Climate Change Impacts on Fire Regimes

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Climate Change Impacts on Fire Regimes
October 3, 2007 Incendios forestales lucha o
adaptación? Spanish Royal Academy of
Sciences Fundación Ramón Areces Anthony
Westerling UC Merced l NOAA OGP USDA Forest
Service California Energy Commission
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Plan of Presentation

• Why did forest fires increase in the Western US,
  but not other kinds of fire?
• What are the links between climate, vegetation,
  and fire?
• What do they imply about fire regime response to
  climate change?
• What is the greatest source of uncertainty about
  wildfire under climate change?

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Federal Agencies Manage Most of the Wests Wild
Lands
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Since the mid-1980s Large Forest Wildfires Have
Increased 300
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Since the mid-1980s Large Forest Wildfires Have
Increased 300
Other Large Wildfires Have Not Changed Substantial
ly
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Why Has Forest Wildfire Increased?

• Management
• Grazing and fire suppression reduced burned area
• Reduced burned area led to biomass accumulation
  in some forests
• Increased biomass reduced the effectiveness of
  suppression efforts

• Climate
• Fire activity driven by antecedent moisture and
  concurrent drought
• Climatologic extremes have been more frequent in
  recent decades

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Consequences? Fuel accumulation reduces
effectiveness of suppression
1867 American River
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1993 American River
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biggest increase in wildfires risks least
affected by suppression
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Western U.S. Population, 1900 - 2020
Census Projection
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but total ignitions have not.
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Why Has Wildfire Activity Increased?

• Management
• fire suppression reduced burned area mid-20th
  century
• Reduced burned area led to biomass accumulation
  in some forests
• Increased biomass reduced the effectiveness of
  suppression efforts

• Climate
• Fire activity driven by antecedent moisture and
  concurrent drought
• Climatologic extremes have been more frequent in
  recent decades

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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
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Woodland Fraction
Forest Fraction
Grassland Fraction
Shrubland Fraction
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Mapping the Western US in terms of Temperature
and Precipitation
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Typical Summer Temperatures
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Warm Places
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Cool Places
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Typical Annual Precipitation
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Dry Places
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Wet Places
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Deserts
Forests
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How dry is the year when large fires burn?
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WET
DRY
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Warm, Dry Deserts Burn in Wet or Neutral Years
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Cool, Wet Forests Burn in Dry Years
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Was the year before the fire burned wet or dry?
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Dry Deserts and Grasslands burn after a wet year
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Cool, Wet Forests do not usually burn after wet
years
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WHY DO WE SEE THESE PATTERNS?
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Climate influences Wildfire through its
effects on Fuel Availability And Fuel
Flammability
Deserts
Forests
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vegetation is sparse here because the climate is
hot and dry so there is not so much
fuel and fire risks are sensitive to the
moisture available to grow more vegetation
Deserts
Forests
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Deserts
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.
Forests
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Grass/Shrub fires follow WET years
Forests burn when they are DRY
Preceding years
Year of Fire
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Percent of West in Drought or Wet Conditions
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Percent of West in Drought or Wet Conditions
Mean Western MAMJJA Temperature
Westerling
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Grass/Shrub Fires and Temperature
Correlation 0.08
Westerling
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Forest Fires Temperature
Correlation 0.70
Westerling
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Westerling et al, 2006 Science
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Forest Fires Timing of Spring
Correlation 0.56
Westerling
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Westerling
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Early - Late Mar to Aug Temp
Early - Late Oct to May Precip
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Most forest fires occur in years with early
Springs
at elevations around 7000 feet
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Late Snowmelt Years
Early Snowmelt Years
1972 - 2003, NPS, USFS BIA Fires over 1000 acres
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A2 PCM
A2 GFDL
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Different Impacts Within CA
The Greatest Increased CA Wildfire Risks
are Concentrated in Northern California Greater
Uncertainty for Wildfire Risks in Southern
California
Wetter Scenarios
Drier Scenarios
Increasing Temperature
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Warming has fostered earlier spring snowmelt and
more forest wildfire in the Western US Spring
and Summer average temperatures increased less
than 1C 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 5C Further
temperature increases will result in more very
active forest wildfire seasons
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Wildfires in fuel-limited regimes did not show
dramatic increases while variability increased,
there was not a clear trend in
precipitation these fires were not sensitive to
observed trends in temperature Uncertainty
about future precipitation in global change
models resulted in significant uncertainty about
the magnitude and direction of change in
fuel-limited wildfire regimes
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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)