The Southwest on Fire: Is Climate Change to Blame?

Natural disasters can provide a dramatic reminder of the impact human activities are having on our planet. In recent months, the Southwestern United States has experienced devastating forest fires. As of June 2nd, over 3 million acres have burned throughout the country this year, representing the largest acreage burned by this point in the year since 2000. According to data from the National Interagency Fire Center (NIFC), the total number of acres burned between 1960 and 2009 has greatly increased. Additionally, the number of acres burned per individual fire event has also increased, at an even sharper rate, demonstrating a trend toward more severe fires. This is true despite the fact that U.S. forest acreage decreased from 1945-1997.

Source: National interagency Fire Center

The Wallow Fire, which began in late May and is still burning, is the largest fire in Arizona’s history covering 538,049 acres. The Las Conchas fire which has thus far destroyed over 100,000 acres and has become the largest blaze in New Mexico’s history is also still burning as of July 1st. There are several other large fires in the Southwestern region.  The increase in number and severity of wildfires makes us wonder:  is climate change to blame? The answer: yes, at least in part. In fact it is a combination of poor management AND climate conditions that is fueling the infernos raging in the American Southwest and elsewhere in the country.

The increase in large-scale forest fires that we have seen in the past several decades is largely caused by increasing temperatures and dry conditions, both impacts of climate change in the American Southwest. Higher summer temperatures coupled with earlier spring snow melt has been shown to increase the frequency, length and severity of wildfires. Spring snowmelt provides soil moisture, and therefore an earlier melt leads to drier summer conditions. The figure below shows the relationship between increasing temperatures and wildfire frequency. Temperatures and wildfire frequency are generally higher from 1987 to 2003 than those from 1970-1986.

Source: Westerling et al., 2006

Fire management practices that have emphasized putting out fires to preserve human property have resulted in increased fuel available for burning when later fires arrive. An interesting interplay between climate conditions and fuel quantity also adds to the problem. Rapid biomass growth during wet years provides excess fuel that burns during dry years. These wet and dry years can be connected to the El Niño Southern Oscillation (ENSO), which results in wetter and cooler weather in the American Southwest. The La Niña phase, on the other hand, is responsible for drier conditions in the Southwest. Until around May 2011, we were considered to be in a La Niña year, following a transition from a strong El Niño year in June 2010. Although there is uncertainty as to whether climate change is responsible for increased El Niño events, connections between climate variability and climate change remain a possibility.

Unfortunately, an increase in wildfires has its own negative effects on the climate system. Fires deplete a valuable sink for absorbing atmospheric carbon dioxide. At the same time the burning of trees and other vegetation releases the carbon dioxide they have sequestered throughout their lifetimes.  The result is a positive feedback loop: greater quantities of carbon dioxide in the atmosphere further amplify warming conditions and in turn increase fire severity and frequency.

The Southwest fires provide just one of many examples of concrete impacts of anthropogenic climate change. As the likelihood of more frequent and severe weather events grows with the changing climate, decision-makers must both try to mitigate climate change, e.g. through a rapid scaling up of energy efficiency and renewable energy development, and be better prepared to adapt to the changes that occur despite all efforts to prevent global warming. In the case of fires, forest management practices should include prescribed burning techniques and other measures that reduce high fuel levels in forests. This summer’s catastrophic fires in the Southwest demonstrate the urgent need for the U.S. to commit to both climate change adaptation and mitigation. At current greenhouse gas concentration levels we are already committed to around 1 degree C of warming based just on past emissions, and we keep adding to the problem every day, without knowing how to deal with it.


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