Estimating present climate in a warming world: a model-based approach |
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Authors: | Jouni Räisänen Leena Ruokolainen |
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Institution: | (1) Department of Physics, Division of Atmospheric Sciences and Geophysics, University of Helsinki, P.O. Box 64, 00014 Helsinki, Finland |
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Abstract: | Weather services base their operational definitions of “present” climate on past observations, using a 30-year normal period
such as 1961–1990 or 1971–2000. In a world with ongoing global warming, however, past data give a biased estimate of the actual
present-day climate. Here we propose to correct this bias with a “delta change” method, in which model-simulated climate changes
and observed global mean temperature changes are used to extrapolate past observations forward in time, to make them representative
of present or future climate conditions. In a hindcast test for the years 1991–2002, the method works well for temperature,
with a clear improvement in verification statistics compared to the case in which the hindcast is formed directly from the
observations for 1961–1990. However, no improvement is found for precipitation, for which the signal-to-noise ratio between
expected anthropogenic changes and interannual variability is much lower than for temperature. An application of the method
to the present (around the year 2007) climate suggests that, as a geographical average over land areas excluding Antarctica,
8–9 months per year and 8–9 years per decade can be expected to be warmer than the median for 1971–2000. Along with the overall
warming, a substantial increase in the frequency of warm extremes at the expense of cold extremes of monthly-to-annual temperature
is expected. |
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Keywords: | Climate change Present climate Climate normals Probability distribution CMIP3 |
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