up carbon dioxide has declined by more than half since the mid-1990s.

The discovery comes three months after researchers reported a similar process occurring in the Southern Ocean (see Polar ocean is sucking up less carbon dioxide), raising the prospect that seas the world over are losing their ability to store the carbon dioxide emitted by human development.

The whole picture

The overall upswing in the rate at which carbon dioxide is accumulating in the atmosphere means that greenhouse-gas levels are now tracking in line with the most pessimistic emissions scenario mapped out by the IPCC.

Canadell calls the observed acceleration "almost rampant". "The longer we wait to implement emissions cuts the harder it will be to curb emissions, because the sinks will be helping us less."

Controversy over the world's approach to controlling climate change has been stoked by recent comments by Australian paleontologist and climate activist Tim Flannery in interviews with the press. He has pointed out that overall levels of the six greenhouse gases covered by the Kyoto Protocol on climate change now represent the equivalent global-warming power of 455 p.p.m. carbon dioxide. This, he says, means that world leaders have already missed one of the crucial thresholds for avoiding "dangerous climate change".

Climatologist Gavin Schmidt of the NASA/Goddard Institute of Space Studies in New York agrees that overall levels of the 'Kyoto gases' — including carbon dioxide and methane — has almost certainly passed 450 p.p.m. CO2 equivalent. "But this is not the number that controls the warming that Earth is going to feel," he explains.

Schmidt notes that Flannery has neglected to mention that other counterbalancing factors, such as the sunlight-reflecting effects of clouds and sulphurous aerosol particles, bring the overall level down to about 370 p.p.m. carbon dioxide equivalent. The theory is "complicated" and "not helped" by people looking at only part of the picture, he says. 

  • References

    1. Canadell, J. G. et al. Proc. Natl Acad. Sci. doi:10.1073/pnas.0702737104 (2007).
    2. Schuster, U. & Watson, A. J. J. Geophys. Res. doi:10.1029/2006JC003941 (2007).
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