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Study Links Warming to Dramatic Increase of Large Wildfires in Western US

Spring and summer moisture availability has declined in many forests in the western United States (left). Most wildfires exceeding 1,000 ha in burned area occurred in these regions of reduced moisture (right). Graphs: Running. Data: Westerling et. al.

A new study led by scientists at Scripps Institution of Oceanography at the University of California, San Diego, implicates rising seasonal temperatures and the earlier arrival of spring conditions in connection with a dramatic increase of large wildfires in the western United States.

In the most systematic analysis to date of recent changes in forest fire activity, Anthony Westerling, Hugo Hidalgo and Dan Cayan of Scripps Oceanography, along with Tom Swetnam of the University of Arizona, compiled a database of recent large western wildfires since 1970 and compared it with climate and land-surface data from the region.

Western United States forest wildfire activity is widely thought to have increased in recent decades, but surprisingly, the extent of recent changes has never been systematically documented. Nor has it been established to what degree climate may be driving regional changes in wildfire. Much of the public and scientific discussion of changes in western United States wildfire has focused rather on the effects of 19th and 20th century land-use history.

...The greatest increases occurred in mid-elevation, Northern Rockies forests, where land-use histories have relatively little effect on fire risks, and are strongly associated with increased spring and summer temperatures and an earlier spring snowmelt.

The new findings, published in the July 6 issue of Science Express point to climate change, not fire suppression policies and forest accumulation, as the primary driver of recent increases in large forest fires.

The increase in large wildfires appears to be another part of a chain of reactions to climate warming. The recent ramp-up is likely, in part, caused by natural fluctuations, but evidence is mounting that anthropogenic effects have been contributing to warmer winters and springs in recent decades.

—Dan Cayan, co-author and director of Scripps’ Climate Research Division

The scientists compiled a comprehensive time series of 1,166 forest wildfires of at least 1,000 acres that had occurred between 1970 and 2003 from wildfire data covering western US Forest Service and National Park Service lands. To investigate what role climate might play, the researchers compared the time series, the timing of snowmelt and spring and summer temperatures for the same 34 years.

For the timing of peak snowmelt in the mountains for each year, they used the streamflow gauge records from 240 stations throughout western North America. The team also used other climatic data such as moisture deficit, an indicator of dryness.

The results point to a marked increase in large wildfires in western US forests beginning around 1987, when the region shifted from predominantly infrequent large wildfires of short duration (average of one week) to more frequent and longer-burning wildfires (five weeks).

The authors found a jump of four times the average number of wildfires beginning in the mid-1980s compared with the 1970s and early 1980s. The comparison showed that the total area burned was six and a half times greater. Also in the mid-1980s, the length of the yearly wildfire season (March through August) extended by 78 days, a 64% rise when comparing 1970-1986 with 1987-2003.

The researchers determined that year-to-year changes in wildfire frequency appear “to be strongly linked to annual spring and summer” temperatures with “many more wildfires burning in hotter years than in cooler years.”

They established a strong association between early arrivals of the spring snowmelt in the mountainous regions and the incidence of large forest fires. An earlier snowmelt, they said, can lead to an earlier and longer dry season, which provides greater opportunities for large fires. Overall, 56% of the wildfires and 72% of the total area burned occurred in early snowmelt years. By contrast, years when snowmelt happened much later than average had only 11% of the wildfires and 4% of the total area burned.

The greatest wildfire increases occurred in the Northern Rockies, where forest ecosystems in middle elevations were found to be highly susceptible to temperature increases. Other significant wildfire increases were found in the Sierra Nevada, the southern Cascades and the Coast Ranges of northern California and southern Oregon.

I see this as one of the first big indicators of climate change impacts in the continental United States. We’re showing warming and earlier springs tying in with large forest fire frequencies. Lots of people think climate change and the ecological responses are 50 to 100 years away. But it’s not 50 to 100 years away—it’s happening now in forest ecosystems through fire.

—Thomas Swetnam, director of the Laboratory of Tree-Ring Research at The University of Arizona in Tucson

Fire-fighting expenditures for wildfires now regularly exceed one billion dollars per year.

The authors state that climate model projections, driven by potential increases in atmospheric greenhouse gas concentrations, indicate that warmer springs and summers will likely continue and intensify in the coming decades, accentuating conditions favorable to large wildfires.

The authors conclude that the increased frequency of large and devastating wildfires may significantly change forest composition and reduce tree densities, transforming the western US forests’ role as a storage sink for sequestering some 20 to 40% of all US carbon to a source for increasing carbon dioxide in the atmosphere.

In a companion Perspective in the same issue of Science Express, Steven Running of the Numerical Terradynamic Simulation Group, University of Montana draws the same conclusion:

Wildfires add an estimated 3.5 × 1015 g to atmospheric carbon emissions each year, or roughly 40% of fossil fuel carbon emissions. If climate change is increasing wildfire, as Westerling et al. suggest, these new sources of carbon emissions will accelerate the buildup of greenhouse gases and could provide a feed-forward acceleration of global warming.

The research was supported by the National Oceanic and Atmospheric Administration’s Office of Global Programs, the National Fire Plan via the United States Forest Service’s Southern Research Station and the California Energy Commission.



Rafael Seidl

Perhaps Smokey the Bear wasn't such a great idea after all. Time to spend one percent of that $400 billion DoD budget on mechanically clearing the underbrush from publically owned forests, and on cutting very wide fire breaks. The biomass reovered could be used to produce electricity or fuel.

In Europe, too, forestry needs different incentives. In bone-dry Portugal, there are huge forests of Eucalyptus trees because they grow fater than the indigenous species. Unfortunately, they also burn a lot hotter when there is a fire, quickly overpowering effort to quench the flames. Spain is similarly at risk, and regularly suffers arson (presumed to be by developers).

allen Z

The factors are, in short:
1) The climate has gotten drier and/or warmer. This may be natural, or man made since climate cycles of the past thousands of yrs have shown dry and wet periods. The boom period of post-WW2 West coincided with a wet period. Now that we are entering a drier period, things are going to change. Warmer climates may reduce the water carryover of winter snows.
2) People are moving into the hills, and up to and against forests/dense scrublands. This makes it easier for fires that otherwise would not be noticed, do harm and damage to human life and property. A 100 acre brushfire in the middle of nowhere is going to do less damage than a 25 acre fire up against a development. People are also careless at times. An intoxicated/ stoned smoker flicks some embers from a cigarette/pot on some dry grass and poof, you got the start of something nasty.
3) The early 20th century policy of snuffing fires, out when they are needed to clear out underbrush, open seed pods, clear out dead wood, etc. during the wet period has made many forests/scrub woodland ripe for infernos. Now that we are entering a dry climate out west, the moisture is not there to help us stop/slow fires. Since more moved out into the wilderness, more people are in the way too.
a) It is like the renewed hurricane cycle. 30 yrs lull, 30 yrs thrashing. During the lull, people moved into storm surge zones. Ask the senior citizens who lived through the 30's-60's on the coastline from Boston Mass. to Miami Fl. to Brownsville Tx. The wet period and systematic wildfire snuffing was the lull. People were lulled into a sense of calm and security against fires. They built ever closer to, then ever farther into woodlands and forests. The built up fire fuel, and dry period makes now the thrashing phase.
4) Several invasive/native species of insects are to blame for enormous tree deaths in the Northwest. They reduce whole forests to upright ready made forest fire fuel. Some have been removed, like in British Columbia, but to great scarring to the mountainsides. Whole mountain ranges were denuded to "Get the wood out".

Harvey D.

We may see a multitude of GHG related 'snow ball effects' on the path to warmer climate. The thawing of huge tundra areas, the multiplication of forest fires, the fast rate of industrialization of China and India, etc etc will most probably accelerate the rate of climate change.

Kyoto's objectives may not be as bad as some (mainly conservative politicians and friends) make it to be. We may need much more to reverse the current trend, if it is not a bit too late already.

Reducing transportation related GHG to zero may not be enough.


These guys need to get out from behind their computers and get out into the woods. Seriously. Or maybe it is just poor reporting (no suprise, that). I have seen little to no mention of the roles of grazing, fire suppression, invasive weeds (especially cheatgrass) and insect outbreaks in developing the fire conditions we are experienceing now, but foresters and firefighters have been talking about them for years. Take a look at the Forest Service Inventory and Analysis data on increment vs withdrawls and mortality - fuel loads are phenomenal all over the west. Or look at repeat photo sets from the west:

I have no doubt that the current warm phase is making a contribution, but how can one report this without mentioning the prevailing theories in the field? How did the fires of 1910 happen?

Oh - and as for costs, there is no mystery there. When the fires start, the checkbook is open and money will be spent in grundles. Until that changes fire costs will continue to skrocket. Note: I have been on a dozen or more wildfires this year alone and have been involved with firefighting for a decade.


Allen z, regarding your first point, check out this discussion of the scientific literature that provides evidence that the current increase in CO2 is due to human activities. It is exteremely convincing if you ask me.

Now how much is this increase in CO2 is likely to contribute to the current warming trend? See here:">


Fire is natural part of living cycle of wild forest. Huge masses of pristine forests in US and Canada will burn no matter what human will do about it. On the whole, massive reforestation is taking place in US/Canada biosphere. Naturally, fluctuations in weather and (possibly) climate influence forest fires very significantly.

If we do not want forest fires in some locations, like close to dwelling and in some particular locations in National Parks, we have actively manage forest, remove accumulated brush, make fire brakes, ignite control fires, etc. Do not doing this and suppressing small fires results only in creation conditions to catastrophic fires, what exactly we see every year.

Rafael: eucalyptus forest “purposely” emits highly flammable esters which significantly decrease self-ignition temperature and promote any fire event into forest fire. This is the way eucalyptus species compete for survival with other trees. Same with sequoia: it seeds lays dormant on the ground until heat of forest fire activates them, and they begin to flourish on cleared and fertilized by forest fire soil.


Interestingly, our successes at combatting forest fire over the past few decades has caused a rise in many species of plants which emit smog forming compounds. (you can find this bit of information in the science journal Nature)

allen Z

Many plant/zooplankton species release volatile chemicals that causes ground level ozone, or NOX or are aerosols. They range from butanol, to dimethylsulfoniopropionate (DMSP) from plankton, to methane from bananas.
There are undergrowth/scrub clearing projects being proposed, some controversial. One that is not that contentious is using goats to selectively clear scrub from the hills above Oakland Ca.
While I do agree humans are affecting the environment/ planetary climate, there are many feedbacks to things that happen which we are responsible, or have no control over. Global Warming and GHGs is now merging with Global Cooling and Aerosols. Then you have the Sun, the giant thermonuclear furnace that while relatively stable, has shifts in output that may be small in percentage terms (0.1%) is enormous in the planetary sence when you count ~1,000 watts per m/sq. Another point is that the planet's climate is very complex, and one forcing may provide an unexpected feedback. There is much we do not know about how the planet responds to all these forcings.
___What we do know is that humans are, by large, the predominant source of GHG's. Human activity is also a major source of aerosols. We also are becoming aware of other factors in our climate, like plankton making DMSP (dimethylsulfoniopropionate) that have an effect on cloud formation. Climate cycles are becoming apparent, and must be considered. In short, things are getting both ever clearer, and ever more complicated at the same time. Now, I do agree, we must reduce polution of every type.
However, how feasible is it to ask India or PRC to go a different route when foreign investment is leery of new (and hence to them risky) development/growth strategy? With capital, there is possibility of growth and modernization. Without, well you get the point. In the West/Developed world, it must be painfully apparent and clear to the citizens that pollution of all types must be reduced/eliminated. The public must also be willing change ingrained habits and shoulder the burden of this transition. For example, instead of spending all surplus funds available and then charge to credit, invest the money in various areas that reduces our dependency on fossil energy/energy intensive processes. A large portion of the population has this "me only" or "me now" mentality; it must change. Aside from the question of global climate change, the basics of wealth creation involves smart saving and investing of surplus capital. Much of the middle class is not doing this. It does buy houses, but often the biggest house a mortgage can buy with less consideration for insulation/ energy efficiency or the develoment's environmental impact. I understand things like a 50" plasma HDTV, or a hot dress can gratifying, but your retirement is important too. So is the future of this planet. It may not be bad for a half century, but there are consequences, and the moral/ethical issues of not tackling a problem while not so bad is irresponsible, even selfish. We either deal with it at our choosing, and grab the bull by the horns, or we can sit and wait for it to go bad, and then have knee jerk reactionary policies. Yes, technology may save us in the end, but humans must decide have a concerted effort to implement it. It is little/some pain now, or much more pain later.



Actually, human activity is responsible for only couple of percents in global carbon uptake. Fossil fuel combustion is even smaller. Actual data varies widely, mostly because the biggest factor in carbon cycle - oceans, is thought to be totally inadequately researched.

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