Amazon rainforest vanishing at twice rate of previous estimates

·6,000 sq miles lost a year as valuable trees removed
·Selective logging causing 25% greenhouse gas boost

The Amazonian rainforest is being destroyed at double the rate of all previous estimates, according to research published today in the journal Science. The destruction is leaving the forest more prone to fires and allowing more carbon dioxide to be released into the atmosphere, according to scientists.

A new analysis of satellite images of the Brazilian part of the Amazon basin, which forms part of the largest contiguous rainforest on Earth, shows that on average 15,500 sq km (6,000 square miles) of forest is being cut down by selective logging each year. This is besides a similar amount clear-cut annually for cattle grazing or farming.

Conservationists have been able to monitor large clear-cut areas using satellite images. But the extent of selective logging, where individual trees of high value, such as mahogany, are felled and smuggled out of the forest, had been unclear, the effects being masked from satellites by the forest's dense canopy.

"People have been monitoring large-scale deforestation in the Amazon with satellites for more than two decades, but selective logging has been mostly invisible until now," said Gregory Asner, of the Carnegie Institution, Washington. He tackled the problem by developing an analytical method named the Carnegie Landsat Analysis System, which allows each pixel of an image to be scrutinised for the amount of forest left to determine the overall ratio of forested to deforested land.

Natalino Silva, of the Brazilian Agricultural Research Corporation, said: "We can now see what's happening from the top of the forest all the way to the soil. We have a whole new picture of the Amazon region and selective logging."

The analysis revealed some surprising facts. "We discovered that annually an area about the size of Connecticut is disturbed this way," said Professor Asner. "Selective logging negatively impacts many plants and animals and increases erosion and fires. Additionally, up to 25% more carbon dioxide is released to the atmosphere each year - above that from deforestation - from the decomposition [of plant material] that the loggers leave behind. Timber harvests are much more widespread than previously thought."

Using images of the Amazon basin taken from 1999 to 2002, Prof Asner studied the five states that account for 90% of deforestation. The extent of selective logging was found to be between 4,685 and 7,973 square miles each year.

Michael Keller, of the US Forest Service, who was the co-author of the research, said: "We expected to see large areas of logging, but the extent to which logging penetrates deep into the frontier is much more dramatic than we anticipated."

A large mahogany tree can fetch hundreds of dollars at the sawmill, making it a tempting target. "People go in and remove just the merchantable species from the forest," said Prof Asner. "Mahogany is the one everybody knows about, but in the Amazon there are at least 35 marketable hardwood species, and the damage that occurs from taking out just a few trees at a time is enormous."

About 400m tonnes of carbon enter the atmosphere every year because of traditional deforestation in the Amazon, and Prof Asner estimates that an additional 100m tonnes of carbon occurs through selective logging. "When a tree trunk is removed, the crown, wood debris and vines are left behind to decompose, releasing carbon dioxide gas into the atmosphere," he said.

A thinned canopy also makes the forest more dry and prone to fire. "On average, for every tree removed, up to 30 more can be severely damaged by the timber harvesting operation itself," said Prof Asner.