Direct and indirect effects of wildfires in the United States

Wildfires in the USA

On December 11th, wildfires have consumed about 94,000 hectares in Ventura and Santa Barbara counties (South West California), poked by Santa Ana strong winds. California is experiencing its worst and most expensive wildfire season on record (9.1 million acres burned from January to November +71% compared to 2016), notably because of a very wet rainy season that provided all the necessary fuel. In addition to vegetation, about 800 structures and 680 homes have been destroyed so far (Reuters). Such a vigour is unexpected in December, as California’s wildfires season normally ends in October. Hereunder figure provides an overview of US wildfire history, in terms of fires number, surface area and suppression costs (source: National Interagency Fire Center). If burnt areas seem to be stabilized over the last 10 years, firefighting costs show a clear growing trend, which suggests that this stabilization is ensured at the expense of increasing fighting means. Compared to previous decades, it also appears that fire activity has significantly increased (although their number has been reduced).

In a 2016 article published in PNAS*, two researchers from the universities of Idaho and Columbia take a look at US Western forest fires and postulate that climate change is the cause of over half of the documented increases in fuel aridity since the 1970s, and that it doubled the cumulative forest fire area since 1984. As it is only the beginning, this would suggest an increasing fire risk in the coming years, with an environment that will be more and more favourable to wildfires. This should materialize, for instance, by an extended fire period.

Besides suppression costs, which almost reached $2 billion in 2016 and probably more than $2.4 billion this year, damage costs represent the main burden of wildfires. From the October fires, more than $9 billion in claims have been incurred by insurers for California only (Insurance Information Institute). Total damages could climb to $85 billion, according to AccuWeather. Verisk consultants estimate that 4.5 million of houses are exposed to high or extreme risk of wildfire in the United States, with more than 2 million in California (second is Texas, with more than 700,000 houses).

What are the effects of wildfires on biodiversity? It is important to know that fires are a natural and vital phenomenon in many ecosystems. Fires allow to maintain biological diversity by limiting the development of more dominant species. They open the canopy and allow the ground to be directly hit by sunlight, which is necessary for many species to proliferate. Some seeds (pyrophile plants) even require to be slightly burnt to germinate. Most plant communities in the Mediterranean Basin, for instance, are fire prone, and some ecosystems, such as the Mediterranean shrublands, are fire dependent. However, changes in frequencies as well as unsuitable management practices can become a threat for their survival. For example, some temperate forests in the US in which fire was deliberately suppressed for management and political reasons have experienced devastating wildfires due to an unnatural accumulation of fuel. On the contrary, too frequent fires can deeply modify ecosystems distribution, and degrade biotopes. In forests where fire is not a natural disturbance, impacts on biodiversity can be disastrous.

It should be reminded that forest fires are a significant source of GHG emissions. Moreover, about 90% of wildland fires are estimated to be caused by humans.

*Proceedings of the National Academy of Sciences of the United States of America (


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