Transcript Seattle Public Library Talk

Decadal Trends
in Extreme
Precipitation,
Winds, and
Snowpack over
the Northwest.
Cliff Mass
University of
Washington
There has been a lot discussion
in the media and in some of the
literature suggesting that
western U.S. extreme
precipitation events have already
increased in intensity or will soon
increase under anthropogenic
global warming
Press Advisory
National Wildlife Federation
November 17, 2009
Contacts:
XXXX (name removed)
Senior Environmental Policy Specialist
National Wildlife Federation, Pacific Region
YYYY (you know her!)
Climate Scientist
National Wildlife Federation
… Global warming is exacerbating extremely heavy
rainfall events, and recent climate change modeling
suggests that these pineapple express storms are no
exception. “Heavier rainfall events combined with significant
snowmelt in midwinter is just what the Pacific Northwest
should expect from global warming,” said Dr. YYYY, climate
scientist, National Wildlife Federation.
…. Recent data[1] suggests that as a result of climate
change, the largest storms (i.e., those that are most likely
to cause flooding) will trend toward producing
increasingly larger quantities of precipitation.
Others Have Suggested We Are
Now Experiencing Extremes in Low
Northwest Snowpack and That
Anthropogenic Warming is A
Significant Cause
XXXXXXXXX
And some have suggested that cold
season storms, like our big
windstorms, will also get stronger
“There is also evidence of an increase in the intensity of storms in
both the mid- and high-latitude areas of the Northern Hemisphere,”
State X
This talk will suggest that some of
these claims are inconsistent with
observations and the latest
modeling results.
The truth is more complex and
nuanced.
Our credibility is at stake
• Anthropogenic global warming IS a serious
problem that must be dealt with.
• But by claiming excessive impacts on extremes
we undermine our credibility and our ability
to enhance society’s efforts to reduce
greenhouse gas emissions and to precede
with adaptation/mitigation.
Extreme Precipitation over the NW
There are several studies in the
literature
Trends of 7-day extreme (1 yr or >): 1931-96
3 and 1-day extremes were similar
Kunkel,
Andsager,
and
Easterling,
J. of
Climate,
1999
Little trend in the NW. Suggesting of small increases in
western WA and decreases in western Oregon (tail
indicates significant at 5% level)
When it Rains it Pours (not reviewed) Used the
Kunkel Approach for 1-day rainfall (1948-2006)
• More over Wa, LESS over Oregon, little trend,
N. CA.
Are there trends in
major precipitation
events?
• Examined top 20,
40, 60 two-day
precipitation events
at stations along
the coast for 19502008.
Trends on
Unregulated
Rivers
1950-2009:
Max Annual
Daily Discharge
Most modeling studies are
inconclusive about whether global
warming has influenced trends of
extreme precipitation during the
past century.
They ran contemporary dynamical downscaling (19702007):
• Hadley regional model forced by Hadley Center GCM
• WRF forced by ECHAM5 GCM
“The lack of correspondence between observed
and simulated trends for extreme precipitation
likely results from the dominance of natural
variability over anthropogenic trends during the
period 1970-2007.
Nearly are all heavy Northwest precipitation events are
associated with the “Pineapple Express” (a.k.a.
atmospheric rivers)
A relatively narrow current of warm, moist air from the
subtropics…often starting near or just north of Hawaii.
Atmospheric Rivers
• Atmospheric rivers are closely associated with
the jet stream (on its southern side)
• Most General Circulation Models suggest the
jet stream will move north and weaken under
global warming.
• Thus, the atmospheric rivers may well move
northward.
• Could our region thus experience a DECREASE
in extreme precipitation?
Regional and Global Climate
Simulations Indicate Substantial
Uncertainty for Extreme
Precipitation over the Northwest
Under Global Warming
• Some even suggest a decline in winter
precipitation
DJF
Simulated seasonal precipitation changes (mm/day) from 1989-1999 to 2045-2055 from the ECHAM5MM5 regional model for a) DJF b) MAM c) JJA and d) SON
Other Research on Future
Precipitation Trends over the NW:
No Agreement
• Chen et al. (2003) examined the impacts of
doubling CO2 using MM5 and RegCM2
regional models to downscale the CCSM GCM
simulation.
– Found an increase in average precipitation over
central California and a decline over the Pacific
Northwest.
Studies
• Kim (2005) used the MAS regional model
downscaled from the HadCM2 GCM for 1990–2000
and 2040–49. Found the largest increases in extreme
precipitation over northern California and Oregon,
with little trend over Washington and southern
California.
• Tebaldi et al. (2006) examined nine general
circulation models found a general increase of
extreme precipitation over the NW, with the
magnitude of the positive trend increasing to the
north, while decreasing or constant intensity was
predicted over central and southern California.
Studies
• Duffy et al. (2006) used four regional climate
models nested within two global ocean–
atmosphere climate models, finding that the
spatial distributions of precipitation vary
substantially.
Extreme Precipitation Bottom Line
• The uncertainty of what will happen over the
NW is huge.
• Simplistic talk about warming temperatures,
causing more water vapor resulting in more
extreme precipitation here should be avoided.
West Coast Windstorms
• Increasing number of major windstorms from
northern Oregon into southern BC
• Decreasing numbers to the south.
West Coast Windstorms
• Northwest windstorms are closely associated
with the strength and position of the jet
stream.
• If the jet stream weakens and moves
northward, what will happen to our winter
storms?
• Will the decline in windstorms over the
southern PNW move northward?
Snowpack
Little Hint of Decline Due to
Anthropogenic Warming
1950-2000
Washington-wide snowpack since 1950,
Relative to 1971-2000 normal
Snowpack in usual maximum month
Washington-wide snowpack period of record,
Relative to 1971-2000 normal
Snowpack in usual maximum month
April 1 Snowpack
Remove Natural Variability
Slow steady decline over the past 80 years—no indication of
anthropogenic global warming signal
The NW is a location in which AGW
temperature effects will be weaker
and delayed
1976-2007 temperature trends
Conclusions
• Changes in weather extremes over the
Northwest have been highly heterogeneous,
increasing some places and decreasing in
others.
• Natural variability appears to be dominant.
• There is no compelling evidence that
anthropogenic warming has had any influence
on changes in Northwest weather extremes.
Conclusions
• The future impact of anthropogenic global
warming on NW extremes is highly uncertain.
• It may well be that extremes in precipitation
and winds could decline over the NW under
global warming.
• We must be very careful in communicating
such uncertainties to our user communities.
The End
60year
Trends
for
Top-60
Events
Individual
Stations
Two-degree
bands