Identification of external influences on temperatures in California

We use nine different observational datasets to estimate California-average temperature trends during the periods 1950-1999 and 1915-2000. Observed results are compared to trends from a suite of climate model simulations of natural internal climate variability. On the longer (86-year) timescale, inc...

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Bibliographic Details
Published in:Climatic change 2008-03, Vol.87 (1), p.43-55
Main Authors: Bonfils, Céline, Duffy, Philip B, Santer, Benjamin D, Wigley, Tom M. L, Lobell, David B, Phillips, Thomas J, Doutriaux, Charles
Format: Article
Language:English
Subjects:
Aerosols
Atmospheric circulation
Atmospheric Sciences
Climate change
Climate Change/Climate Change Impacts
Climate models
Climate variability
Cooling
Datasets
Earth and Environmental Science
Earth Sciences
Global warming
Greenhouse effect
Greenhouse gases
Noise
Simulation
Surface temperature
Temperature
Trends
Urbanization
Weather
Winter
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Summary:We use nine different observational datasets to estimate California-average temperature trends during the periods 1950-1999 and 1915-2000. Observed results are compared to trends from a suite of climate model simulations of natural internal climate variability. On the longer (86-year) timescale, increases in annual-mean surface temperature in all observational datasets are consistently distinguishable from climate noise. On the shorter (50-year) timescale, results are sensitive to the choice of observational dataset. For both timescales, the most robust results are large positive trends in mean and maximum daily temperatures in late winter/early spring, as well as increases in minimum daily temperatures from January to September. These trends are inconsistent with model-based estimates of natural internal climate variability, and thus require one or more external forcing agents to be explained. Observational datasets with adjustments for urbanization effects do not yield markedly different results from unadjusted data. Our findings suggest that the warming of Californian winters over the twentieth century is associated with human-induced changes in large-scale atmospheric circulation. We hypothesize that the lack of a detectable increase in summertime maximum temperature arises from a cooling associated with large-scale irrigation. This cooling may have, until now, counteracted summertime warming induced by increasing greenhouse gases effects.
ISSN:0165-0009
1573-1480
DOI:10.1007/s10584-007-9374-9