Firefighters in Arizona are reporting the Wallow fire, which has burned nearly 470,000 acres, is now the most extensive in the state on record, surpassing the Rodeo Chediski fire of 2002. Experts project numerous factors have contributed to the fire’s severity, ranging from forest management practices to this year’s drought. Perhaps more troubling, a report from the National Academy of Sciences (NAS) indicates a modest climate warming may lead to startling increases in wildfire impacts in the West.

Fire acreage is now 469,407 making it the largest fire in AZ history, containment remains at 18% #Wallowless than a minute ago via web Favorite Retweet ReplyArea Command Team 3

Tweet from National Interagency Area Command Fire and Emergency Response Team 3. Hat tip: AccuWeather Twitter feed (@breakingweather)

The NAS report, Climate Stabilization Targets: Emissions Concentrations, and Impacts over Decades to Millenia, presents an analysis finding the area burned each year in the western United States from 1ºC warming may increase 73 percent to over 600 percent compared to recent levels.


From NAS report: “Map of changes in area burned for a 1ºC increase in global average temperature, shown as the percentage change relative to the median annual area burned during 1950-2003. Results are aggregated to ecoprovinces (Bailey, 1995) of the West.” (Figure from Rob Norheim in Climate Stabilization Targets: Emissions Concentrations, and Impacts over Decades to Millenia)

Aggregating all 14 ecoprovinces [i.e. regions] in which fire is most sensitive to temperature variations, the net area burned by the median fire increases from 572,000 ha for the reference period 1970-2003, to 1,800,000 ha with a 1ºC global average temperature increase.

If you consider the most conservative future global warming projections published in scientific assessments exceed 1ºC over the next century and as much as 6ºC under more aggressive greenhouse gas emissions scenarios, the risk of more severe wildfires appears clear and present.

A more conservative analysis cited in the NAS report (Spracklen et al., 2009) projects a 54 percent increase in area burned in the West by mid-21st century compared to the late 20th century (for a mid-range emissions scenario, i.e. A1B).

The NAS report caveats that these projections are dependent on the availability of fuel (i.e. vegetation or combustible material) which will vary according to changes in precipitation. It states:

For example, if climate change further dries already arid grasslands, then the grasslands will wither to deserts and fire will no longer be supported. On the other hand, if climate change changes the distribution of pest and pathogens such that forests become diseased, then additional fuel will be made available due to forest dieback

The current models don’t take into account these possible changes in fuel availability.

The large amount of available “fuel” has been a key contributor to the severity of this year’s Wallow fire says the New York Times, which described a number of possible explanations for the surplus:

Some complained that it was environmentalists who had caused the forests to become tinderboxes by preventing the thinning of trees as they sought to protect wildlife. Others, like William Wallace Covington, a forestry expert at Northern Arizona University, countered that the leading factor was the grazing of forest grass for generations. The government’s longstanding practice of quickly extinguishing forest fires was also seen as adding to the thick clusters of highly combustible trees.

In addition to the availability of combustible material, the drought in portions of the Southwest has no doubt exacerbated the fire situation. The entirety of eastern Arizona is in moderate to exceptional drought according to the U.S. Drought Monitor.

Are climate change and global warming responsible for the current drought situation? Are the NAS projections already bearing out in the current climate?

That answer is far less clear.

In a blog post examining whether the historic drought in Texas (which has also experienced major wildfires) could be linked to climate change, climatologist John Nielsen-Gammons told the Houston Chronicle’s the SciGuy, Eric Berger, no or sort of:

Climate model projections of increased drought severity across Texas are based on a combination of decreased rainfall, increased evaporation, and possibly changes in weather patterns. Since there’s not at present any evidence of decreasing precipitation, the present drought notwithstanding, total rainfall changes are unattributable at this point.
Climate change has probably had a couple of secondary effects, though. For one thing, some of the burned areas, especially the one in northern Mexico, received some excessive rainfall during the summer. Global warming probably produced a slight enhancement of the rainfall, leading to a little extra plant growth. Also, the warm temperatures during the past couple of months are probably a degree or two warmer than they would have been without the rise in global temperatures, thereby increasing the dryness.

Satellite imagery of wildfire smoke plumes originating from eastern Arizona on June 9, 2011 (NOAA)

[The unusual precipitation pattern] fits the type of precipitation pattern climate models expect to occur over the U.S. by the end of the century due to human-caused warming of the climate ... This drying of the Southern U.S. and increased precipitation in the Northern U.S. is expected to occur because of a fundamental shift in the large scale circulation of the atmosphere. The jet stream will retreat poleward, and rain-bearing storms that travel along the jet will have more moisture to precipitate out, since more water vapor can evaporate into a warmer atmosphere. The desert regions will expand towards the poles, and the Southern U.S. will experience a climate more like the desert regions of Mexico have now, with sinking air that discourages precipitation. A hotter climate will dry out the soil more, making record intensity droughts like this year’s in Texas more probable. So, is it possible that the record extremes of drought and wetness this spring in the U.S. were due to a combination of La Niña and climate change.

But Masters cautions it’s a challenge to “disentangle” the contributions of climate change and La Nina to this year’s extreme weather without conducting time-consuming modeling studies.

Not only will be modeling studies be required to better understand the causes of extreme weather, but also more observational data to detect meaningful trends. So we’ll have to wait quite a while to credibly attribute drought and wildfire events to climate change.

In the mean time, policymakers will have to make decisions on anecdotal evidence and projections - i.e. imperfect information. Will they take the appropriate actions to deal with the risk? Only time will answer that question as well.