With fires raging in multiple western states and in Canada, there is this assumption that there is a single cause for these large, hot wildfires that, in some cases, are creating their own weather.  There is a tendency by different media outlets and political figures to say it’s climate change, or fire suppression has created a fuels problem, or if people just didn’t build houses in the forest there wouldn’t be a problem.  The basic fact of the matter is this:

1. Climate change is making vegetation more flammable
2. In some forest types, fire suppression has increased fuel loads
3. When we build houses in flammable environments, there is a good chance they will burn

 
Now, I’ll unpack these three things.

​Climate Change
Climate change influences wildfire in several ways.  When vegetation is wet, it takes way more energy to get it to burn because essentially all of the water has to boil off before the vegetation will burn.  This is why you don’t use wet wood to make a camp fire.  As the air temperature goes up due to climate change, snow in the mountains melts out earlier in the year and increases the length of the fire season.  Once the snow is gone, higher temperatures continue to make vegetation more flammable by drying it out.  High air temperature means the more water is evaporating from the soil and dead vegetation and that live plants are using more water (transpiration) to photosynthesize.  Human-caused climate change is responsible for approximately half of the fuel drying in the western US.  So now we’ve got high temperatures making vegetation available to burn for a longer part of the year and making it more flammable by drying it out.  But wait, there’s more…
 
The other climate change effect is that nighttime temperatures are remaining high.  This is important for fire behavior and firefighting.  When the temperature drops at night and the relative humidity increases, fire behavior tends to calm down.  This is typically when much progress is made by wildland firefighters in containing the fire.  With high nighttime temperatures, we are now seeing fires spread faster at night.
 
Fuels
There are some forests that used to burn every few years to decades before we started putting fires out.  This has increased fuels in these forest types and in some cases harvesting smaller trees to reduce the chance that fire burns through the forest canopy will reduce the chance of a large, hot fire.  However, we also have to return more frequent fires to these forest types.  If all we do is cut down smaller trees, we’re putting a band-aid on the situation because fuels will just build back up.
 
There are other forest types, like lodgepole pine forests, that are supposed to burn infrequently and when they do, they burn hot.  The 1988 Yellowstone fires are a classic example.  When these forests burn every 100-300 years, that is natural.  Fire suppression has not drastically altered these forests and thinning to reduce the chance of a large, hot wildfire is not appropriate.
 
Humans
Human behavior creates three main problems when it comes to wildfire.  The first is that humans cause many wildfires.  We have increased the number of ignitions and because of the timing of the ignitions, increased the length of the fire season.  The second problem that we’ve created is that we like to build houses in flammable environments and don’t take responsibility for managing the risks around our properties by building with non-flammable material and maintaining a fuel buffer around our homes and communities.  This map shows where the wildland urban interface is located in the continental US.  We like to live in beautiful places and that puts us at risk.  The third problem is that we don’t want to have a bunch of smoke in the air from fires.  Now, that’s like buying a house next to a freeway and getting pissed off when you hear traffic noise.  Estimates of area burned in California before the 1800s suggest most of the state is quite flammable.  If 6,900 square miles were burning each year, California was probably a pretty smokey place.  For comparison, there are roughly 1015 square miles burning in California today (8/13/2018) and the smoke analysis looks like this:

​The basic facts of the matter is that we are facing a challenge that we have created through human-caused climate change, fire suppression, and not thinking through our actions.  Any solution that we develop will require addressing all three of these factors if it is going to be successful.

Note: This was originally submitted to the LA Times and rejected.
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Unfortunately, “A wildfire plan that makes things worse” has misrepresented a number of the findings from the scientific community that provide the basis for the Forest Carbon Plan and Governor Brown’s proposal.  
 
Governor Brown’s May 10 proposal represents a forward-looking policy response to the challenges facing California’s forests and communities.  The forests of the Sierra Nevada are facing a number of challenges including hotter temperatures, drought, insect outbreaks, and uncharacteristic wildfire.  Numerous scientific studies in the Sierra Nevada have documented that mid- and low-elevation forests were historically maintained by frequent surface and mixed-severity fires that killed individual or small groups of trees.  The forests we have today are the result of a century of fire suppression. The large wildfires burning today are uncharacteristic for these forests because they kill large patches of trees.  Large, hot wildfires negatively impact wildlife habitat for some species like the California spotted owl and Pacific fisher, they kill the vegetation on steep slopes that stabilizes soil and protects water quality, and they decrease the amount of carbon that these forests remove from the atmosphere and store.  Large patches of fire-killed trees can be a source of carbon to the atmosphere for decades, contributing to climate change.  Studies across the western US show that as global warming has increased, the fire season has gotten longer.  More large wildfires are occurring across the west and in California.  With continued warming, we expect this trend to continue. 
 
Many studies have demonstrated that we can reduce the risk of large, tree-killing wildfires by using a combination of thinning and prescribed fire.  In some locations, such as adjacent to communities thinning prior to burning is required to reduce fire hazard.  In other locations, fire alone can do the necessary work to restore these forests.  Thinning can in fact remove many trees, but the type of thinning the Governor’s proposal includes is focused on small trees.  This is an important point because large trees remove more carbon from the atmosphere and store more carbon than small trees.  Further, trees compete for resources and when there is a drought and large trees are competing with small trees for water, they become stressed and more susceptible to insects and death.  California’s most recent drought demonstrated this fact, with 102 million dead trees in the Sierra.
 
Hanson and Miller also misrepresent our scientific understanding of forests, fire, and carbon.  While true that only a small fraction of a live tree killed by wildfire is combusted during the fire, they do not acknowledge the basic fact that live trees remove carbon from the atmosphere and dead trees emit carbon to the atmosphere. 
  
My research group has conducted studies at both the watershed scale and for the entire Sierra Nevada to evaluate the influence of these treatment options on the ability of the forest to remove carbon from the atmosphere and store it.  Our results show that thinning and burning treatments do reduce the amount of carbon stored in the forest in the near-term.  However, over time a more natural forest condition stores much more carbon than the overly-dense forests we currently have.  The two determining factors are the proportion of trees killed by wildfire and the size of harvested trees. 
 
When we reduce tree density and restore surface and mixed-severity fire, we significantly reduce the chance of a large, tree-killing wildfire.  When wildfire burns through treated forest, many more trees survive the fire and continue helping to regulate the climate by removing carbon from the atmosphere.  Our research demonstrates that because we expect more wildfire with more climate change, implementing these treatments over a larger area at a faster pace leads to lower total wildfire emissions and more carbon stored in Sierran forests. 
 
Investing in reducing the risk of large, tree-killing wildfires in the Sierra Nevada carries both ecological and societal benefits.  The scientific community has done the research to identify management options that will benefit forests. 
 
Governor Brown’s proposal is based on decades of scientific research.  It acknowledges that the climate is changing and that California’s forests will be significantly challenged by continued warming.  The proposal also recognizes the role of California’s forests in regulating the climate and that forest management based on scientific evidence can help sustain these forests and the services that they provide to society.  

​ The area burned by wildfire in the Sierra Nevada has increased by 274% over the last 40 years and the area impacted by stand-replacing fire has also increased.  The forests in the Sierra Nevada are important for provision of clean water and are also part of the state’s climate action plan.  As a result, figuring out how to reduce the chances of large, hot fires presents a large challenge. 
 
We know that the current pace and scale of forest treatments to reduce the risk of large, hot fires is inadequate given the scale of the problem and the area burned by wildfire is projected to increase with on-going climate change.  In a recent study led by Shuang Liang, we set out to determine how the pace of large-scale treatment implementation would alter carbon storage across the Sierra Nevada.  We ran simulations under projected climate and wildfire and two management scenarios.  Both management scenarios included applying thinning and prescribed burning treatments to low and mid-elevation forests.  These are forests that have been most impacted by fire suppression.  In the distributed scenario, we simulated an equal portion of the area treated at each time-step and with full treatment implementation by the end of this century.  In the accelerated scenario, we simulated the same treatments over the same area, but schedule the treatments so they were complete by 2050.  We included a control scenario that assumed no active management for comparison.
 
The area burned between all three scenarios was fairly consistent because we used the same fire size distributions in our simulations (black line in Figure 1).  However, the proportion of burned area that was burned by stand-replacing fire (severity 4 and 5) decreased substantially.  The faster pace of treatment under the accelerated scenario increased the proportion of area burned by surface fire (severity 1 and 2) and decreased the area burned by stand-replacing fire at a much faster rate than the distributed scenario.

Both the accelerated and distributed treatments ended up storing more carbon than the control by 2100 (Shown by the difference in Figure 2).  

​However, what was most striking was how these treatments influence the carbon balance of Sierra Nevada forests as a percentage of California’s 2020 emissions limit from the Governor’s Climate Action Plan.  Initially, total carbon losses are higher in the treatment scenarios, with the accelerated treatment having the largest loss (Figure 3).  However by 2030, carbon loss is similar amongst all three scenarios and by 2050 the accelerated scenario has lower emissions than the wildfire emissions under the control.

​As we demonstrated in a previous study, changing climate and the increase in area burned has the potential to increase wildfire emissions by 19-101% by later this century. The results from this study demonstrate that restoring surface fires to the low and mid-elevation forests in the Sierra Nevada can reduce the magnitude of future emissions and maintain a larger amount of carbon stored in these forests.