STANFORD (US)—More than half a million square miles of new farmland—roughly the size of Alaska—was carved out of what was primarily tropical forests between 1980 and 2000, in the name of global agricultural expansion.

“This has huge implications for global warming, if we continue to expand our farmland into tropical forests at that rate,” says Holly Gibbs, a postdoctoral researcher in the Department of Environmental Earth System Science at Stanford University.

An analysis of Landsat satellite data and images from the United Nations is the first to map and quantify what types of land have been replaced by the immense area of new farmland developed across the tropical forest belt during the 1980s and 1990s.

While this huge increase was happening within the tropics, agricultural land in the non-tropical countries actually decreased in area.

The study appears in in the online early edition of the Proceedings of the National Academy of Sciences.

The United Nations Food and Agriculture Organization estimates that to keep pace with increasing demand, global agricultural 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 warns.

Most of the carbon released comes from burning 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 says.

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,” Gibbs says. “If we continue cutting down these forests, there is a huge potential to further contribute to climate change.”

The increasing demand for agricultural production stems in part from the ever-growing number of people on the planet.  Additionally, members of the growing middle class in emerging economies such as China and India are showing interest in eating more meat, which further intensifies demand.  And incentives to grow crops for biofuel production have increased.

Gibbs says there are some encouraging signs. The patterns of change in the locations they analyzed 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.

In Brazil, where a pattern had developed of expanding soy production by direct forest clearing and by pushing cattle ranching off pastureland and into forested areas, a campaign by Greenpeace and others resulted in agreements by key companies to rein in their expansion. Instead, they worked to increase production on land already in agricultural use.

“These farmers effectively increased the yield of soy on existing lands and they have also increased the head of cattle per acre by a factor of 5 or 6,” Gibbs says. “It is exciting that we are starting to see how responsive industry can be to consumer demands. We really are seeing positive changes in this area.”

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

Other studies that analyzed land use changes between 2000 and 2007 have shown that the pace of cutting down the tropical forests has begun to slow in some regions.

But as long as the human population on the planet continues to grow, the pressure to put food on the table, feed in the barnyard, and fuel in the gas tank will continue to grow, too.

“It is critical that we focus our efforts on reducing rates of deforestation while at the same time restoring degraded lands and improving land management across the tropics,” Gibbs says.

“The good news is that pressure from consumer groups and nongovernmental organizations combined with international climate agreements could provide a real opportunity to shift the tide in favor of forest conservation rather than farmland expansion.”

Researchers from the University of Minnesota contributed to the study.

More news from Stanford University: