Download Summary_of_extremes_SIO_Sasha

Expected Changes in Key
Weather-related Extreme Events
in
California
Presented by: Alexander Gershunov (Scripps/UCSD)
Co-authors: Mike Dettinger (USGS)
Ed Maurer (Santa Clara University)
Michael Mastrandrea (Stanford)
Alex Hall (UCLA)
Norm Miller (LBNL)
Lisa Sloan (UC Santa Cruz)
Kelly Redmond (WRCC/DRI)
Guido Franco (CEC)
Robin Webb (NOAA)
David Pierce (Scripps/UCSD)
Dan Cayan (USGS and Scripps/UCSD)
Kristen Guirguis (Scripps/UCSD and UCAR-PACE)
Heat Waves are Already Changing in
California
INDIVIDUAL HEAT WAVES
The heat wave of
July 2006 was an
unprecedented
deadly event
4
6
daytime magnitude
nighttime magnitude
0
2
California heat wave
activity is increasing
1950
1960
1970
1980
1990
2000
8
TOTAL HEAT WAVE ACTIVITY
6
Day > Night magnitude
Specifically, humid,
nighttime-accentuated
heat waves are on the
rise
0
2
4
Night > Day magnitude
1950
1960
1970
1980
1990
HEAT WAVES ARE BECOMING
MORE HUMID!
2000
Gershunov, Cayan and Iacobellis, 2009, Journal of Climate
California Heat Waves and
Global Climate Change: CNRM
PAST
PRESENT
FUTURE
8
50
ONE CLIMATE MODEL UNDER THE MILD EMISSIONS “B1” SCENARIO
20
0
2
30
4
40
6
observations
1950
1960
1970
1980
1990
2000
Night > Day magnitude
0
10
Day > Night magnitude
1900
1950
2000
2050
2100
California Heat Waves and
Global Climate Change: CNRM
PAST
PRESENT
FUTURE
8
50
ONE CLIMATE MODEL UNDER THE MILD EMISSIONS “B1” SCENARIO
SUGGESTS THAT THE OBSERVED CHANGE IS A TIP OF THE ICEBERG
20
0
2
30
4
40
6
observations
1950
1960
1970
1980
1990
2000
Night > Day magnitude
0
10
Day > Night magnitude
1900
1950
2000
2050
2100
Mean Warming and
Temperature
Extremes
SUMMERTIME DAILY TMAX
Along the
coast, extreme
heat is
projected to
become more
extreme
relative to
mean
temperatures.
ºC
Gershunov and Guirguis, work in progress
Mean Warming and
Temperature
Extremes
JANUARY TMIN
• The high end of the
distribution would
experience more
pronounced
warming
• Very cold
temperatures (low
end of the
distribution) would
not change
• Average
temperatures (e.g.,
Tmin average for
Jan) are not a good
Warm Tmin
extremes are
projected to
become more
extreme
relative to
mean
temperatures
everywhere
Pierce et al. 2011. Probabilistic estimates of California climate change by the 2060s using statistical and dynamical downscaling
Water Resources and
Flood Regimes
MOST PRECIPITATION FALLS
IN THE MOUNTAINS AND
CURRENTLY AS SNOW: HERE
WE HAVE A TREND!
Snow-to-Rain conversion changes
the frequency, intensity and timing
of floods
Warming affects the
type of precipitation
historically:
“Cool” storms
contribute
immediate runoff
from smaller areas
of the river basin
(the rest goes into
snowpack for later)
In a warmer climate:
Less snow,
more rain
Warm storms
contribute
immediate runoff
from larger areas of
the river basin
1°C (1.8 °F) warming causes snowlines to rise 500 feet
More frequent and intense rainfall-driven floods in
winter, fewer snowmelt-driven floods in spring
Das et al. (2011) project increases in both the magnitudes of annual-peak
floods and in the frequencies of generally high flows from both the Northern
and South-Central Sierra. The reasons for those increases reflect
some combination of effects from increased storm
intensities, warmer storms, more rain,
and even wetter winter soils.
Das et al., 2011, Climatic Change
COLORADO RIVER DROUGHT
•Drought, as expressed in Colorado River flow, is projected to become more
frequent, more intense and longer-lasting, resulting in water deficits not seen
during the instrumental record. This is due primarily to the effects of warming,
e.g. declining snowpack, earlier melt, and enhanced evapotranspiration.
•The Southern 2/3 of California is expected to be more drought-prone than in
the recent past.
•Northern watersheds in the Sierra Nevada, however, may become wetter
with climate change.
Cayan et al. 2010, PNAS
Santa Ana Winds
Santa Ana winds of coastal
Southern California are expected
to diminish in frequency and
intensity, but at the same time
become drier and hotter.
Hughes, Hall and Kim, 2011, Climatic Change
Looking ahead
• The concept of frequency and intensity of extremes is key.
• Extreme impacts often involve a convergence of different causal mechanisms, e.g.
heat and humidity.
• Global change does not automatically equate to increased storminess or increases in
other measures.
• It is crucial to sustain high quality observations, not only of physical climate
measures but also of key impact variables.
• Models are invaluable tools in looking forward. Careful scrutiny of model
simulations is needed to ascertain if models make realistic results for the "right"
reason. This requires mechanistic understanding.
• Finer scale climate and atmospheric models, which perform realistically, are needed
to elucidate processes and occurrence of extreme impact events. These models are
still being developed.
Summary of Extremes
•
•
•
•
•
•
•
Heat waves: A tendency towards not only stronger, longer lasting and more intense, but also more
humid heat waves and specifically accentuated nighttime temperatures is observed and projected for
California. In coastal regions, heat waves may become more intense even relative to the mean seasonal
warming.
Cold spells might not necessarily decline much in intensity.
Storms: Global change does not automatically equate to increased storminess or increases in other
measures. Taking a global view, storminess changes vary considerably, and along the low mid latitudes
of the North Pacific and the southern 2/3 of the California coast storminess may actually decline.
However, the intensity and frequency of very intense Atmospheric Rivers is projected to increase.
Floods generated by atmospheric rivers on the western slopes of the Sierra Nevada range are expected
to increase in intensity and in frequency in winter. It is also expected that snowmelt-driven spring and
summertime floods will diminish in both frequency and intensity.
Drought, as expressed in Colorado River flow, is projected to become more frequent, more intense and
longer-lasting, resulting in water deficits not seen during the instrumental record. This is a highconfidence result. The Southern 2/3 of California is expected to be more drought-prone than in the
recent past. Northern watersheds in the Sierra Nevada, however, may become wetter with climate
change.
Santa Ana winds of coastal Southern California are expected to diminish in frequency and intensity, but
at the same time become drier and hotter.
Sea level rise is expected to cause more frequent and intense coastal erosion and salt-water intrusion
and extreme impacts on coastal infrastructure, tourism and water resources.