Western fires are burning higher in the mountains and at unprecedented rates as the climate warms

<figure><img src=”https://images.theconversation.com/files/420425/original/file-20210910-21-1pbdpxm.jpg?ixlib=rb-1.1.0&amp;rect=0%2C0%2C3000%2C1994&amp;q=45&amp;auto=format&amp;w=1440&amp;fit=clip” /><figcaption><span class=”caption”>Heat and dryness are leaving high mountain areas more vulnerable to forest fires.</span> <span class=”attribution”><a class=”source” href=”https://www.gettyimages.com/detail/news-photo/the-241-square-mile-station-fire-continues-to-blacken-news-photo/90351732″> David McNew/Getty Images</a></span></figcaption></figure><p>The Western U.S. is experiencing <a href=”https://disasterphilanthropy.org/disaster/2021-north-american-wildfire-season/”>another severe fire season</a>, and a recent study shows that even high mountain areas once considered too wet to burn are at increasing risk as the climate warms.</p>

<p>With <a href=”https://www.nifc.gov/fire-information/nfn”>more than 5 million acres already burned by early September</a>, the 2021 U.S. fire season is <a href=”https://www.nifc.gov/fire-information/nfn”>about on pace</a> with the extreme fire season of 2020. This summer has been <a href=”https://www.ncei.noaa.gov/news/national-climate-202108″>the hottest</a> on record and one of the driest in the region, with <a href=”https://droughtmonitor.unl.edu/data/jpg/20210907/20210907_usdm.jpg”>80% of the Western U.S.</a> in severe to exceptional drought. That combination of heat and dryness is a recipe for <a>disastrous wildfires</a>. </p>

<figure class=”align-center zoomable”>
<a href=”https://images.theconversation.com/files/420624/original/file-20210912-46896-ipnb6j.png?ixlib=rb-1.1.0&amp;q=45&amp;auto=format&amp;w=1000&amp;fit=clip”><img alt=”Map of the contiguous United States with historic temperature ranks on each state. Several were the hottest on record or close to it.” src=”https://images.theconversation.com/files/420624/original/file-20210912-46896-ipnb6j.png?ixlib=rb-1.1.0&amp;q=45&amp;auto=format&amp;w=1440&amp;fit=clip” srcset=”https://images.theconversation.com/files/420624/original/file-20210912-46896-ipnb6j.png?ixlib=rb-1.1.0&amp;q=45&amp;auto=format&amp;w=600&amp;h=431&amp;fit=crop&amp;dpr=1 600w, https://images.theconversation.com/files/420624/original/file-20210912-46896-ipnb6j.png?ixlib=rb-1.1.0&amp;q=30&amp;auto=format&amp;w=600&amp;h=431&amp;fit=crop&amp;dpr=2 1200w, https://images.theconversation.com/files/420624/original/file-20210912-46896-ipnb6j.png?ixlib=rb-1.1.0&amp;q=15&amp;auto=format&amp;w=600&amp;h=431&amp;fit=crop&amp;dpr=3 1800w, https://images.theconversation.com/files/420624/original/file-20210912-46896-ipnb6j.png?ixlib=rb-1.1.0&amp;q=45&amp;auto=format&amp;w=1440&amp;h=542&amp;fit=crop&amp;dpr=1 754w, https://images.theconversation.com/files/420624/original/file-20210912-46896-ipnb6j.png?ixlib=rb-1.1.0&amp;q=30&amp;auto=format&amp;w=1440&amp;h=542&amp;fit=crop&amp;dpr=2 1508w, https://images.theconversation.com/files/420624/original/file-20210912-46896-ipnb6j.png?ixlib=rb-1.1.0&amp;q=15&amp;auto=format&amp;w=1440&amp;h=542&amp;fit=crop&amp;dpr=3 2262w” sizes=”(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px”></a>
<figcaption>
<span class=”caption”>Several states saw their hottest June-August period since consistent record keeping began in the 1890s.</span>
<span class=”attribution”><a class=”source” href=”https://www.ncei.noaa.gov/news/national-climate-202108″>NOAA</a></span>
</figcaption>
</figure>

<p>In a study in the Proceedings of the National Academy of Sciences released in May 2021, <a href=”https://scholar.google.com/citations?user=S1J4kAoAAAAJ&amp;hl=en”>our team of fire</a> and <a href=”https://scholar.google.com/citations?user=tGGNDyUAAAAJ&amp;hl=en”>climate scientists</a> <a href=”https://scholar.google.ca/citations?user=ZaW8ZbsAAAAJ&amp;hl=en”>and engineers</a> found that forest fires are now <a href=”https://www.pnas.org/content/118/22/e2009717118″>reaching higher, normally wetter elevations</a>. And they are burning there at rates unprecedented in recent fire history. Two fires burning in northern California in 2021 – the Dixie and Caldor fires – are examples: They were the first and second wildfires on record to cross the Sierra Nevada crest and burn on both sides.</p>

<p>While <a href=”https://theconversation.com/how-years-of-fighting-every-wildfire-helped-fuel-the-western-megafires-of-today-163165″>historical fire suppression</a> and other forest management practices play a role in the West’s worsening fire problem, the high-elevation forests we studied have had little human intervention. The results provide a clear indication that climate change is enabling these normally wet forests to burn. </p>

<p>As wildfires creep higher up mountains, another tenth of the West’s forest area is now at risk, our study found. That creates new hazards for mountain communities, with impacts on downstream water supplies and the plants and wildlife that call these forests home.</p>

<figure class=”align-center “>
<img alt=”Map showing how high-elevation forest fires advanced uphill.” src=”https://images.theconversation.com/files/402231/original/file-20210523-21-1xdrd55.png?ixlib=rb-1.1.0&amp;q=45&amp;auto=format&amp;w=1440&amp;fit=clip” srcset=”https://images.theconversation.com/files/402231/original/file-20210523-21-1xdrd55.png?ixlib=rb-1.1.0&amp;q=45&amp;auto=format&amp;w=600&amp;h=627&amp;fit=crop&amp;dpr=1 600w, https://images.theconversation.com/files/402231/original/file-20210523-21-1xdrd55.png?ixlib=rb-1.1.0&amp;q=30&amp;auto=format&amp;w=600&amp;h=627&amp;fit=crop&amp;dpr=2 1200w, https://images.theconversation.com/files/402231/original/file-20210523-21-1xdrd55.png?ixlib=rb-1.1.0&amp;q=15&amp;auto=format&amp;w=600&amp;h=627&amp;fit=crop&amp;dpr=3 1800w, https://images.theconversation.com/files/402231/original/file-20210523-21-1xdrd55.png?ixlib=rb-1.1.0&amp;q=45&amp;auto=format&amp;w=1440&amp;h=787&amp;fit=crop&amp;dpr=1 754w, https://images.theconversation.com/files/402231/original/file-20210523-21-1xdrd55.png?ixlib=rb-1.1.0&amp;q=30&amp;auto=format&amp;w=1440&amp;h=787&amp;fit=crop&amp;dpr=2 1508w, https://images.theconversation.com/files/402231/original/file-20210523-21-1xdrd55.png?ixlib=rb-1.1.0&amp;q=15&amp;auto=format&amp;w=1440&amp;h=787&amp;fit=crop&amp;dpr=3 2262w” sizes=”(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px”>
<figcaption>
<span class=”caption”>Forest fires advanced to higher elevations as the climate dried from 1984 to 2017. Every 200 meters equals 656 feet.</span>
<span class=”attribution”><span class=”source”>Mojtaba Sadegh</span>, <a class=”license” href=”http://creativecommons.org/licenses/by-nd/4.0/”>CC BY-ND</a></span>
</figcaption>
</figure>

<h2>Rising fire risk in the high mountains</h2>

<p>In the <a href=”https://www.pnas.org/content/118/22/e2009717118″>new study</a>, we analyzed records of all fires larger than 1,000 acres (405 hectares) in the mountainous regions of the contiguous Western U.S. between 1984 and 2017.</p>

<p>The amount of land that burned increased across all elevations during that period, but the largest increase occurred above 8,200 feet (2,500 meters). To put that elevation into perspective, Denver – the mile-high city – sits at 5,280 feet, and Aspen, Colorado, is at 8,000 feet. These high-elevation areas are largely remote mountains and forests with some small communities and ski areas.</p>

<p>The area burning above 8,200 feet more than tripled in 2001-2017 compared with 1984-2000. </p>

<figure class=”align-center “>
<img alt=”Fire lights up a ridge behind a farm.” src=”https://images.theconversation.com/files/402236/original/file-20210523-15-1hxi37i.jpg?ixlib=rb-1.1.0&amp;q=45&amp;auto=format&amp;w=1440&amp;fit=clip” srcset=”https://images.theconversation.com/files/402236/original/file-20210523-15-1hxi37i.jpg?ixlib=rb-1.1.0&amp;q=45&amp;auto=format&amp;w=600&amp;h=343&amp;fit=crop&amp;dpr=1 600w, https://images.theconversation.com/files/402236/original/file-20210523-15-1hxi37i.jpg?ixlib=rb-1.1.0&amp;q=30&amp;auto=format&amp;w=600&amp;h=343&amp;fit=crop&amp;dpr=2 1200w, https://images.theconversation.com/files/402236/original/file-20210523-15-1hxi37i.jpg?ixlib=rb-1.1.0&amp;q=15&amp;auto=format&amp;w=600&amp;h=343&amp;fit=crop&amp;dpr=3 1800w, https://images.theconversation.com/files/402236/original/file-20210523-15-1hxi37i.jpg?ixlib=rb-1.1.0&amp;q=45&amp;auto=format&amp;w=1440&amp;h=431&amp;fit=crop&amp;dpr=1 754w, https://images.theconversation.com/files/402236/original/file-20210523-15-1hxi37i.jpg?ixlib=rb-1.1.0&amp;q=30&amp;auto=format&amp;w=1440&amp;h=431&amp;fit=crop&amp;dpr=2 1508w, https://images.theconversation.com/files/402236/original/file-20210523-15-1hxi37i.jpg?ixlib=rb-1.1.0&amp;q=15&amp;auto=format&amp;w=1440&amp;h=431&amp;fit=crop&amp;dpr=3 2262w” sizes=”(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px”>
<figcaption>
<span class=”caption”>One of Colorado’s largest wildfires, 2020’s East Troublesome Fire, crossed the Continental Divide and was burning at elevations around 9,000 feet in October, when snow normally would have been falling.</span>
<span class=”attribution”><a class=”source” href=”https://newsroom.ap.org/detail/Exchange-ColoradoWildfires-Blow-ups/8e10c8213c3847f3a7ef14e7ff81eddf/photo”>AP Photo/David Zalubowski</a></span>
</figcaption>
</figure>

<p>Our results show that climate warming has diminished the high-elevation flammability barrier – the point where forests historically were too wet to burn regularly because the snow normally lingered well into summer and started falling again early in the fall. Fires advanced about 826 feet (252 meters) uphill in the Western mountains over those three decades.</p>

<p>The <a href=”https://inciweb.nwcg.gov/incident/6964/”>Cameron Peak Fire in Colorado</a> in 2020 was the largest fire in the state’s history, burning over 208,000 acres (84,175 hectares), and is a prime example of a high-elevation forest fire. The fire burned in forests extending to 12,000 feet (3,658 meters) and reached the upper tree line of the Rocky Mountains. </p>

<p>We found that rising temperatures in the past 34 years have helped to extend the fire territory in the West to an additional 31,470 square miles (81,507 square kilometers) of high-elevation forests. That means a staggering 11% of all Western U.S. forests – an area similar in size to South Carolina – are susceptible to fire now that weren’t three decades ago. </p>

<h2>Can’t blame fire suppression here</h2>

<p>In lower-elevation forests, several factors contribute to fire activity, including the presence of more people in wildland areas and a history of fire suppression. </p>

<p>In the early 1900s, Congress commissioned the U.S. Forest Service to <a href=”https://foresthistory.org/research-explore/us-forest-service-history/policy-and-law/fire-u-s-forest-service/u-s-forest-service-fire-suppression/”>manage forest fires</a>, which resulted in a focus on suppressing fires – a policy that continued through the 1970s. This caused flammable underbrush that would normally be cleared out by occasional natural blazes to accumulate. The increase in biomass in many lower elevation forests across the West has been associated with increases in <a href=”https://doi.org/10.1890/ES13-00217.1″>high-severity fires and megafires</a>. At the same time, <a href=”https://doi.org/10.1073/pnas.1607171113″>climate warming has dried out forests in the Western U.S.</a>, making them more prone to large fires. </p>

<figure class=”align-center “>
<img alt=”Illustration of two mountains showing fires higher, less snow and more dead trees” src=”https://images.theconversation.com/files/402233/original/file-20210523-19-46yvai.png?ixlib=rb-1.1.0&amp;q=45&amp;auto=format&amp;w=1440&amp;fit=clip” srcset=”https://images.theconversation.com/files/402233/original/file-20210523-19-46yvai.png?ixlib=rb-1.1.0&amp;q=45&amp;auto=format&amp;w=600&amp;h=413&amp;fit=crop&amp;dpr=1 600w, https://images.theconversation.com/files/402233/original/file-20210523-19-46yvai.png?ixlib=rb-1.1.0&amp;q=30&amp;auto=format&amp;w=600&amp;h=413&amp;fit=crop&amp;dpr=2 1200w, https://images.theconversation.com/files/402233/original/file-20210523-19-46yvai.png?ixlib=rb-1.1.0&amp;q=15&amp;auto=format&amp;w=600&amp;h=413&amp;fit=crop&amp;dpr=3 1800w, https://images.theconversation.com/files/402233/original/file-20210523-19-46yvai.png?ixlib=rb-1.1.0&amp;q=45&amp;auto=format&amp;w=1440&amp;h=519&amp;fit=crop&amp;dpr=1 754w, https://images.theconversation.com/files/402233/original/file-20210523-19-46yvai.png?ixlib=rb-1.1.0&amp;q=30&amp;auto=format&amp;w=1440&amp;h=519&amp;fit=crop&amp;dpr=2 1508w, https://images.theconversation.com/files/402233/original/file-20210523-19-46yvai.png?ixlib=rb-1.1.0&amp;q=15&amp;auto=format&amp;w=1440&amp;h=519&amp;fit=crop&amp;dpr=3 2262w” sizes=”(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px”>
<figcaption>
<span class=”caption”>On average, fires have spread 826 feet (252 meters) higher into the mountains in recent decades, exposing an additional 31,470 square miles (81,507 square kilometers) of forests to fire.</span>
<span class=”attribution”><span class=”source”>Mojtaba Sadegh</span>, <a class=”license” href=”http://creativecommons.org/licenses/by-nd/4.0/”>CC BY-ND</a></span>
</figcaption>
</figure>

<p>By focusing on high-elevation fires in areas with little history of fire suppression, we can more clearly see the influence of climate change. </p>

<p>Most high-elevation forests haven’t been subjected to much fire suppression, logging or other human activities, and because trees at these high elevations are in wetter forests, they historically have long <a href=”https://www.fs.fed.us/database/feis/fire_regime_table/PNVG_fire_regime_table.html”>return intervals between fires</a>, typically a century or more. Yet they experienced the highest rate of increase in fire activity in the past 34 years. We found that the increase is strongly correlated with the observed warming.</p>

<p><iframe id=”87j2d” class=”tc-infographic-datawrapper” src=”https://datawrapper.dwcdn.net/87j2d/1/” height=”400px” width=”100%” style=”border: none” frameborder=”0″></iframe></p>

<h2>High mountain fires create new problems</h2>

<p>High-elevation fires have implications for natural and human systems. </p>

<p>High mountains are natural water towers that normally provide a sustained source of water to millions of people during dry summer months in the Western U.S. The scars that wildfires leave behind – known as burn scars – affect how much snow can accumulate at high elevations. This can influence the timing, <a href=”https://doi.org/10.1021/es500130g”>quality and</a> <a href=”https://www.fs.fed.us/rm/pubs/rmrs_gtr042_4.pdf”>quantity of water</a> that reaches reservoirs and rivers downstream.</p>

<p>High-elevation fires also remove standing trees that act as anchor points that normally stabilize the snowpack, <a href=”https://doi.org/10.1016/j.foreco.2009.01.050″>raising the risk of avalanches</a>.</p>

<p>The loss of tree canopy also exposes mountain streams to the Sun, <a href=”https://doi.org/10.1890/09-0822.1″>increasing water temperatures</a> in the cold headwater streams. Increasing stream temperatures can harm fish and the larger wildlife and predators that rely on them. </p>

<p>Climate change is increasing fire risk in many regions across the globe, and <a href=”https://doi.org/10.1073/pnas.1003669107″>studies show that this trend will continue</a> as the planet warms. The increase in fires in the high mountains is another warning to the U.S. West and elsewhere of the risks ahead as the climate changes.</p>

<p><em>This is an update to a story published <a href=”https://theconversation.com/drafts/159699/edit”>May 24, 2021</a>.</em></p>

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This article is republished from The Conversation, a nonprofit news site dedicated to sharing ideas from academic experts. It was written by: Mojtaba Sadegh, Boise State University; John Abatzoglou, University of California, Merced, and Mohammad Reza Alizadeh, McGill University.

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<p class=”fine-print”><em><span>John Abatzoglou receives funding from the National Science Foundation and the National Oceanic and Atmospheric Administration.</span></em></p><p class=”fine-print”><em><span>Mohammad Reza Alizadeh does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>

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