Fire - Are we facing an increase in wildfires? [Present]

Anthony L. Westerling

Sierra Nevada Research Institute, University of California, Merced, USA; This email address is being protected from spambots. You need JavaScript enabled to view it.

Large wildfires in a diverse array of forests are typically associated with drought during the fire season (Krawchuk and Moritz 2011). Drought occurrence is driven by variability in both temperature and precipitation. Climate change is expected to increase temperature over land, with greater increases in continental interiors and high northern latitudes. However, precipitation is more varied and uncertain, with increases projected in the tropics and high latitudes, and decreases in mid-latitudes (Dai 2010). Warming combined with changes in precipitation is projected to produce permanent severe drought conditions by mid-to-late century for much of the Americas, Africa, Southern Europe, Central and Southeast Asia, and Australia, although uncertainty is high (Dai 2010). Changes of this magnitude could substantially alter ecosystems, with wildfire constituting a mechanism effecting abrupt changes in response to more gradual climate forcings. The lack of analogues for transitions of this speed and magnitude limit the capacity to robustly model climate-fire-vegetation interactions in coming decades.


Figure 1: The number of large forest wildfires (vertical axis) versus average March through August temperature anomalies (horizontal axis) for 1972-2008 in Western US forests. Anomalies were constructed subtracting the long-term mean for 1972-1990. Fires are all large (> 200 ha) fires reported by the United States’ Bureau of Indian Affairs, National Park Service, and Forest Service as burning in forests. All fires were classified as “action” fires on which suppression was attempted (fires used to manage vegetation were excluded). See Westerling et al. 2006 and Westerling et al 2011 for data and methodology.

Ecosystems highly sensitive to recent climate trends include cool, moist forests with infrequent, stand-replacing fire where warming has led to longer fire seasons and/or increased evapotranspiration. Examples include substantially increased fire in mid-elevation Rocky Mountain forests of the USA (Westerling et al. 2006) and Canadian and Alaskan boreal forests (Soja et al. 2007) (Fig. 1). High-severity burned area in Siberian boreal forests may also have increased, but historical baseline data are less reliable (Soja et al. 2007). Fire is likely to further increase in these forests with continued warming (e.g. Krawchuk et al. 2009; Wotton et al. 2010; Westerling et al. 2011). However, as climate shifts and fire becomes more frequent, changes in regeneration and productivity for forest species could transform vegetation assemblages and the fire regimes they can support (Soja et al. 2007; Krawchuk et al. 2009; Westerling et al. 2011). An additional uncertainty for these ecosystems is how fire may interact with other disturbance types such as bark beetle outbreaks that alter the structure of forest fuels.

Dry, warm forests may still be sensitive to warming that exacerbates periodic drought. Large areas burned in conjunction with recent drought and warming in mountain forests of the southwestern United States (Williams et al. 2010). The largest fires there coincided with reduced precipitation, higher temperatures and earlier spring snowmelt (Westerling et al. 2006). Land use and fire suppression in southwestern forests also led to fuel accumulation and changes in fuel structure. The interaction of fuel changes with climate change and variability likely contributed to increased fire and fire severity, but the relative importance of these causes is not known (Williams et al. 2010). Recurrent severe drought could convert large portions (>50%) of Southwestern U.S. forests to non-forest vegetation due to fire, beetles and other climate-related dieoff (Williams et al. 2010), substantially altering fire regimes.

As in higher latitude forests, drought-driven increases in fuel flammability drive increased fire in tropical forests. However, short-term reductions in precipitation, rather than elevated temperatures, are the dominant influence on wildfire in tropical forests due to their higher temperatures (Goldammer and Price 1998). While on average increased aridity is projected for Amazon, Mexico and Congo forests across many climate models, the greater uncertainty associated with projected patterns of precipitation make future fire predictions in these tropical forests more uncertain as well.

Diverse forests in many regions of the globe have the potential for increased fire in the coming decades due to changes in temperature, precipitation or both. Changes in climate and disturbance may substantially alter vegetation in ways that feed back to or limit changes in forest wildfire.

Selected references

Full reference list online under:


Dai A (2011) Wiley Interdisciplinary Reviews: Climate Change, 2(1): 45-65

Krawchuk M and Moritz M (2011) Ecology 92(1): 121-132

Soja A et al. (2007) Global and Planetary Change 56: 274-29

Westerling A et al. (2011) PNAS 108(32): 13165-13170

Williams A et al. (2010) PNAS 107(50): 21289-21294

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