Estimates of global forest loss typically come from the United Nations Food and Agriculture Organization (FAO). But researchers have recognized for some time that those figures are highly inaccurate because they rely on data collected according to a hodge-podge of different techniques and standards. Now the results of a new remote-sensing study, published today in the journal Science, may help to refine those estimates.

An international team led by Frdric Achard of the Joint Research Center in Ispra, Italy, used satellite images to assess deforestation rates between 1990 and 1997, sampling 100 patches representing 6.5 percent of the planets humid tropical forests. The good news is that the rates were 23 percent lower than expected. The bad news is that the losses still amounted to about 5.8 million hectares a year on averagean area almost twice the size of Belgium.

The findings could help ecologists balance the carbon books. So far, investigators have been unable to explain where a significant chunk of the carbon released by human activities--notably fossil fuel burning and deforestation--actually goes. According to the Intergovernmental Panel on Climate Change, about a quarter of it (2.3 petagrams) is probably soaked up by temperate forests. Yet scientists havent actually found enough vegetation to do that. But if less deforestation has occurred, as Achard and his collaborators argue, then less carbon has been released, and less vegetation is required to absorb it. In that case, because climate models take into account how much carbon plants absorb, global warming predictions may change.

Only further probing will reveal exactly what is going on. In the meantime, habitat destruction "costs the human enterprise, in net terms, on the order of $250 billion that year, and every year into the future," argues another group of scientists in the same issue. Andrew Balmford of the University of Cambridge and his colleagues looked at case studies of the economic productivity of ecosystems before and after they were converted to human use. They found that in each case the value of the wild land far outweighed that of its altered counterpart. In fact, the team estimates that global conservation of the natural habitats that remain would have an overall benefit to cost ratio of at least 100 to 1.

"People are hearing a message that nature is being eroded, but it takes a while to sink in, even for me," Balmford remarks. "One third of the world's wild nature has been lost since I was a child and first heard the word 'conservation'--that's what keeps me awake at night."