Historically, the frequency and global distribution of wildfires have been strongly influenced by the climate, but according to new research published in Nature Geoscience, during the past 200 years, humans exerted a far stronger influence – until now.

After analysing charcoal remnants contained within 406 sediment cores from six continents, Professor Sandy Harrison of the University of Bristol and colleagues found that up until around 1750, there was a gradual decrease in wildfires worldwide, probably a result of cooling leading up to the so-called Little Ice Age. Fires then became more frequent, increasing in line with rising levels of atmospheric carbon dioxide and a growth in population during the Industrial Revolution, before declining sharply until 1970. This decline is believed to reflect the global expansion of agriculture and intensive grazing, which reduced fuel availability and led to landscape fragmentation, which helped to prevent fires from spreading.

Although the research only goes up to 1970, other evidence suggests that global warming and a build-up of fuel are driving a new increase in wildfires, as illustrated by those that raged in California and southern Europe last year. ‘Warmer climates will naturally cause more fires,’ said Harrison. ‘It looks as if this is overcoming human suppression and we are entering a new fire regime. If, as our work has shown, people were inhibiting climate-induced fires until recently, then the increase in fires during the next decade will be spectacular.’

December 2008