Download The new findings about the potential magnitude of heat

NOTES ON THE PNAS PAPER ON CLIMATE CHANGE IMPACTS IN
CALIFORNIA 8/17/04
Michael Hanemann
Chancellor’s Professor
Department of Agricultural & Resource Economics, and
Goldman School of Public Policy
Director, California Climate Change Center
University of California, Berkeley
This study takes results from the latest version of two global climate models that have
just been published to serve as input for the next international (IPCC) report on global
climate change that will appear two or three years from now, and translates them into
specific impacts in California.
The work, by a team of 19 researchers, appears in the Proceedings of the National
Academy of Sciences and can be found at
http://www.pnas.org/cgi/doi/10.1073/pnas.0404500101
From my perspective, there are two important messages from our research.
First, it is a mistake to think that California will not be experiencing some significant
adverse effects from climate change until the latter part of this century. The evidence now
indicates that, as a consequence of past emissions of greenhouse gasses, it is inevitable
that some significant adverse effects will be experienced in California within the next 30
years.
Second, because of the time lags in the global climate system, if we do not take action
now to reduce emissions of greenhouse gasses, the consequences for California after
about 2050 will become significantly more harmful than if we do take action now.
The new findings differ from results based on the previous generation of global climate
models in two ways. First, while it was long recognized that there will be an increase in
winter-time temperatures in California, it was assumed that the effect in the summer
would be no greater than in the winter; the new results indicate that summer temperatures
will increase sharply more than those in winter. Second, some of the previous models
suggested that climate change could actually bring an increase in our precipitation; the
new models are notably more pessimistic in this regard.
These changes will have serious consequences for people’s health and wellbeing in
California. The new PNAS study focuses specifically on two main areas: summer heatrelated mortality and water supply.
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Increased temperature
Under the study’s lower emissions scenario, summer temperatures in California will rise
4 - 8 degrees Fahrenheit by the end of the century. If nothing is done to curb our use of
fossil fuels, summer temperatures rise a dramatic 7.5-15 degrees Fahrenheit, according to
the study.
Average winter temperatures are projected to rise by 3.5 – 6 F by the end of the century
under low emissions scenario, and by 5.5 – 9 F under the higher emissions scenario
Those figures are higher than previous models had predicted, particularly in the summer
months.
Reduced snowpack
The hotter weather will trigger reductions in the Sierra Nevada snowpack, which feeds
into California’s streams and reservoirs. By the beginning of April in a "good" water
year, the total amount of water stored naturally in the Sierra snowpack just about matches
the total amount stored in the lakes behind our major reservoirs; thus, the snowpack
effectively doubles our ability to store water for warm-season uses.
By mid-century, the snowpack is projected to decline by about 25-40%. Toward the end
of the century, the loss of snowpack could reach 30-70% (3.5 – 9 MAF of storage) under
the lower emissions scenario, and a stunning 70 – 90% (9-11 MAF) under the higher
emissions scenario.
The combination of delayed snow accumulation and earlier snowmelt could shorten the
Sierra Nevada ski season by 3-6 weeks by 2050. Toward the end of the century, the ski
season would start 3-6 weeks later and end 4-9 weeks earlier under the lower emissions
scenario. Under the higher emission scenario, skiing with natural snow in the Sierras
would vanish except possibly at the very highest elevations.
Water supply impacts
Reduced stream flow: By mid-century, spring and summer (especially April – July)
stream flow is projected to decline by about 10-20% under the lower emissions scenario,
and 20-25% under the higher emissions scenario. Towards the end of the century, spring
and summer stream flow could be reduced as much as 40% under the lower emissions
scenario and 45-55% under the higher emissions scenario.
More frequent and more severe droughts: In the historical period, 31% of the years were
dry or critical. For the PCM model with low emissions, the proportion of years projected
to be dry or critical at the end of the century falls to about 8%, but under the high
emissions scenario, and for the Hadley model under both low and high emissions, the
proportion of years projected to be dry or critical rises to 50-64%. Moreover, the severity
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of some of the projected droughts would exceed what was experienced over the past
century.
Precipitation Impacts
Precipitation in California is something of a climate conundrum because it occurs within
a limited seasonal window; over about 120 days California must accumulate 2/3 of its
annual precipitation, and this tends to come episodically – a few discrete events deliver a
major fraction of the total precipitation. More so than anywhere else in the country, the
difference between a good precipitation year and bad year in California can be the arrival,
or not, of just a couple of large, wet storms.
This makes long-term forecasts of precipitation in California quite tricky. Another
complicating factor is the existence of natural oscillations in annual precipitation;
forecasting the precise timing of these oscillations is difficult.
The previous round of climate models produced quite divergent results for precipitation,
unlike temperature where there was relative consensus. The previous versions of the two
models used in our study showed in one case a sharp increase in total annual precipitation
in California, and in the other case a small reduction. The new versions of these models
show either no increase or a small reduction.
This is consistent with the general observation by climate scientists that a warmer climate
is likely to produce a dryer climate and greater frequency of dry spells
It should be noted that, even if there were to be a significant increase in total annual
precipitation in California, by itself this would not be an immediate benefit to
California’s water supply. This is because of the mismatch between the timing of
precipitation in California and the timing of water use. About three quarters of all water
use in California occurs in the late spring and summer, between April and September.
However, about 80% of California’s precipitation occurs between October and March.
Even if we were to get more rain from winter storms in February and March, say, this
does not translate into a corresponding increase in usable water unless we invest in more
storage and conveyance infrastructure, which is costly and problematic.
Caveat
These projections are likely to understate the severity of any future droughts because (1)
the climate scenarios themselves do not account for climate variability, (2) there is no
allowance for population growth which is expected to raise California’s population from
34 million in 2000 to about 90 million by the end of the century, and (3) the projected
increases in temperature could lead to increased summertime demand for water for
outdoor irrigation by both farmers and urban households. Another factor not being
considered is the possibility that, if adequate investments are not made to protect the
existing levies in the San Francisco Bay Delta, a future rise in sea levels might lead to the
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flooding of some of the Delta Islands, with adverse consequences for the amount of high
quality drinking water that can be supplied to urban users.
Note that the PNAS paper does not provide any detail regarding the water supply impacts
for individual water agencies in California, nor does it give an estimate of the economic
consequences for agricultural and urban water users in various parts of California. These
are being analyzed as part of the research under way at the California Climate Change
Center at UC Berkeley, funded by the California Energy Commission. This research is
being conducted by a team led by myself, Professor John Dracup in the Department of
Civil and Environmental Engineering at UC Berkeley, and Dr. Larry Dale, in the Energy
& Environment Division at Lawrence Berkeley National Laboratory.
For more information about the California Climate Change Center at UC Berkeley,
please go to http://gspp.berkeley.edu/programs/cccc.htm
Implications for California Water Policy
These forecasts are bad news for water managers in California.
The predicted reduction in the snowpack and the earlier timing of snowmelt will greatly
complicate the task of managing our reservoirs, and make for a more difficult tradeoff
between filling reservoirs to capture runoff for warm-season uses versus leaving empty
space for flood control in the event of a possible late winter storm.
The high emissions scenario implies the loss of a very substantial fraction of the State’s
effective water storage capacity towards the end of the century – natural plus manmade.
At the same time, the population of California then is likely to be at least twice what it is
now. Thus storage – whether above ground or below ground via conjunctive use -becomes much more valuable.
There is also a greater need for water conservation and smarter, more science-based
operation of water supply infrastructure. Water markets will play an increasing role. But
changes in the reliability of water rights resulting from altered stream flow could create
legal disputes that would need to be resolved before water rights could be transferred (see
next item).
The sharp reduction in late spring and summer stream flow could create havoc for the
state’s water rights system. Both water rights and also water supply contracts for the two
big water projects in California are heavily oriented around the traditional time pattern of
stream flow and water diversions. With the shrinking of the snowpack, the traditional
pattern of stream flow will no longer be there. This is going to create major complications
for water users. Because of incomplete record keeping for many water rights in California
there is a real potential for institutional gridlock.
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Reductions in surface water supplies could lead to increased pumping of groundwater by
farmers, but that resource is already overtaxed in many areas. Protecting groundwater
supplies from overuse and managing conjunctive use projects to prevent unauthorized
extractions and protect water quality will become a high priority.
Heat Waves
Rising temperatures also means more frequent heat waves and a longer heat wave season.
Using Los Angeles as an example, we found that the number of heat wave days could
increase 2-3 times before mid-century. By the end of the century, the length of the heat
wave season could increase from its current 4 months to 5 months under the lower
emission scenario, and 6-7 months under the higher emission scenario, with
corresponding increases in the number of heat wave days of 4 times under the lower
scenario and 6-8 times under the higher scenario.
The heat wave season will likely grow considerably longer, particularly in the coastal and
more southern locations. In Los Angeles, for example, the heat wave season is projected
to increase from about 14 weeks during the 1990s to about 19 to 25 weeks by the 2090s
under the lower-emissions scenario and 31 to 37 weeks—a total of nearly eight to nine
months—under the higher-emissions scenario. Much of the increase is due to heat waves
occurring earlier in the year, when people are more vulnerable to extreme-heat events
High-temperature extremes will likely rise and occur more often. Under the higheremissions scenario, for example, high temperatures that occurred on the 18 hottest days
of the year during the 1990s are projected to occur on as many as 90 to 130 days each
year during the 2090s. Under the lower-emissions scenario, such temperatures are
projected to occur on about 40 to 65 days each year
Increases in extreme heat have implications for human health. By 2090s, heat-related
deaths in Los Angeles could increase from an average of 165 per year during the 1990s
by 2-3 times under the lower scenario and 5-7 times, or ~800 to 1100 per year, under the
higher scenario.
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