One possible solution is to manage this deforestation prudently, taking only some of the trees, allowing the forest to regenerate, or planting fast-growing species. The new growth will absorb much of the extra carbon dioxide.
But this may in fact be adding to the problem of global warming, recent research indicates - and the humble termite may be to blame.
Termites are social insects, related to cockroaches. They are well-known for building extraordinary cathedral-like mounds with clever ventilation systems, and notorious for eating wooden houses - in the US they cause more damage to homes than storms and fires combined. Some termites eat soil, and others eat dead wood in tropical forests, aiding the decomposition process. Some termites give off methane when digesting food, by fermentation in the gut, in a similar way to cattle. Methane is another of the greenhouse gases.
In 1982 three American scientists, Zimmerman, Greenberg and Wandiga, published a paper in Science, reporting experiments in which they had measured the amount of methane produced by termites in test-tubes, then calculated from this the amount they produced in their natural habitats. Their estimate was large enough to cause concern - they concluded that termites could produce as much as 150 million tonnes of methane a year. At the same time the amount of methane in the atmosphere was - and still is - increasing faster than other greenhouse gases, by 1 per cent a year since 1978.
Although there were clearly other sources of methane - paddy-fields, coal-mines and rubbish dumps among them - termites were blamed for at least part of the increase, with estimates ranging from less than 5 per cent to more than 40 per cent of the total annual global methane production. Termite numbers were thought to be increasing because the forests were being destroyed and termites were thriving in the resulting savannah areas.
The paper's conclusions were based on slim evidence - a laboratory study of just one type of North American termite. No one had made an in-depth field study of what happens to tropical forest- dwelling termites when their habitats are disturbed or destroyed, so an important part of the picture was missing. In fact, forest termites are more abundant than those in the savannah.
Now, however, a group of British scientists is carrying on where the Americans left off. Funded by the Natural Environment Research Council, the Terrestrial Initiative in Global Environmental Research (Tiger) project has been sending scientists from the Natural History Museum, Queen Mary and Westfield College, Imperial College, and the Natural Resources Institute, Chatham, Kent, to Cameroon, which has some of the best 'primary' (undisturbed) tropical forest in Africa, and is home to up to 100 million termites per hectare.
Over the past two years the group has sampled extensively at five forest sites around the logging town of Mbalmayo, in southern Cameroon. An immediate effect of logging is to destroy termite habitats, but invariably only the largest trees are taken away. The rest are usually left broken on the ground, where they decay and dry out. Then either a slow regrowth takes place, or new trees are planted. Once there is enough cover to allow shelter from the sun, the termites return to feed on the plentiful dead wood on the forest floor.
Dr Paul Eggleton, from the Natural History Museum, has been identifying and counting the termites collected in Cameroon. 'We were surprised to find that termite abundance and species richness is highest in two 'secondary' forest areas, one where the forest was logged manually and left to regenerate and one where it was replanted with fast-growing trees. We were expecting the termites to do best in the primary forest.'
If termites really are a major source of methane, this could have important implications for global warming. At least 40 per cent of cleared forest is allowed to regenerate across the whole of Africa, and this figure is likely to increase with more agro-forestry projects. In these regenerating areas there could be more termites producing methane. But do termites really produce the levels of methane previously estimated?
Measuring methane production is not as straightforward as Zimmerman and his colleagues believed. Dr David Bignell from Queen Mary and Westfield College, leader of the Tiger termite team, says: 'Termites in the lab react differently to termites in their natural habitats, and the amount of methane escaping from termite nests in the field is much less than might be expected.' Scientists are divided on why this is so. Some believe that termites operate an 'internal methane cycle' whereby soil bacteria in and around the mounds oxidises the methane they produce, rendering it harmless. Others think that the act of measuring emissions disturbs the termites so that they leave the nests and go into subterranean foraging tunnels. Some termite mounds are built wholly under the ground, making it harder still to measure emissions. At any time fewer than 10 per cent of termites in an area will be inside mounds, so the likelihood of accurate overall measurements is small.
Termites may also contribute to greenhouse gases in another way. Termite mounds appear to contain denitrifying bacteria which release nitrogen from the soil. Small amounts of two particularly potent greenhouse gases, nitric and nitrous oxide, are also emitted. Although this goes on in mounds in undisturbed forest, the bacteria seem to be stimulated when the forest is disturbed, especially when fertilisers are used on cleared land.
While it remains uncertain exactly how much termites contribute to greenhouse gases, it is certain that for millions of years termites have existed in tropical forests and have done no harm. The danger lies in upsetting the natural balance of the forest eco- system. Dr Bignell concludes: 'Termites are the ultimate guardians of the gas exchange between the soil and the atmosphere. Destroy the natural habitat of these guardians, upset the system and the consequences could be very serious.'
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