"During the late 1970s, Indonesian Borneo changed from being highly fire-resistant to highly fire-prone during drought years, marking the period when one of the world's great tropical forests became one of the world's largest sources of pollution," said Robert Field, a University of Toronto doctoral student who led the research. "Ultimately, this abrupt transition can be attributed to rapid increases in deforestation and population growth. The resulting occurrences of haze currently rank among the world's worst air pollution episodes, and are a singularly large source of greenhouse gas emissions."
Although Sumatra has suffered large fires at least since the 1960s, Indonesian Borneo seemed to have been resistant to large fires even in dry years. That lasted until land use changed from small-scale subsistence agriculture to large-scale industrial agriculture and agro-forestry, said Field.
"This should give pause to further agro-forestry expansion in Indonesia, particularly for oil palm as a source of biofuel," he said.
The study, which included researchers Guido van der Werf of the Free University of Amsterdam and Sam Shen of San Diego State University, is to appear in the March issue of Nature Geoscience and is available online now at the journal's Web site.