A new study has revealed that more than 80 percent of the farmlands created in the tropics between 1980 and 2000 came into existence after cutting forests.

As a result it sends carbon into the atmosphere and drives global warming.

Stanford researcher have also noted that big agribusiness has largely replaced small farmers in doing most of the tree cutting in Brazil and Indonesia, which may make it easier to rein in the trend.

More than half a million square miles of new farmland - an area roughly the size of Alaska - was created in the developing world between 1980 and 2000, of which over 80 percent was carved out of tropical forests, according to Stanford researcher Holly Gibbs.

“This has huge implications for global warming, if we continue to expand our farmland into tropical forests at that rate,” said Mr. Gibbs, lead author of the study.

Dr. Gibbs and colleagues at several other universities analysed Landsat satellite data and images from the United Nations to reach their conclusions.

“Every million acres of forest that is cut releases the same amount of carbon into the atmosphere as 40 million cars do in a year,” Dr. Gibbs said.

Most of the carbon released comes from burning the forests, but even if the trees are simply cast aside, the bulk of the carbon from the plants makes its way into the atmosphere during decomposition, she said.

Dr. Gibbs and colleagues found that about 55 percent of the tropical forests that had been cut between 1980 and 2000 were intact forests and another 28 percent were forests that had experienced some degradation, such as some small-scale farming, logging or gathering of wood and brush for cooking or heating fuel.

“The tropical forests store more than 340 billion tons of carbon, which is 40 times the total current worldwide annual fossil fuel emissions,” Dr. Gibbs said.

“If we continue cutting down these forests, there is a huge potential to further contribute to climate change.” But Dr. Gibbs and her colleagues also observed some encouraging signs. The patterns of change in the locations they analysed made it clear that during the 1990s, less of the deforestation was done by small family farms than was the case in the 1980s and more was done by large, corporate-run farms.

Big agribusiness tends to be more responsive to global economic signals as well as pressure campaigns from advocacy organizations and consumer groups than individual small farmers.

Along with wiser use of land already cleared, Dr. Gibbs said, improvements in technology and advances in yield intensification also could slow the expansion of farming into the forests.

The study was published in the online Early Edition of the Proceedings of the National Academy of Sciences.