Determining the Local
Implications of Global Warming
Clifford Mass
University of Washington
Global Warming 101
What is a Greenhouse Gas?
• A greenhouse gas is a gas that that is relatively
transparent to solar radiation, but absorbs and emits in
the infrared…the type of radiation the earth emits.
• Some examples:
– Water vapor
– Carbon dioxide
– Nitrous oxide
– Methane
Partly
(infrared)
Greenhouse Gases Make the Earth Warmer by Slowing the Loss of Infrared Radiation
The Problem:
Rapidly Rising Greenhouse
Gases Due to Mankind
Amplifiers of Global Warming
• There are a number of natural “amplifiers” of
mankind’s emission of greenhouse gases.
• The warming due to increased carbon
dioxide, methane, and other greenhouse gases
will cause more water to be evaporated from
the earth’s oceans.
• Water vapor is the most potent greenhouse
gas and thus causes even MORE
warming.
• This is called a positive feedback
Even more…
• Warming temperatures melt snow and ice.
• Snow and ice help cool the planet because they reflect much of
the sun’s radiation….that is why you need sunglasses while
skiing.
• As the snow melts, less radiation is reflected to space and more
is absorbed.
• Thus, the earth gets warmer, which melts more snow.
• Another positive feedback!
1979
2003
Is Global Warming Already
Happening?
• Finding the global warming signal is made more
difficult because the earth’s climate has a substantial
amount of natural variability.
• Also the warming due to man was relatively small
prior to the middle of last century before the large
increase in fossil fuel usage.
• The consensus of most atmospheric scientists is that
some global warming due to man is now evident.
Eleven of the twelve warmest years
in the past 150 years have
occurred in the past twelve years
(1995-2006)
But is greenhouse warming
occurring in the Northwest?
• Finding the signal is made more difficult
here by large interdecadal variability--e.g.,
the Pacific Decadal Oscillation.
Greenhouse Warming in the NW?
• Also there are large influences by the
regions high terrain and proximity to the
ocean. How do they change the story?
• There have been some major claims of large
local effects--e.g., snowpack reduction of
50%….but are they really true?
But what about the future?
The Technology of Prediction
• Atmospheric scientists use complex climate
prediction models…called General Circulation
Models…to predict the future climate.
• These models are similar to weather forecast
models, but allow the gases in the atmosphere
to change.
• They also simulate the evolution of the oceans.
Technology of Prediction
• Have to assume the future emission of
greenhouse gases by mankind…a major
uncertainty.
• These models are not perfect and cannot
exactly replicate the current climate….but
they are close enough for useful results and
getting better each year.
Which Scenario Will Mankind Follow?
Sample Climate Model Output for 2100
But what about the NW?
What are the implications of global
warming for the Northwest?
How will our mountains and land-water
contrasts alter the story?
Do global models tell us the full story?
Will there be any surprises?
Regional Climate Prediction
• As noted earlier, to understand the impact of
global warming, one starts with general
circulation models (GCMs) that provide a view
of the global evolution of the atmosphere.
• Even leading GCMs only describe features
of roughly 500 km or larger in scale.
•Northwest weather is
dominated by terrain
and land-water
contrasts of much
smaller scale.
•In order to understand
the implications of
global changes on our
weather, downscaling
of the GCM predictions
considering our local
terrain and land use is
required.
Model Topography and Resolution
MM5 Topo (15 km)
ECHAM5 Topo (150km)
Downscaling Is Needed
Downscaling
• The traditional approach to use GCM
output is through statistical downscaling,
which finds the statistical relationship
between large-scale atmospheric structures
and local weather.
• Statistical downscaling either assumes
current relationships will hold or makes
simplifying assumptions on how local
weather works.
Downscaling
Such statistical approaches may be a
reasonable start, but may give deceptive or
wrong answers… since the relationships
between the large scale atmospheric flow
and local weather might change in the
future.
Downscaling
• There is only one way to do this right…
running full weather forecasting models at
high resolution over extended periods, with
the large scale conditions being provided by
the GCMs
...this is called dynamical downscaling.
• Such weather prediction models have very
complete physics and high resolution, so
they are capable of handling any “surprises”
Downscaling
• Computer power and modeling
approaches are now powerful enough
to make dynamical downscaling
realistic.
• Takes advantage of the decade-long
work at the UW to optimize weather
prediction for our region.
UW Regional Climate Simulations
• Makes use of the same weather prediction
model that we have optimized for local
weather prediction: the MM5.
• 10-year MM5 model runs nested in the
German GCM (ECHAM).
• MM5 nests at 135 km, 45 km, and 15 km
model grid spacing.
MM5 Model Nesting
• 135, 45, 15 km MM5 domains
• Need 15 km grid spacing to model local weather features.
Regional Modeling
• Ran this configuration over
several ten-year periods:
• 1990-2000-to see how well the
system is working
• 2020-2030, 2045-2055, 20902100
Details on Current Study: GCM
• European ECHAM model with resolution roughly
equivalent to having grid points spaced ~ 150 km apart.
Can resolve features of roughly 600 km size or more.
• IPCC climate change scenario A2 -- aggressive CO2
increase (doubling by 2050)
IPCC Report, 2001
IPCC Report, 2001
Now, The Future
Why Such Strong Warming on
Mountain Slopes..Particularly in
Spring?
• Probable Answer: Snow melt
resulting in more solar heating.
Change in
Water
Of
Snowpack
(%)
Snow and Ice Reflect Much of
The Incoming Solar Radiation
Solar Radiation
Now
Global Warming Causes Snow level to Rise
Resulting In Absorption of Solar Energy on
Melted Slopes
Solar Radiation
Future
=WARMING
Why Relative Cooling West of
Cascades in Spring?
• Low clouds due to more onshore flow from
off the cool, cloud Pacific.
• The Montereyization of the western
lowlands!
Precipitation
• Bottom Line: Annual trends relatively
small, but some seasonal shifts
Early fall sees the greatest enhancement
Summary
• The viability of the approach…using high
resolution numerical prediction models forced
by large-scale general circulation climate
models (GCMs)… has been demonstrated.
• Careful evaluation of the GCM output is
required…there are deficiencies.
• Although there is general warming over the
region for all seasons, the terrain and land
water contrasts of the region enhance or
weaken the warming in certain areas.
Summary
• Warming is enhanced on the upper windward slopes
due to snow melt.
• Springtime warming is lessened west of the Cascade
crest due to more low clouds.
• Many more hot days during the summer.
• Precipitation changes are more modest then
temperature changes.
• There will be a substantial loss of snowpack,
reaching catastrophic decreases by 2090.
The END
Project Support
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King County
Seattle City Light
EPA STAR Program
Climate Impacts Group
We can evaluate our models by
simulating the last 100 years
Annual
Precipitation
Global Forcing: Surface Temperature
First things first
• But to make this project a reality we needed
to conquer some significant technical
hurtles.
• Example: diagnosing and predicting future
deep soil temperatures
• Example: requirements for acquiring GCM
output every 6 h and storing massive
amounts of output.
• Example: insuring long-term mass
conservations
• Evaluating the 1990-2000 simulations
Evaluating of Model Fidelity
• We have carefully evaluated how well the GCM
and the MM5 duplicated the 1990-2000 period.
• We previously had run the system using another
GCM…the Parallel Climate Model…with
unsatisfactory results….crazy cold waves during
the winter.
• ECHAM Model appears far better…but not
perfect.
1990-2000
Too Cold
• Cold episodes occurred 1-2 times per winter
with temperature getting unrealistically cold
(below 10F) in Puget Sound:
• Also a general cold bias to minima
• Better than previous attempts, but still a
problem.
Why Cold Outbreaks?
• Unrealistic surges of arctic air into NW in
ECHAM5, likely owing to poorly-resolved terrain
(Cascades and Rockies).
• Extreme cold air inherited by MM5.
• Results from previous experiments with lowerresolution (T42) GCM indicate that higher resolution
reduces frequency and severity of unrealistic cold
events.
• Can possibly fix by extending our 45-km domain
• Also problem in model physics--probably more
important on a daily basis
The Physics Fix
• Our research during the past few months
suggests the problem was a bug in the land
surface model.
• Fixed in the current version of model and
will be used in next production runs.
Evaluation of Future Runs
Because there are biases in the GCM runs,
results for future decades (2020s, 2040s,
and 2090s) will be evaluated against the
ECHAM5-MM5 1990-2000 baseline
But will have to be all redone once we are
sure we have addressed all the problems.