In September 2007, 1,000 square kilometres of tundra near the Anuktuvuk River on Alaska’s North Slope was burnt. After the fire, Feng Sheng Hu of the University of Illinois travelled to the region in the hope of determining whether the fire was an anomaly or if large fires such as this were a regular occurrence in the region. ‘If such fires occur every 200 years or every 500 years, it’s a natural event,’ Hu said. ‘But another possibility is that these are truly unprecedented events caused by, say, greenhouse warming.’

Hu collected sediment cores from two nearby lakes and then he and his colleagues analysed the distribution of charcoal particles in the cores. The team found no evidence of a fire of a similar scale and intensity in sediments representing about 5,000 years for that area.

The researchers then created a model that linked the area of tundra burned to mean temperature and precipitation during the warmest period of the year (June–September) using 60 years of data from the Alaskan tundra. The analysis, which is published in the Journal of Geophysical Research, uncovered a striking pattern. ‘There is a dramatic, non-linear relationship between climatic conditions and tundra fires, and what one may call a tipping point,’ Hu said.

Once the mean temperature rises above a mean threshold of 10°C, the results suggested, fires become significantly more frequent. For the past 60 years, mean summer temperatures have fluctuated between about 6°C and 9°C, trending upwards since 1995. In 2007, the mean was a record 11°C, while precipitation and soil moisture were at an all-time low.

January 2011