大象传媒

Will the world's forests help to lessen the impact of global warming, or will they make it worse? Peter Evans looks at the latest research over the question of whether trees will tame or fuel the greenhouse effect.

At the moment, mankind's fossil fuel burning puts about 6.5 billion tonnes of carbon dioxide into the atmosphere each year. Fortunately, only half of this remains in the air. One quarter dissolves into the oceans, and other quarter is absorbed by the Earth's photosynthesising plants - in the most part trees, because of their bulk. The evidence suggests our rising CO2 emissions have been having a fertilising effect on trees in recent decades. Our carbon dioxide has boosted their growth and the amount of carbon they take out of the atmosphere and lock into their wood.

But for how long will this carbon sequestering service last? Peter Evans visits two experiments aimed at providing answers. In southeastern USA , Duke University has been exposing large circular plots of 25 year old pine trees to the atmospheric concentration of carbon dioxide expected for the year 2050. This experiment has been going for nine years, and the trees have maintained an increased growth rate of 10 to 15 percent compared to trees growing under today's atmospheric conditions.

In a mature broad-leaved forest in Switzerland however, rather different results are emerging from an experiment run by the University of Basel . The deciduous trees - some more than 100ft tall - in this 500 m2 plot have not profited in any extra growth after breathing 2050 air for four years.

With or without the CO2 fertilisation effect, the amount of carbon trees can take out of the atmosphere is also influenced by the climatic conditions under which they grow. Findings from European forests during the hot and dry summer of 2003 suggest that in the future, trees could be putting very large quantities into the global atmosphere - adding to man-made emissions rather than reducing them. The same applies to the vast tracts of forest in Amazonia, according to the global change computer simulations of the UK 's Hadley Centre. By the later decades of this century, some of the world's forests could be fuelling global warming rather than putting a brake on it.