Gaia's Lungs

Natural History, March, 1999 by William F. Laurance

Are rainforests inhaling Earth's excess carbon dioxide?

Most scientists agree that global warming, or the greenhouse effect, is primarily a result of an increase in carbon dioxide in the earth's atmosphere. But researchers have been puzzled, because given all the burning of fossil fuels and forests during the past few decades, there should actually be a lot more [CO.sub.2] in the air than there is. Something seems to be sucking up millions of tons of the gas each year.

Now it seems that the mystery of the "missing" carbon dioxide may be solved--at least partly. Working at fifty sites scattered throughout the Amazon and Central America, our research team, which includes British ecologists Oliver Phillips, of the University of Leeds, and Yadvinder Malhi, of the University of Edinburgh, and others, has compiled studies of more than a hundred thousand trees over the past thirty years. To our surprise, we found that the total mass of living trees in each acre of rainforest at our sites has increased by an average of 17 metric tons (37,000 pounds) since the beginning of our study. Not only are these forests producing more trees per acre, but the existing trees are growing larger faster. To produce that much extra plant tissue over the course of thirty years, each acre would have had to utilize an additional twenty tons of carbon dioxide. In the Amazon basin alone, intact rainforests could be absorbing over one billion tons of carbon dioxide each year. Our conclusions are in line with earlier studies of two small areas of intact forest in the Amazon that also use dramatically increased quantities of carbon dioxide.

Why are the forests suddenly producing more and bigger trees per acre? One possibility is that extra plant nutrients, such as nitrogen and phosphorus, have been raining down on the forests as other areas burn. But more likely is the startling conclusion that the accelerated growth is a direct response to high levels of carbon dioxide in the atmosphere. To grow, plants need sugars, which they synthesize from carbon dioxide. A greater quantity of available [CO.sub.2] may thus mean more and faster growth.

The implications of this discovery are profound: without tropical rainforests, the greenhouse effect would be more troublesome today and considerably worse in the future. But I must emphasize that only large, unbroken tracts of forest--between two thousand and fifteen thousand acres--absorb the extra carbon dioxide. Forests that have been burned, logged, or fragmented do not. In fact, disrupted forest areas actually lose carbon dioxide to the atmosphere.

About forty million acres of tropical forest are cleared and burned annually--the equivalent of over seventy football fields a minute. Another fifteen million acres are logged each year, and vast stretches are becoming mosaics of denuded land and isolated forest patches. Such destruction causes a quarter of all greenhouse gas emissions. In addition, cattle ranching--a common land use in these once-forested areas--is a major source of methane, a heat-absorbing gas that also contributes to global warning. Our studies show that tropical rainforests are even more important for human' welfare than had previously been suspected. Preserving small forest fragments as parks and reserves while the remaining forest is destroyed or degraded--as is commonly done in the name of conservation--is a demonstrably inadequate strategy for protecting the global ecosystem. We need vast intact tracts of forest to protect watersheds and soil, to maintain regional and global climes, and especially to soak up and utilize the atmosphere's excess [CO.sub.2]. Since we now have hard evidence that rainforests play a vital role in keeping our planet livable, it is time for us to start returning the favor.

William F. Laurance is senior research scientist at the National Institute for Research in the Amazon, located in Manaus, Brazil.