Dan Cayan

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Climate Change challenges looming for
western water resouces
Western States Water Council, Irvine, CA Oct 15,
2006
Dan Cayan Scripps
Institution of Oceanography, Climate Research
Division and US Geological Survey
with input from Mike Dettinger
Iris
Stewart
Noah Knowles sponsors NOAA OGP
RISA element California Energy Comm PIER
program http//meteora.ucsd.edu/cap
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What changes are evident from observations?
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The result of the natural Greenhouse effect can
be seen by comparing the temperature of the
moon with that of the earth which are about the
same distance from sun but moon is 33C (60F)
cooler
The average surface temperature of the moon is
-18C. The moon, of course, has no atmosphere.
. By contrast, average surface temperature of
the Earth is 15C.
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Humans have greatly Influenced the Atmosphere
during the Industrial Era
Source IPCC TAR 2001
While these are relatively low concentrations,
these trace gases are critical because they
absorb infrared radiation and thus change the
temperature of the atmosphere and the earths
surface
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Observations suggest that global temperatures
have already risen at a extremely rapid pace.
Northern Hemisphere temperatures
Mann et al., 2000
1990s warmest decade in instrumental record
(NASA/NOAA) 1. 1998 warmest year 2. 2002 3.
2003 4. 2004 2005 2nd (or 1st) warmest
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Very broad winter and spring warming 1950-1997
California warmed 0.5-1.5C
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• To put these projected levels of of warming into
  perspective SENSITIVITY OF HYDROCLIMATE TO A
  3ºC WARMING

What fraction of each years precipitation
historically fell on days with average
temperatures just below freezing?
YOSEMITE
More vulnerable
Less vulnerable
Rain vs Snow
In Yosemite, detailed simulations suggest 30
historically --gt 45 by 2100
Computed by Mike Dettinger from gridded
historical US weather data (from Bates et al, in
rev)
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historically Cool storms contribute immediate
runoff from smaller areas of the river basin (the
rest goes into snowpack for later)
runoff
In a warmer climate Warm storms contribute
immediate runoff from larger areas of the river
basin
runoff
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Earlier spring flows last 2-3 decades
center time
Center Timing of snowmelt watersheds have
advanced by 1-5 weeks earlier across West
Iris Stewart
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Downward Trends in April 1 Snow Water Equivalent
1950-1997
Source Mote et al. (2004)
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Snow losses (1950-2000) have been greatest
in lower and middle elevation zones
where warming is most effective
Phil Mote, U. Washington
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More Rain Less Snow WY 1949-2004
Winter (Nov-Mar) SFE/P trends at western
US weather stations symbol area is proportional
to study-period changes, measured in standard
deviations as indicated circles indicate high
trend significance (plt0.05), squares indicate
lower trend significance (pgt0.05).
Noah Knowles et al. 2006 in press J. Climate
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since 1985 the number of large wildfires in
western U.S. increased by 4X
Anthony Westerling et al. Science August 2006
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Large wildfire threat is aggravated by
warmer springs and summers Anthony
Westerling et al. Science August 2006
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Springs with earlier snowmelt have produced
summers with higher wildfire
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What are recent climate model simulations
telling us?
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But, while future emissions will continue, their
levels could be could be more or could be less,
and climate would respond accordingly Global CO2
Emissions ScenariosIntergovernmental Panel on
Climate Change (IPCC)
Higher (A1fi)
Previous (IS92a)
Lower (B1)
PNAS study Hayhoe et al Summer 2004
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We face significant losses of spring snowpack

• Less snow, more rain
• Particularly at lower elevations
• Earlier run-off
• More floods
• Less stored water

By the end of the century California could lose
half of its late spring snow pack due to climate
warming. This simulation by Noah Knowles is
guided by temperature changes from PCMs
Business-as-usual coupled climate simulation.
(a middle of the road emissions scenario) better
understanding of present day and future snow
accumulation and melt processes is needed
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6 different climate models
Strong concensus for warming But large
uncertainty with precipitation
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Projections of Precipitation Trends Precipitation
generally projected to increase at high
latitudes decrease in subtropics patchy
uncertain in between (e.g., thruout lower 48)
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Observed SFO (left) and modeled Global (right).
Sea level rise estimates based upon an envelope
of output from several GHG emission scenarios
Projected envelope of global s.l. rise
observed
Climate models Only provide loose guidance
on The amount of sea level rise, but It is very
likely that rates will increase
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Seasonally intensified warming some
models suggest amplified summer warming does
this bear out in land surface models? great
implications for Ag!
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Climate models project ocean warming by end of
century of 1.5-2.C greater warming on land than
oceans would amplifythermal gradient across
California coast-interior In GCMs, Summer
land-surface warming is accentuatedcan we
resolve this and understand changes in sea-land
circulations and associated phenomena?
GFDL CM2.1 Jun-Aug air temp change 2070-2099
minus 1961-1990
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• OBSERVATIONS AND MODELS INDICATE
• Humans have altered atmospheric composition and
  thus are altering the earths climate GH gases
  have long lifetimes, so choices made now and in
  future will determine future climate.
  Warming already underway and coming fast.
• Western temperature projections are broadly in
  consensus (2 to 6ºC by 2100),
• Warming would produce more rain, less snow,
  earlier flows, more floods, higher sea level,
  drier summers.
• California precipitation projections scattered,
  with MOST showing small (drier?) changes but a
  couple of wet outliers.
• Shoulders of watershed elevations at 6000-8000
  would generate
• more immediate runoff
• Better monitoring and modeling crucially needed.

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(No Transcript)
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The extinction of the cool part of the
temperature distribution
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Douglas Alden UCSD-SIO 2005
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Ocean Beach , February 1983
Extreme storm-forced sea levels during an extreme
tide
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Projected total exceedances of San Francisco
hourly sea level height (SLH) above historical
99.99 percentile (black), and number that are
coincident with sea level pressure anomalies less
than -7mb, Projected sea level from GFDL model
weather and Nino3.4 SST with a linear trend of
30cm over 2000-2100.
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Changes in Sierra Nevada Outflow, 2060 2100
and, in many settings, more severe winter
floods
Knowles and Cayan, 2004 http//www.cgd.ucar.edu/c
as/ACACIA/workshops/precip/dettinger.pdf
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Historical Floods in Yosemite
Winter warm storms have produced largest floods
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Other major floods
New Years 1997
Large dots Major winter floods Open circles
Warm-wet storms that didnt yield floods (mostly,
soils too dry)
NOTE About half of the extremely warm-wet winter
storms have yielded major floods.
From http//www.cgd.ucar.edu/cas/ ACACIA/workshops
/precip/dettinger.pdf
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Simulated Merced River Flood Responses to Global
Warming--using a climate model that has unusually
(among climate models) large precipitation events
in response to global warming ---gt A whole
new population of winter floods!
Observed timing size of floods, 84 yrs
Simulated HISTORICAL timing size of floods, 400
yrs
Simulated FUTURE timing size of floods, 400 yrs
From Wilby Dettinger, 2001
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Flood Responses to Global Warming? Broad
mid-altitude shoulders of the range are going to
be critical! Warm storms put rain up onto
extremely flat, broad mid-altitude shoulders.
6500-7500 1980-2300 m 7500-8500 2300-2600 m
Hetch Hetchy
Mono Lake
Tioga Rd (120)
Yosemite Valley
You are here
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400,000 years of temperatures, carbon dioxide and
methane
Pushing a climate system well beyond its
norms..
From Hansen, Climatic Change 2005, based on
Petit, Nature 1999 (slide borrowed from Brad
Udall, CIRES Western Water Assessment)
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Global Energy Balance The overall state of the
global climate is determined by the balance
between energy the Earth receives from the Sun
and the energy which the Earth releases back to
space. Absorption of long wave Terrestrial
radiation by gasses in the atmosphere causes a
greenhouse effect.
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The Greenhouse Effect