Planet Earth Online

CO2 emissions harm ocean's ability to absorb carbon

3 February 2009, by Tom Marshall

The paper, published in Nature Geoscience, also suggests that now is a particularly bad time to be disturbing the climate system with rocketing CO₂ emissions, as it is particularly sensitive to them at present - more so than it's been for much of the last 400 million years.

The research was led by scientists at Liverpool University, in collaboration with colleagues at Bristol University and the Massachusetts Institute of Technology in the US. The authors set out to investigate how the sensitivity of the climate system to increases in atmospheric CO₂ - that is, how much a given amount of CO₂ warms the globe - varies with changes in the way carbon is cycled between sea and atmosphere.

Their conclusion was deceptively simple - radiative forcing varies linearly with carbon dioxide emissions, at least over long timescales when the atmosphere has had time to reach an equilibrium with the ocean. Radiative forcing is the difference between the solar energy striking the planet and the heat energy it radiates back into space. If radiative forcing is a positive number, the planet tends to get warmer over time.

The researchers calculate that for every 1000 gigatonnes of carbon emitted, the Earth will be affected by an extra 1.5 watts per square metre of radiative forcing. And other feedback processes such as melting permafrost releasing its carbon contents into the atmosphere could increase this by a further 30 per cent.

It's estimated that burning all remaining fossil fuel reserves would release as much as 5000 gigatonnes of carbon into the atmosphere, if techniques to capture and store the carbon, such as pumping CO₂ into the undersea caverns left behind by oil and gas extraction, were not employed.

The paper's authors estimate this would subject the Earth to additional heating of 7.5 watts per square metre - as much as five times the additional heating of 1-2 watts per square metre that has already taken place. This heating would last for thousands of years.

'Other researchers using different methods to look at the problem have shown that emissions can raise atmospheric carbon dioxide for many thousands of years,' says Professor Ric Williams, an oceanographer at the University of Liverpool and one of the paper's lead authors along with Dr Phil Goodwin, who was a PhD student at Liverpool during the research and has now moved to the University of East Anglia. 'But our method provides a very clean analytic way of looking at the relationship between climate and carbon emissions,' Williams adds.

The linear relationship between carbon emissions and radiative forcing is complicated by other factors, though.

Much of the CO₂ added to the atmosphere is absorbed by the oceans. The total amount of carbon stored here is typically 60 times greater than the total found in the atmosphere and 20 times greater than that in land-based ecosystems and soil.

'We should be thankful that the oceans absorb a great deal of the CO₂ that we emit,' says Williams. 'But there's a sting in the tail - as this happens the oceans get more acidic, and this acidity changes the way they store carbon. This will inhibit uptake of CO₂ in future.'

He explains that carbon can be stored in the ocean in three forms - as aqueous CO₂, bicarbonate or as carbonate. Normally the first form is the rarest, but as atmospheric levels of CO₂ rise it becomes more common. Ultimately changes in this balance, which scientists refer to as 'chemical buffering' will reduce the oceans' ability to absorb more CO₂.

Effectively, the more CO₂ we emit, the more we weaken the ocean's capacity to absorb it. And now is not an opportune time to be doing this. Williams says that the climate's sensitivity to further emissions is now at its highest level for much of the last 400 million years.

'If we'd somehow released a lot of carbon into the atmosphere 200 million years ago, it wouldn't have had such an impact on the climate,' he explains. 'There was more carbon in the system at that time, and there was a kind of saturation effect meaning further emissions would have had a more limited effect.'

By contrast the present-day climate has less carbon in the system so our carbon emissions will lead to more warming. And this sensitivity of the climate to emissions is likely to remain even as we emit more and more CO₂, because of the diminishing ability of the oceans to absorb carbon.

Keywords: Atmosphere, Environmental change, Hazards, Natural resources, Oceans, Pollution,

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