THE soil-to-air cycle of carbon dioxide or, more precisely, soil respiration is a major source of carbon dioxide emission. The soil is likely to respire even faster in future as temperatures rise due to global warming.
Plants photosynthesize during the day and produce oxygen from carbon dioxide. At night a different metabolic cycle takes place which results in the release of carbon dioxide. This, along with microbial respiration and chemical processes that lead to oxidation of soil minerals are the ways in which the soil respires.
Ecologist Ben Bond-Lamberty and his colleague, Allison Thomson, at the Pacific Northwest National Laboratory conducted a study to prove the soil-to-air carbon cycle is accelerating, meaning the exchange of carbon dioxide between soil and air is occurring faster. They estimated the total emission due to soil respiration was 98 billion metric tonnes, or 98 petagrammes (1 petagramme is 10 14 grammes) in 2008. The number rose 15 per cent in the last 20 years.
The duo went through data from 306 studies conducted on soil respiration between 1989 and 2008. All the studies used standardized equipment to obtain their results. It was calculated that the contribution of temperate and tropical areas to the global annual emission was 20 and 67 per cent respectively, while the Artic region added 13 per cent. The temperate and tropical soil respiration increased two and three per cent within the last two decades which is in sync with the global rise in temperatures. That of the northern Arctic (boreal) area increased the maximum over the past two decades—seven per cent. This was surprising as temperatures in that region are relatively lower. It is, however, in agreement with the Arctic having large amounts of stored carbon under its layer of permafrost. It has also seen recurring waves of climate change. Other lines of evidence, too, suggest warming is unlocking the old stores.
“The study proves soil respiration is increasing. An interesting area of research will be to compare the data, which were actually observed in the field, to results from global climate models,” said Bon-Lamberty.
“It is well accepted that respiration increases as temperature rises. It is very important to put the changes in context. Temperature-induced change in atmospheric carbon dioxide is much smaller than changes brought about by humans burning fossil fuels,” said Gavin Schmidt, climate scientist at the Columbia University in New York.
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