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B.
D. Santer, C. Doutriaux, J. S. Boyle, J. J. Hnilo, K. E. Taylor, and M.
F. Wehner, PCMDI, Lawrence Livermore National Laboratory
T. M. L. Wigley and G. A. Meehl, National Center forAtmospheric Research
D. J. Gaffen, National Oceanic and Atmospheric Administration Air Resources
Laboratory
L. Bengtsson, M. Esch, and E. Roeckner, Max Planck Institute for Meteorology,
Hamburg, Germany
P. D. Jones, University of East Anglia, Norwich, UK
Research Objectives
The principal
mission of the Program for Climate Model Diagnosis and Intercomparison (PCMDI)
is to develop improved methods and tools for the diagnosis, validation,
and intercomparison of global climate models and to engage in research on
a wide variety of outstanding problems in climate modeling and analysis.
In this project, we tested various interpretations of the apparent difference
between estimated global-scale temperature trends at Earth's surface (as
recorded by thermometers) and in the lower troposphere (as monitored by
satellites).
Computational
Approach
We performed
three tests — NOMASK, VARMASK, and FIXMASK — to investigate
the effect of coverage differences on the estimated trend differential
between two data sets: the satellite-based Microwave Sounding Unit (MSU)
tropospheric temperature data, and the Intergovernmental Panel on Climate
Change (IPCC) surface data. We then analyzed data from 300-year control
integrations performed with three models — ECHAM4/OPYC, PCM, and
CSM — to explore whether the residual trend difference of roughly
0.1 °C per decade could be explained by natural variability of the
climate system on decadal time scales. Finally, we used a set of three
perturbation experiments to test the effects of external forcing.
Accomplishments
Our results
show that the observed difference between surface and tropospheric temperature
changes cannot be fully explained by coverage differences between satellite-
and surface-based measurement systems and/or the effects of natural internal
climate variability. However, we find that both effects may make substantial
contributions to the observed trend difference. A recent model result
suggests that the observed warming of the surface relative to the lower
troposphere may be a response to combined forcing by well-mixed greenhouse
gases, sulfate aerosols, stratospheric ozone, and the effects of the Pinatubo
eruption in June 1991. Further simulations of the climate of the past
two decades are needed to determine the precise causes of the temperature
trend difference.
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Least-squares
linear trends and associated 95% confidence intervals in modeled
and observed surface, 2LT (lower troposphere), and surface — 2LT
temperature time series (panels A, B, and C, respectively). Observed
trends and confidence intervals over 1979—1998 were computed with
annual-mean, spatially-averaged IPCC surface and MSUd 2LT data,
using FIXMASK masking. Model-based results are from experiments
with anthropogenic and natural forcings performed with ECHAM. Model
data were processed such that results are given for only one 20-year
period in GSDIO ("1979—1998" in model years) and for one 19-year
period in GSO1, GSO2, and GSOP ("1979—1997" in model years). Model
results are also based on FIXMASK sampling. Confidence intervals
are adjusted to account for temporal autocorrelation in the data.
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Significance
Understanding
the difference between surface and tropospheric temperature trends is
crucial for modeling climate, explaining and attributing climatic changes,
and planning for future climate monitoring.
Publications
B. D. Santer, T. M. L. Wigley, D. J. Gaffen, L.
Bengtsson, C. Doutriaux, J. S. Boyle, M. Esch, J. J. Hnilo, P. D. Jones,
G. A. Meehl, E. Roeckner, K. E. Taylor, and M. F. Wehner, "Interpreting
differential temperature trends at the surface and in the lower troposphere,"
Science 287, 1227 (2000).
http://www-pcmdi.llnl.gov/
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