More than half a million square miles of new farmland - an area as big as Alaska - was created in the developing world between 1980 and 2000.

More than 80 percent of such farmland was hacked out of tropical forests which sends carbon into the atmosphere and drives global warming, according to Stanford University researcher Holly Gibbs.

'The tropical forests store more than 340 billion tons of carbon, which is 40 times the total current worldwide annual fossil fuel emissions,' said Gibbs, post-doctoral researcher in the Environmental Earth System Science, who led the study.

'This has huge implications for global warming, if we continue to expand our farmland into tropical forests at that rate,' added Gibbs, according to a Stanford statement.

Gibbs and colleagues at several other universities analysed Landsat satellite data and images from the United Nations to reach their conclusions, reports the journal Proceedings of the National Academy of Sciences.

Theirs is the first study to map and quantify what types of land have been replaced by the immense area of new farmland developed across the tropical forest belt during the 1980s and 1990s.

The United Nations Food and Agriculture Organisation estimates that with increasing demand, global agri-production will have to keep increasing, possibly even doubling by 2050.

That would likely lead to millions of additional acres of tropical forest being felled over the next 40 years.

'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,' Gibbs said.

Gibbs and her 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.

'If we continue cutting down these forests, there is a huge potential to further contribute to climate change,' he added.