THIS century's biggest - and worst - contribution to environmental degradation has definitely been global warming: the much-debated and discussed phenomenon arising from build-up of "greenhouse" gases, such as carbon dioxide (CO₂). These gases trap heat and consequently lead to an increase in global temperatures. So far, those concerned by global warming have blamed the industries and automobile manufacturers since they have been responsible for emitting the greatest amounts of greenhouse gases into the atmosphere. But now, experts have found another, previously overlooked source of these greenhouse gases.

The adverse effect of clearing rainforests may have been seriously underestimated. A team from Brazil has recently found that CO₂ emissions from felling of tropical forests are generally seven per cent higher than previously thought, and as much as 42 per cent higher in a few regions.

Recent estimates suggest the worldwide deforestation releases some 2,000,000,000 tonnes of CO₂ into the atmosphere every year through burning and decomposition, a large chunk of which comes from the tropics. However, William Laurence at the National Institute for Research in the Amazon in Manaus, Brazil, says the figures do not account for the extra CO₂ produced by fragments of forests after clearing.

An 18-year study of more than 66 forest fragments published in Science last year, Laurence and his team found that forest fringes - within 100 metres of the edge - lose significantly more vegetation than their inner areas since these fringe parts are exposed to higher winds and other climate extremes. The decomposition resulting from such vegetation losses emits considerable amounts of CO₂ and methane, the two major greenhouse gases. The loss of trees also means there is less biological matter capable of absorbing CO₂ from the atmosphere during the growth.

Now, in a follow-up project, the researchers have calculated the amount of CO₂ emitted by these tropical forest fragments. They simulated three patterns of forest clearing: the small-farmer settlement schemes in Amazonia, the large cattle-ranching clearances typical of eastern Amazonia, and a random scenario imitating unplanned forest loss. The first two account for most forest clearance in the region.

In each case, the team tested various levels of clearance, from five to ninety-five per cent of the land area. At every level, the random and the small-farmer scenarios suffered the highest carbon losses - between two and five times more than for the large cattle-ranching models - because they led to greater fragmentation. The randomly cleared landscapes, in which irregular edges were common, were especially prone to vegetation loss, even at very low levels of clearance.

The team calculates that annual carbon emissions from fragmentation of forests are between three and 15.6 million tonnes in the Brazilian Amazon and between 22 and 149 million tonnes for all tropical forests at present rates of clearance. This suggests that current estimates for CO₂ emissions from the felling of tropical forests are up to seven per cent too low, and as much as 42 per cent too low in some regions, says Laurance.

"This prediction seems rather modest until one realises that this is the equivalent of clearing and burning an additional 150,000 to one million hectares of rainforest each year." If anything, says he, the figures underestimate the problems. The experimental plots were surrounded by regrowth forest which provided some protection from wind. Fragments surrounded by cattle pasture and crops would be fully exposed. Laurence says that governments should encourage patterns of forest clearance that minimise fragmentation.