Transcript Document

SEEMS TO ME, THE
ICE USED TO BE A
LOT THICKER THIS
TIME OF YEAR…
YOU MEAN DESPITE
THE MOUNTAIN OF
HARD EVIDENCE,
YOU STILL DON’T
BELIEVE IT?
OH, HERE WE GO AGAIN…
ANOTHER BORING GLOBAL
WARMING RANT
OH, PLEASE… CAN
YOU POINT TO
ANYTHING THAT
SHOWS IT’S AS
DANGEROUS TO ME
AS THEY CRACK IT UP
TO BE?
Effects of global warming on humans
(interpreted broadly)
Wildfires have consumed increasing
areas of western U.S. forests in
recent years, and fire-fighting
expenditures by federal landmanagement agencies now regularly
exceed US$1 billion/year. Hundreds
of homes are burned annually by
wildfires, and damages to natural
resources are sometimes extreme and
irreversible.
Recent, very large wildfires (>100,000
ha) burning in western forests have
garnered widespread public attention,
and a recurrent perception of crisis
has galvanized legislative and
administrative action.
A few examples…
Oakland Hills Fire in October 1991 destroyed 2,550 single-family homes, and 37
apartment and condominium units, and was one of the most deadly fires in
recent history, killing 25 people including one police officer and one firefighter.
It caused an estimated $1.5 billion in direct damage and burned 1,600 acres.
In 2003 and again in 2007
there were enormous wildfires
near San Diego, each at the
end of especially dry years.
Were the brutal San Diego wildfires directly caused by global warming?
Princeton’s Michael Oppenheimer put it this way:
The weather we’ve seen… may or may not be due to the global warming trend,
but it’s certainly a clear picture of what the future is going to look like if we
don’t act quickly to cut emissions of the greenhouse gases.
The next few slides are modified from
an important paper that links wildfires to
global warming:
Warming and Earlier Spring Increase
Western U.S. Forest Wildfire Activity
A. L. Westerling, H. G. Hidalgo, D. R.
Cayan, T. W. Swetnam
Science 313, 940-943 (2006).
Forest wildfire activity in the western US is widely
thought to have increased in recent decades, yet neither
the extent of recent changes nor the degree to which
climate may be driving regional changes in wildfire has
been systematically documented.
Much of the public and scientific discussion of changes in
wildfire frequency has focused instead on the effects of
19th- and 20th-century land-use history.
Which is most responsible for the recent increases?
Competing explanations: Land-use versus climate
Land-use
•Fire frequency reduced by livestock grazing and fire suppression
methods in the 20th century
• Forest regrowth after extensive logging and absence of extensive
fires increased the fuel load, making it harder to fight fires. Thus
they are bigger and last longer.
Climatic explanations
•Increasing variability in moisture conditions (wet/dry oscillations
promoting biomass growth, then burning),
•And/or a trend of increasing drought frequency,
•And/or warming temperatures
have led to increased wildfire activity
In some cases these may be complementary.
Increased forest
wildfire activity.
The incidence of
large wildfires
(>400 ha) in
western forests
increased in the
mid-1980s
Subsequently,
wildfire frequency
was nearly four
times the average
of 1970 to 1986,
and the total area
burned was more
than six times its
previous level.
Wildfire frequency
is strongly
associated with
spring and summer
temperature.
The frequency of
fires also
correlates with the
timing of the spring
snowmelt. Early
melting is related
to temperature.
Overall, 56% of
wildfires and 72%
of area burned in
wildfires occurred
in early snowmelt
years. Only 11% of
wildfires and 4% of
area burned
occurred in late
snowmelt years.
The length of the
wildfire season also
increased in the
1980s, both
because of earlier
ignition and later
control of fires.
Regardless of past trends, virtually all climate-model
projections indicate that warmer springs and summers will
occur over the region in coming decades. These trends will
reinforce the tendency toward early spring snowmelt and
longer fire seasons. This will accentuate conditions favorable
to the occurrence of large wildfires, amplifying the
vulnerability the region has experienced since the mid-1980s.
The Intergovernmental Panel on Climate Change's consensus
range of 1.5° to 5.8°C projected global surface temperature
warming by the end of the 21st century is considerably larger
than the recent warming of less than 0.9°C observed in spring
and summer during recent decades over the western region.
Current estimates indicate that western U.S. forests are responsible
for 20 to 40% of total U.S. carbon sequestration. If wildfire trends
continue, much of this carbon will be released, suggesting that the
forests of the western United States may become a source of
increased atmospheric carbon dioxide rather than a sink, even under
a relatively modest temperature-increase scenario.
Hence, the projected regional warming and consequent increase in
wildfire activity in the western United States is likely to magnify the
threats to human communities and ecosystems, and substantially
increase challenges in reducing greenhouse gas emissions.
This is a good example of positive feedback that
accelerates global warming.
• increased temperature increases fires;
• fires release CO2;
• CO2 increases temperature.
The interaction of increased
temperatures, the mountain pine
beetle, and both lodgepole and jack
pine trees provides another good
example of positive feedback.
The adult mountain pine beetle
(Dendroctonus ponderosae) is less than one
centimeter long. Damaged trees turn red.
By the end of 2006, the
mountain pine beetle
(Dendroctonus ponderosae) had
ravaged 130,000 square
kilometers of forest in western
Canada.
Though not the first time an
outbreak has occurred in the
region, the latest is an order of
magnitude larger than any
previous attack and brings the
total area of forest destroyed
between 1997 and 2007 to 13
million hectares.
Beetle populations have exploded because a series of mild winters has
allowed the larvae to survive from one year to the next. There is now
concern that they will be able to cross the Rocky Mountains and
infest timber in the east.
Not only is this bad news for the affected trees, whose fate is sealed once
the beetle takes hold; the infestation also packs an atmospheric punch.
According to a recent report (Kurz et al, (2008) Nature 452, 987-990) the
assault on British Columbia's pine trees could cause the region to release
more carbon dioxide than it absorbs from the atmosphere over the coming
decade.
With fewer healthy trees available to absorb the greenhouse gas and more
trees decaying and dying, this will further contribute to the warming that is
facilitating the pest's territorial spread.
According to the new calculations, by 2020 the beetle outbreak alone will
have released 270 megatonnes of carbon dioxide into the atmosphere.
That's exactly the amount of greenhouse gas emissions that Canada is
committed to reducing by 2012 under the Kyoto Protocol. And given that
Canada is far from meeting that target, it may be even harder than once
thought for the nation to offset its emissions through forest management.
Stating the results another way, the predicted emissions are larger than
the total CO2 absorbed by all of Canada's managed forest over the last
decade.
Extending the positive feedback cycle further, pines
infested by the beetle are much more susceptible to
wildfires. Because the fires can spread into adjacent
uninfested forest, they will cause even more CO2
release.
CO2
warming
beetles
dead trees
fire
decay
live trees