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The world's peat bogs are haemorrhaging carbon dioxide into the atmosphere, accelerating global warming, warns a UK researcher.
And worse still, the process appears to be feeding off itself, as rising atmospheric levels of carbon dioxide are triggering further releases from the bogs.
Billions of tonnes of carbon could pour into the air from peat bogs in the coming decades, says Chris Freeman of the University of Wales at Bangor, UK. "The world's peatland stores of carbon are emptying at an alarming rate," he says. "It's a vicious circle. The problem gets worse and worse, faster and faster."
Peat bogs are a vast natural reservoir of organic carbon. By one estimate, the bogs of Europe, Siberia and North America hold the equivalent of 70 years of global industrial emissions. But concern is growing that such bogs are releasing ever more of their carbon into rivers in the form of dissolved organic carbon (DOC).
"There seems to be an increase of DOC in rivers of about 6 per cent a year at present," says Fred Worrall of the University of Durham in the UK, who collates global data on DOC levels in rivers. Worrall suspects the rise in DOC began about 40 years ago.
Summer droughts
Bacteria in the rivers rapidly convert DOC into carbon dioxide that bubbles into the atmosphere. But speculation has been rife about why the peat bogs are giving up their carbon in the first place. In 2001, Freeman proposed that global warming was the cause (New Scientist print edition, 25 August 2001). But that hypothesis failed to stand up in field trials. A second suggestion, that increased river flows were flushing more carbon out of the bogs, also failed.
So Freeman tested a third idea - that summer droughts cause more vegetable matter in bogs to decompose, freeing up more carbon that is released into the rivers. But that too failed when Freeman simulated drought conditions in a bog in central Wales, and found that this reduced the DOC in rivers, rather than increasing it.
The trials indicate that there may be another culprit altogether: the direct effects of carbon dioxide in the atmosphere. Freeman grew plants on soil from peat bogs in igloo-like glass structures, some containing normal air and others with a CO2-rich atmosphere.
He found that plants in the CO2-rich atmosphere began to assimilate much larger amounts of CO2, which in turn was released into the soil moisture. There it can feed bacteria in the water that break down the peaty soil itself, releasing stored carbon from the bog into the rivers.
Rate of acceleration
After three years, the proportion of DOC in the CO2-rich soil was 10 times that within the normal soil. And there was no sign of the increase tailing off. "This shows that even without global warming, rising CO2 can damage our environment," says Freeman. "The peat bogs are going into solution."
Recent data from the Centre for Ecology and Hydrology in Lancaster, UK shows a 90 per cent increase in DOC levels in Welsh mountain rivers since 1988.
"The rate of acceleration suggests that we have disturbed something critical that controls the stability of the carbon cycle in our planet," Freeman says. "On these trends, by the middle of the century, DOC emissions from peat bogs and rivers could be as big a source of CO2 to the atmosphere as burning fossil fuels."
Journal reference: Nature (vol 430 , p 195)
Fred Pearce