Tim Barnett
February,
2002
Question:What
is the impact of global warming on water resources inthe
Western United States?
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Slide
Number
Description
1
ACPI title page showing PIs.Note
3 DOE labs, 3 Universities, 1 DOD lab, USGS and NCAR participated in the
project.
2
Project schematic.(Top)Observed
ocean state of the mid-1990s is(Middle)used
to initialize CGCM forced by BAU scenario.This is
gives data(precip,
snow,temperature,
etc.) that is
(Bottom)downscaled to 40km grid
over the western US for subsequent impact analysis.Impacts:The
lower right panels
shows the large decrease in Sierra snow pack in 2050.The
lower left panel shows the large reduction in annual mean snow pack in
the Pacific Northwest.
3
The ocean initialization of the climate model is necessary since the oceans
have already begun to warm as illustrated in this slide which shows the
temperature change to a depth of 2000m in each of the major oceans relative
to 1900. Taking
this warming into
account avoids the ‘cold start’ problem.
4
Strength of the global ocean warming signal from PCM(Parallel
Climate Model) forced with observed/BAU anthropogenic forcing beginning
in 1870 relative the signal predicted in 2100.Three
different realizations are shown(horizontal
bars) for decadal average signal strength.Blue/greens
are from observed/estimated GHG emissions, while yellow/reds are from BAU
scenarios.Observed signal strength
from ocean data assimilation for6
years in the mid-1990s is shown with green circles.The
model captured the observed warming quite well.
5
Does the PCM produce realistic simulation of present climate?If
not, how could we believe its predictions of the future climate?Slide
shows the observed and PCM simulated winter
temperature and precipitation fields over the US, especially the western
1/3 which is the subject of this study.Conclusionde:The
model, while not perfect, does a good job of reproducing the observations.Most
CGCMs do not do as well.
6
The need for downscaling:The PCM
grid (280 km) overlaid on the geographic drainage basins of interest..Note
California is only two grid points wide and so does not resolve the Sierra.
7
The high resolution grid (40 km) of the downscaling routines now does crudely
resolve major geographic features.This
is important for simulating orographic precip and snow effects.
8
Results show downscaling adds a lot of realism to the PCM results.The
PNNL-MM5 and RSM results for summer and winter precipitation are compared
with the PCM field used to run the two downscaling models.
9
Verification of the two down scaling models (RSM and PNNL-MM5) against
observed (PRISM) data.The models
were initialized with reanalysis products from NCEP and/or ECMWF.The
results show the two models capture well the observed monthly, annual and
interannual variations in the precip and temperature fields over the main
basins of interest.Note that the
models do better in winter than in summer and
that neither model is capable of simulating the SW monsoon.But
since the main precip period important for this study occurs during the
winter/spring these are not serious flaws for this study.
10
Areal comparison of PNNL-MM5 and RSM with observations of mean daily rainfall
and the 95% extrema of the daily rainfall pdf for the winter and summer
seasons.Again, both models do a
good job of reproducing the observations.Reanalysis
data was used to forced
the regional models.
11 Estimating
anthropogenic impacts on water resources.The
flow diagram shows the downscaled ‘weather’ data forcing hydrological models
which in turn give information on snowpack, run off and streamflow.
13
RESULTS:The snow water equivalent(SWE)
for the Sierra (top) compared with the BAU simulation 1,2,…,5 decades into
the future.Note the large reduction
in future snow pack and the large decade-to-decade variability predict 14
RESULTS:The reduced snow pack is
reflected in the reduced Spring time run off in the California drainage.Reductions
of 30-50% are typical. 15
RESULTS:The calendar day of maximum
runoff is shown.Note it retreats
nearly one month in the annual cycle in the next 40-50 years. This is particularly
true for rivers fed by snow melt from intermediate elevations such as the
Carson River.Rivers fed by higher
elevation snows (American River) show less impact.Retreats
of a few days to a week have been already observed relative to 1970 (not
shown) over most of western N.
America.The point is that this retreat
is already in progress. 16
RESULTS:The stream flow problems
lead to biological problems.Due
to increased warming, the salmon of the PNW will have to delay their run
upstream until the waters cool sufficiently with the onset of Fall.But
the freshet (maximum stream flow associated with snow melt) needed to flush
the young smolt out of the rivers in the Spring of 2050 comes a month earlier
than present.Essentially the reproductive
cycle of these endangered
fish is compressed (lower time line arrow).Can
they adapt to this change within 50 years?If
not, they will
become extinct in the PNW. 17
RESULTS:The Sierra snow melt (backdrop)
feeds streams (blue lines) which inturn
flow into the Delta where they eventually encounter the salinewater
pumped into the San Francisco Bayfrom
the ocean. The inter- 18
RESULTS:$ impacts and management….the
Sacramento System natural inflows now and over the next 50 years.Note
the reduction in water volume. 19
RESULTS:If the above water reduction
is met with forced 50% reduction in water demand in California’s Central
Valley in what are dry years, then current
agricultural demands would experience a 730 thousand acre feet. short fall
or 23% less than is needed,
in spite of the mandatory reduction.The
associated current
metropolitan and industrial (MI)demand
will exceed what is available by 17%.Population
growth in the central valley is projected at approximately 8 million in
the next 20 years. The increased water demand of this growth and projected
supply reduction are on a collision course.This
is a bleak economic picture for California’s future. 20
RESULTS:Assuming the 50% enforced
reduction in water usage will still lead to a reduction in winter-time
hydro power production of order 20% (blue line).The
monthly $ loss associated with the water reduction (bars) is of order 20
million dollars/month during the winter. 21
RESULTS:The summer-time wildfire
activity in the Sierra will increase by approximately 50% by 2050.This
is due to reduction in summer soil moisture associated with the earlier
seasonal snow melt. Basically the fire season will start sooner and last
longer. SUMMARY
The
impacts will manifest themselves in a number of ways.The
snow pack will melt earlier.As a
result, maximum Spring-time streamflows
will happen about one month earlier in the calendar year.This
causes numerous water resource problems.It
also impacts the life cycle of salmon in the PNW, as well as the entire
ecosystem of the Sacramento Delta.Additional
impacts will be found in reduced ability to generate hydro electric power
in the winter and large increases in wildfire activity in the summer. The
above results are particularly disturbing since the computer model from
which they originally spring is known to have a relatively low sensitivity
to GHGs.This means the above statements
should be considered as conservative; if anything, they are under estimates
of what will happen.ied
by the model.mix
of the fresh and saline waters creates an unique and richbiological
environment.The reduced summer
flow will increase salinity in the entire California delta region(right)
compared to what it is today (left).The
increase is so large that it will invoke a biological disaster, plus effect
trans-California water distributions.