The world’s peat bogs are haemorrhaging carbon dioxide into the atmosphere, accelerating global warming. Worse, the process appears to be feeding off itself, as rising atmospheric levels of CO2, are triggering further releases from the bogs.
That’s the claim of a British researcher this week who is warning that billions of tonnes of carbon could pour into the air from peat bogs in the coming decades. “The world’s peatland stores of carbon are emptying at an alarming rate,” says Chris Freeman of the University of Wales at Bangor. “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.
Bacteria in the rivers rapidly convert DOC into CO that bubbles into the atmosphere. But speculation has been rife about why the peat bogs are giving up their carbon in the first place. Three years ago, Freeman proposed that global warming was the cause (New Scientist, 25 August 2001, p 8). 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, has also bitten the dust.
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 rate of acceleration suggests we have disturbed something critical that controls the stability of the carbon cycle in our planet”
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 C02, 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.
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!
Researchers from the Centre for Ecology and Hydrology in Lancaster, UK, have been measuring DOC levels in water for some years as part of a programme to monitor river chemistry. But the study is also providing critical evidence of the impact of rising levels of C02 in the atmosphere.
“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!
Freeman says the dissolved carbon also poses a potential health threat. DOC can react with chlorine disinfectant at water treatment works to produce cancer-causing chemicals called trihalomethanes. “Apart from the global warming implications, this means we will have to pay higher water bills for removing these toxins,” Freeman says.