« Researchers measure global decrease in fire despite upward trend in forests »

Growing populations, shrinking burn acreages: scientists measure global decrease in fire despite upward trend in forests

By Paul Zedler, TPOS Principal Investigator

Image acquired Sept. 4, 2017. NASA Earth Observatory images by Joshua Stevens and Jesse Allen, using Suomi NPP OMPS data provided courtesy of Colin Seftor (SSAI) and VIIRS data from the Suomi National Polar-orbiting Partnership.

World-wide, the area burned has decreased significantly in the period 1998-2015, according to a paper in a recent issue of Science (Andela et al. 2017). The area of vegetation burned each year has consistently decreased by 24.3 ± 8.8% from 1998-2015, as estimated using satellite data.

The trend, however, is not uniform over the globe. The researchers note that, “Globally, decreases were concentrated in regions with low and intermediate levels of tree cover [especially tropical savannas of South America and Africa and grasslands across the Asian steppe], whereas an increasing trend was observed in closed-canopy forests.” This pattern explains how their conclusion for the globe a whole does not conflict with the fact that area burned has been increasing, probably in part because of climate change, in the western U. S. and other forested regions (see, for example, Dennison et al. 2014).

The authors considered aspects of climate that might explain the decrease, most importantly variation in rainfall. They conclude that rainfall does explain much of the year-to-year variation in area burned, but not the consistent downward trend.

So what is the explanation? In two words: human activity. They point out that over the 18 years of the study the human population has increased, and we know that this is especially true for tropical regions. The drop in area burned is greatest in regions in which agricultural activity has expanded and intensified and especially in areas of low to moderate tree cover – savannas and grasslands. This is plausible given the history of fire over large areas in our upper Midwest region. The landscape level fires that once were the terror of early settlers have disappeared as agriculture expanded and human infrastructure broke up the continuity of fuels and shifted the natural vegetation that remained toward less flammable states.

The consequences of decreased fire are not all bad, at least from a human-centered perspective. The concentration of aerosols has declined and terrestrial carbon sinks have increased. But, as we know, the combination of loss of habitat by conversion to crop land, the change in the vegetation and the frequency of fire in natural habitats are likely to cause loss of biodiversity over large areas. The authors conclude with these observations: “Achieving a balance between the conservation of fire-dependent ecosystems and increasing agricultural production to support growing populations will require careful management of fire activity in human dominated landscapes.”

This conclusion is not startling to those of us concerned with fire management in our region, but now the pattern has been described at a global level. 

Works Cited 

Andela, N., D. C. Morton, L. Giglio, Y. Chen, G. R. van der Werf, P. S. Kasibhatla, R. S. DeFries, G. J. Collatz, S. Hantson, S. Kloster, D. Bachelet, M. Forrest, G. Lasslop, F. Li, S. Mangeon, J. R. Melton, C. Yue, and J. T. Randerson. 2017. A human-driven decline in global burned area. Science 356:1356-1362.


Dennison, PE, Brewer, SC, Arnold, JD and Moritz, MA. 2014. Large wildfire trends in the western United States, 1984-2011. Geophysical Research Letters 41(8): 2928-2933