Did climate change contribute to the recent (2020) wildfires in the Western U.S.?

The New York Times (NYT) published an excellent story today about the massive 2020 fires on the West Coast. You can read the story here. This article does a great job of documenting the dramatic rise in fires in the western states. Of course, it doesn’t hurt that they quoted me in the story….

A recent story in the Seattle Times documents that the health effects from these fires are staggering–not just direct loss of life, but lives lost due to the extreme smoke inhalation (link).

Wild land fires are on the rise in the 21st century. This plot shows annual area burned in acres for the U.S. going back to 1960. This data is from the National Interagency Fire Center (https://www.nifc.gov) who is considered the most reliable source of data on wildfires in the U.S.

Acres burned in the U.S., 1960–2020 (as of 9/21/2020) (Data source: National Interagency Fire Center, https://www.nifc.gov).

Note that for 2020 this data is current as of 9/21/2020, so this year will certainly exceed 8 million acres. I have used this number in many public and scientific talks to frame the discussion. It’s a number that was never exceeded between 1960–2000. Going back to earlier times, it is likely that wild land fires did burn more area, but our records are limited so it’s not possible to compare with the current data. What’s important here is the number of large fire years, defined as greater than 8 million acres burned. From 1960–2003, we never had a year that exceeded 8 million acres burned. But recently, things have really started changing. Since 2004, 10 of the past 17 years (including 2020) have exceeded this number. In other words, a bit more than every other year has been a massive wild land fire season in the U.S. I use the term “wild land fires” rather than wildfires. This is because in the current world, many of our fires are not “wild” in the traditional sense of the word. Fires can be started by lightning, campfires, faulty cars, industrial equipment, power lines, arson, etc. Sometimes we don’t know what started a fire. So the fire community has moved to the term “wild land fire” to refer to fires that are burning mostly forest, grasses, shrubs, or other vegetation.

I recently led a peer-reviewed scientific analysis on smoke and fires. Doing this was really fun as I got to work with some of the top experts on smoke and fires from around the country. Each of us came with our own perspective and expertise (mine is smoke chemistry), and I learned a lot from the process. You can read the summary here. Email me if you would like to see a full version of this report. Importantly, we concluded that the recent increase in fires is due to three causes:

  1. Past forest management and fire suppression.
  2. An increase in direct human ignitions (campfires, vehicles, arson, fireworks, etc.).
  3. Climate change (higher temperatures, reduced snowpack, increased droughts, etc.).

Fires are ignited by things like lightning, campfires, fireworks, etc. It’s important to recognize that past forest management and climate change don’t ignite fires. These are contributing factors that result in faster fire spread and make fires more difficult to put out.


So did climate change contribute to the recent explosive growth in fires in the California, Oregon and Washington? In short, the answer is yes.

A recent post by Cliff Mass (link here) suggests that the Oregon fires this past summer did not have a direct link to climate change. While this post may be correct for this one fire, I disagree on the broader implications. 

Just like the strong consensus on climate change (Cook et al., 2016), there is a strong scientific consensus that increasing temperatures, pests (like the pine bark beetle), and lower humidity are adding to the risk of wildfires in the West (Abatzoglou and Kolden, 2013; Abatzoglou and Williams, 2016; Dennison et al., 2014; Westerling, 2016; Williams et al., 2019). But there are other factors. Fusco et al. (2019) point out the importance of invasive species and Balch et al. (2017) point out the important role for human ignitions. Did I say it was complicated?

Its also important to note that this does not imply that all fires have a climate connection.  How could they?    A random match could set a fire anywhere at any time. So to argue that any one fire is or is not due to climate is missing the forest for the burning trees.   As the cartoon above implies, climate change does not ignite fires, but it does increase the odds of a large fire, one that cannot be controlled, and this is what we are seeing more and more of in the western U.S.   A  good analogy here is that one hot or cold day does not prove/disprove climate change.   It’s the long term statistical pattern of events that is important. 

Let’s take a look at some climate factors to see how they might have influenced fires in 2020. Here is a figure from the publication by Abatzoglou and Williams (2016). It shows a couple of important patterns. First, fuel aridity, or fuel dryness, is clearly linked to greater amount of area burned. Seems pretty obvious. Second, the increasing temperatures in the recent two decades are linked with fuel dryness and greater areas burned. So what causes fuels to dry? Simple. Warm temperatures and low humidities.

Relationship of annual wild land fire area and fuel aridity (dryness) for the area shown in the inset map (Source: Abatzoglou and Williams, 2016).

So what was the climate picture in 2020?

Temperatures have been warming around the country and globally for decades. We are now in the hottest decade ever recorded in modern times.

Global land-ocean temperature index, showing temperature anomalies, relative to the 1951–1980 average. (Source: NASA Goddard Institute for Space Studies, https://climate.nasa.gov/vital-signs/global-temperature/).

In the western U.S., the summer of 2020 fit the pattern quite well, with very warm conditions, well above the long-term mean.

Plot showing temperature anomaly for July 25–September 22, 2020, relative to the 1981-2010 average (Data source: Western Regional Climate Center, https://wrcc.dri.edu/).

Fires in the West in 2020

Washington, California, and Oregon suffered devastating fires this year. Dozens of people were killed, including an 11-year old boy, and thousands of families lost their homes. This is serious stuff. To date, more than 5 million acres have burned in Washington, California, and Oregon alone. The NYT story has a nice graphic showing the dramatic increase in the area burned in these three states.

It’s well known that smoke covered the West Coast and led to horrible air quality. In fact, Seattle suffered its worst single-day air quality (as measured by PM2.5, 264µg/m3) ever. The previous high value was in 2017, another horrible fire year, but that year reached “only” 152 µg/m3. The smoke flowed into the Seattle area mostly from Oregon and California. Areas in Oregon and California had much worse air quality, exposing millions of people to “hazardous” air quality for several weeks, as defined by the EPA “Air Quality Index” or AQI (see here for more info).

Here is a view of downtown Seattle from the Space Needle webcam on September 14, 2020, the worst air quality day for us and a plot showing the highest PM2.5 value at any monitor in the Seattle area, since 1999, when these measurements were first recorded.

View of downtown Seattle from the Space Needle webcam (https://www.spaceneedle.com/webcam).

While the largest fires this year were in California and Oregon, there were plenty of fires burning in Washington. Here is a map of Washington fires, published by the Seattle Times.

Washington state fires (Source: Seattle Times, published 9/10/2020, updated 9/18/2020,

Two of the largest fires in the state were the Cold Springs and Pearl Hill fires in Okanogan County. So let’s take a look at long-term temperature trends and an indicator of humidity called the “vapor-pressure deficit, or VPD” for this region. For the geeks reading this, VPD is the difference between how much gaseous water air could hold and how much it actually holds and it’s reported in pressure units (hPa), since this is how we report water vapor concentrations. If that last sentence makes no sense to you, don’t worry about it. Just think of VPD as “dryness”.

Annual average summer and VPD (July–August) temperatures for Okanogan County, Washington State. (Data source: Oregon State University, Prism Climate Group, https://prism.oregonstate.edu/).

Wow. These data show a striking (and statistically significant) trend in both summer temperatures and VPD. Now the summer of 2020 in Okanogan County was neither the hottest nor the driest on record. Does that matter? No. The 2020 temperatures were 2.3o­C warmer than the long-term mean and the VPD was 0.5 hPa higher (dryer) than its long-term mean. Climate change is driving these trends in temperature and VPD and is a major “threat multiplier” for fires in the western U.S. Put another way, this means that climate change is increasing the severity and area burned of wild land fires in the U.S. But still, climate change doesn’t start a fire and this is a common point of confusion.

So the claim that climate change had nothing to do with fires in 2020 would be, well, laughable, if it weren’t for the fact that these events are so serious, harmful, and even deadly to so many people. And climate change will continue to kill some and cause misery to many through flooding, fires, changes in agriculture, spread of disease pests (like mosquitoes), and in many other ways.

Dealing with climate change (and its impacts) on fires is not a simple problem and will take decades to solve. There are faster solutions around forest management and, maybe, prescribed burning, that we have to consider. But let’s put facts first and understand the problem, because without a clear understanding of the problem, any solutions we devise will be on shaky ground.


Abatzoglou, J. T., and C. A. Kolden. 2013. Relationships between climate and macroscale area burned in the western United States. Int. J. Wildland Fire 22 (7):1003-1020. doi: 10.1071/wf13019.

Abatzoglou, J. T., and A. P. Williams. 2016. Impact of anthropogenic climate change on wildfire across western US forests. P. Natl. Acad. Sci. USA 113 (42):11770-11775. doi: 10.1073/pnas.1607171113.

Balch, J. K., B. A. Bradley, J. T. Abatzoglou, R. C. Nagy, E. J. Fusco, and A. L. Mahood. 2017. Human-started wildfires expand the fire niche across the United States. P. Natl. Acad. Sci. USA 114 (11):2946-2951. doi: 10.1073/pnas.1617394114.

Cook J. et al. 2016 Consensus on consensus: a synthesis of consensus estimates on human-caused global warming. Environ. Res. Lett. 11 048002, doi: 10.1088/1748-9326/11/4/048002

Dennison, P. E., S. C. Brewer, J. D. Arnold, and M. A. Moritz. 2014. Large wildfire trends in the western United States, 1984–2011. Geophys. Res. Lett. 41 (8):2928-2933.

Fusco, E. J., J. T. Finn, J. K. Balch, R. C. Nagy, and B. A. Bradley. 2019. Invasive grasses increase fire occurrence and frequency across US ecoregions. P. Natl. Acad. Sci. USA 116 (47):23594-23599. doi: 10.1073/pnas.1908253116.

Westerling, A. L. 2016. Increasing western US forest wildfire activity: sensitivity to changes in the timing of spring. Phil. Trans. R. Soc. B 371 (1696). doi: 10.1098/rstb.2015.0178.

Williams, A. P., J. T. Abatzoglou, A. Gershunov, J. Guzman-Morales, D. A. Bishop, J. K. Balch, and D. P. Lettenmaier. 2019. Observed impacts of anthropogenic climate change on wildfire in California. Earth’s Future 7 (8):892-910. doi: 10.1029/2019EF001210.

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Joan Tornow

Thank you, Dr. Jaffe, for this analysis. And thank you for calling out Cliff Mass on the disinformation on his blog. Indeed, his blog overall displays his ideological (Libertarian-type) bias. Hope readers will watch the film, Merchants of Doubt, which beautifully describes the methods and motives of climate change minimizers like Cliff Mass — mainly their aversion to government policies (that could address climate change) which they see as “socialistic.” They allow their politics to distort their science, such that it is not really even “science” any more, but propaganda. Thank you, Dr. Jaffe, for your fact-based reporting. Only with FACTS, can voters make informed decisions.


What about BAD forest management? or the antifa THugS that started the bonney lake fire…