Wildfire trends in the West are clear. More large fires are burning now than at any time in the past 40 years, and the total area burned each year has also increased.
Climate Central's 2012 report, Western Wildfires, analyzed federal wildfire data stretching back to the 1970s to see how fires have changed in the American West. In some states, such as Arizona and Idaho, the number of large fires burning each year has tripled or even quadrupled. And in other states, including California, Colorado, New Mexico, Nevada and Wyoming, the number of large fires has doubled.
Over the same span, average spring and summer temperatures across 11 Western states have increased by more than 1.5 degrees Fahrenheit, contributing to the higher fire risks. Spring temperatures in Arizona have warmed faster than any other state in the U.S., rising nearly 1 degree per decade since 1970, which has likely played a key role in Arizona's rapid increase in fires over the past two decades.
In addition, years with abnormally warm spring and summer temperatures tend to be years with more and bigger fires. For example, 2012 was the hottest spring and summer on record for Colorado, and the state also saw its second-
highest number of large fires.
Not surprisingly, years with low spring snowpack (measured as the amount of water in snowpack on the ground as of April 1) also tend to be years with more fires. When there is a relatively thin snowpack come spring, it can melt quickly as the weather warms, leaving the forest drier earlier and much more likely to burn.
Across the Southwest in particular, several recent years of below-average spring snowpack have extended the region's drought and fueled more big fires.
Research has shown that there has been a persistent decline in Western snowpack since the 1920s. And in looking at the past 30 years, it's clear that from one year to the next, below-average snowpack raises the risk of wildfires.
Western wildfires are also influenced by year-to-year climate variability and how the U.S. Forest Service manages fires, but the trends of rising temperatures and lesser snowpack hint at what fires will look like in the coming decades.
Researchers predict that the area burned in the West will quadruple for every additional 1.8 degrees of temperature rise. According to the draft National Climate Assessment report, the most recent climate model projections show that temperatures will rise between 2 degrees and 4 degrees across most of the U.S. within the next few decades, and as much as 8 dgrees by 2100.
Here are a few states that have seen the most dramatic changes in wildfires since the 1970s:
* Utah -- In the past decade, more wildfires have burned in Utah in most of those years than any year in the 1970s. And in 2012, thin spring snowpack and above-average spring temperatures contributed to the biggest wildfire year in a decade.
* Idaho -- Idaho has seen the biggest increase in wildfires since 1970 than any other Western state. Year-to-year there is still a lot of variability, but in the past decade, several years with above-average temperatures and low spring snowpack have led to dozens of large wildfires on federal land.
* Nevada -- Spring snowpack has been decreasing dramatically in Nevada since the early 1980s. And in 2012, the lowest spring snowpack in more than 30 years, combined with above-average spring temperatures, helped fuel the biggest wildfire year in more than a decade.
* Arizona -- Spring temperatures have warmed faster in Arizona than any other state since 1970. These warming temperatures, matched with decreasing spring snowpack, have helped increase wildfire risk. Every year since 2004 has seen more fires burn in Arizona than in any year in the 1970s.
* Colorado -- Colorado has the second-fastest warming spring temperatures of any state. And in 2012 -- the hottest spring and summer on record for Colorado -- the state saw its second-highest number of large wildfires.
* New Mexico -- Spring warming in New Mexico is among the top three fastest in the country. In the past decade, many more wildfires burned in New Mexico than during the 1970s and 1980s, but the year-to-year variations in wildfires depend on several factors, including temperatures, spring snowpack and how large the previous year's wildfires were.
Wildfire data is based on fires larger than 1,000 acres that burned on U.S. Forest Service land in the 11 Western states.