Causal relationships versus emergent patterns in the global controls of fire frequency
- 1Forest Research Centre, School of Agriculture, University of Lisbon, Tapada da Ajuda 1349–017 Lisbon, Portugal
- 2Geography and Environmental Sciences, School of Archaeology, Geography and Environmental Sciences (SAGES), Reading University, Whiteknights, Reading, RG6 6AB, UK
- 3Department of Biological Sciences, Macquarie University, North Ryde, NSW 2109, Australia
- 4AXA Chair of Biosphere and Climate Impacts, Grantham Institute for Climate Change and Grand Challenges in Ecosystems and the Environment, Department of Life Sciences, Imperial College London, Silwood Park Campus, Buckhurst Road, Ascot, SL5 7PY, UK
Abstract. Global controls on month-by-month fractional burnt area (2000–2005) were investigated by fitting a generalised linear model (GLM) to Global Fire Emissions Database (GFED) data, with 11 predictor variables representing vegetation, climate, land use and potential ignition sources. Burnt area is shown to increase with annual net primary production (NPP), number of dry days, maximum temperature, grazing-land area, grass/shrub cover and diurnal temperature range, and to decrease with soil moisture, cropland area and population density. Lightning showed an apparent (weak) negative influence, but this disappeared when pure seasonal-cycle effects were taken into account. The model predicts observed geographic and seasonal patterns, as well as the emergent relationships seen when burnt area is plotted against each variable separately. Unimodal relationships with mean annual temperature and precipitation, population density and gross domestic product (GDP) are reproduced too, and are thus shown to be secondary consequences of correlations between different controls (e.g. high NPP with high precipitation; low NPP with low population density and GDP). These findings have major implications for the design of global fire models, as several assumptions in current models – most notably, the widely assumed dependence of fire frequency on ignition rates – are evidently incorrect.