Interpretation of Recent Temperature Trends in California Page: 4 of 7
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Warming measured by thermometer ,is consistent with findings from other types Deleted: seen in
of temperature indicators. California river flows have shifted towards earlier in the year Deleted: measurements
(Stewart et al., 2005), indicating earlier melting of snow, and a shift in precipitation from
snow to rain. Snow water content has declined (Mote, 2004), and certain plant species are
blooming earlier in Spring (Cayan et al., 2001). Loss of snow and altered river flows
indicate warming specifically in the higher elevations, in contrast to the findings of
Christy et al. (2006).
What is causing these trends? Bonfils et al. (2006a) showed that rapid increases in
observed daily average and daily minimum temperatures in California exceed those
possible from natural internal climate variability alone, as estimated by global climate
models. This means that some external factor(s)-. g. urbanization or greenhouse Deleted:
warmingxmust be acting to increase temperatures in California. Deleted:
Models are of limited help in identifying these factor(s), or in interpreting observed
temperature trends in California. Global model simulations of the 20th century archived
the IPCC database at LLNL do not in general reproduce observed temperature trends in
California (Bonfils et al, 2006a). While all simulations show warming, none reproduce
the observed seasonal or diurnal cycle of temperature increases. However, these
simulations do not treat all relevant forcings: none includes effects of irrigation and few
include other types of land-use changes (such as urbanization), most use simplified
treatments of only
limited types of aerosols. (Agricultural aerosols, for example, are omitted.) Some
regionally-varying forcings are not characterized well enough to allow them to be
represented in climate models. The biggest problem, however, is probably the coarse
spatial resolutions used in the IPCC simulations, which prevent simulation of the
complex topography and ocean-land interactions in California. (For example, coarse
resolution will reduce or eliminate any snow-albedo feedback, since snow amounts are
much less than observed, and may be zero.) Also, coarse resolution
makes it difficult to represent regionally-varying forcings or responses to them.
Thus, even ignoring possible inherent inadequacies in how models represent relevant
physical processes, it is hardly surprising that these simulations do not accurately
reproduce observed temperature trends in California. Finer resolution global or limited
domain models should be able to better represent regional climate forcings, processes,
and responses; however most of the simulations that would be needed to interpret
observed trends in California have not been performed. To guide interpretation of
observed trends, simulations of individual forcings (as have been performed with the
DOE/NCAR Parallel Climate Model
(PCM) (EXPLAIN ACRTONYM) global model), as well as simulations of multiple
forcings in combination, are
Based on the apparent contrast between temperatures trends found in the Central Valley
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Duffy, P B; Bonfils, C & Lobell, D. Interpretation of Recent Temperature Trends in California, article, September 21, 2007; Livermore, California. (digital.library.unt.edu/ark:/67531/metadc899789/m1/4/: accessed January 16, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.