Climate Change and Wildfire
PACLIM 2007
Asilomar, CA
May 14, 2007
Anthony Westerling
Sierra Nevada Research Institute
UC Merced
NOAA OGP
USDA Forest Service
California Energy Commission
Each fire is matched
with the average
Annual PRECIPITATION
&
Summer TEMPERATURE
at its location,
and an index of
DROUGHT
during the year
the fire burned
Mapping the Western US
in terms of Temperature and Precipitation
Typical Summer
Temperatures
Warm Places
Cool Places
Typical Annual
Precipitation
Dry
Places
Wet
Places
Deserts
Forests
Forest Fraction
Woodland Fraction
Shrubland Fraction
Grassland Fraction
Deserts
Forests
How dry is the year when
large fires burn?
WET
DRY
Warm, Dry Deserts
Burn in Wet or Neutral Years
Cool, Wet Forests
Burn in Dry Years
Was the year before the fire
burned wet or dry?
Dry Deserts and Grasslands
burn after a wet year
Cool, Wet Forests do not
usually burn after wet years
WHY DO WE SEE THESE PATTERNS?
Climate influences
Wildfire
through its effects on
Deserts
Fuel Availability
And
Forests
Fuel Flammability
vegetation is sparse here
because the climate is hot
and dry…
Deserts
… so there is not so much
fuel
Forests
and fire risks are sensitive
to the moisture available
to grow more vegetation
Deserts
Forests
vegetation is dense here
because the climate is cool
and moist…
… so there is a lot of fuel
… and fire risks are very
sensitive to factors that
dry the fuels.
Since the mid-1980s
Large Forest Wildfires
Have Increased ~300%
Since the mid-1980s
Large Forest Wildfires
Have Increased ~300%
Other Large Wildfires
Have Not Changed
Substantially
Forest Fires & Temperature
Correlation: 0.70
Grass/Shrub Fires and Temperature
Correlation: 0.08
Western US Spring and Summer Temperature
Percent of Western US in Drought or Wet Conditions
Forest Fires &
Timing of Spring
Correlation: 0.56
Most forest fires
occur in years with
early Springs…
… at elevations
around 7000 feet
Late Snowmelt Years
Early Snowmelt Years
1972 - 2003, NPS, USFS & BIA Fires over 1000 acres
Dry Winters
+
Warm Springs
Early Snow Melt
Drier Soils in
Early Summer
Longer Dry Season
Drier Vegetation
More Fires
Western US Forest Area versus Forest Wildfire
Fires > 200 hectares, reported as forest fires, within our domain
We used moisture reconstructions,
We used moisture reconstructions,
Surface Characteristics,
We used moisture reconstructions,
Surface Characteristics,
And Temperature Reconstructions
To model the probability
large wildfire occurrence
Cor. With NR fire perimeters
Cor. With NR Fire Scars
Cor. With Utah Fire Scars
Future Wildfire Scenarios
Our Scenarios Envision a Wide Range of Possible Futures…
PCM B1 - Wetter, Warmer
GFDL A2 - Drier, Much Warmer
Climate Change Impacts Within CA
100
The Greatest Increases in
CA Wildfire Risks are
Concentrated in
Northern California
80
So Cal
No Cal
60
40
Greater Uncertainty for
Wildfire Risks in
Southern California
20
0
B1 PCM
A2 PCM
B1 GFDL
A2 GFDL
-20
Wetter Scenarios
-40
Drier Scenarios
Increasing Temperature
Organic Carbon
Black Carbon
Spracklen et al, GRL (in review)
Warming has fostered earlier spring snowmelt and more
forest wildfire
Spring and Summer average temperatures
increased less than 1°C
The number of large fires increased ~300%
Area burned in these fires increased ~600%
Fire season length increased 78 days (64%)
Fires burn on average 5 weeks (up from 1 week)
IPCC 4th Assessment projects June - August
temperature increases of 2° to 5°C
Further temperature increases will result in more very
active forest wildfire seasons
Western mountain forests account for 20% - 40% of
annual carbon taken up by US ecosystems (Schimmel)
Wildfires annually add ~40% of fossil fuel carbon
emissions (van der Werf)
Increased frequency and severity of forest wildfires may
accelerate the buildup of greenhouse gases and could
provide a feed-forward acceleration of global warming
(Running)
Have fires increased because Population has increased?
Western U.S. Population, 1900 - 2020
90
80
Census
population (millions)
70
Projection
60
50
40
30
20
10
0
1900
1910
1920
1930
1940
1950
1960
1970
1980
1990
2000
2010
2020
Population increased… but total ignitions did not.
Consequences?
1867
American River
Consequences? Fuel accumulation reduces effectiveness of suppression
1993
American River
less
vulnerable
to warming
most
affected by
suppression
vulnerable
affected by
suppression
largest
vulnerable
forest area
biggest
increase
in wildfires
risks least
affected by
suppression
If lilacs bloom
before ~May 20…
Western US Forest Area versus Federal Lands
Our domain of analysis: intersection of forest and reporting areas
Western North American
Large Fire History
(fires greater than 200 hectares)
Timing of Spring
snowmelt in western
streamflow gauges is
a regional signal,
correlated with
Forest Wildfire