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ABSTRACT: 1] Objective measures of climate model performance are proposed and used to assess simulations of the 20th century, which are available from the Coupled Model Intercomparison Project (CMIP3) archive. The primary focus of this analysis is on the climatology of atmospheric fields. For each variable considered, the models are ranked according to a measure of relative error. Based on an average of the relative errors over all fields considered, some models appear to perform substantially better than others. Forming a single index of model performance, however, can be misleading in that it hides a more complex picture of the relative merits of different models. This is demonstrated by examining individual variables and showing that the relative ranking of models varies considerably from one variable to the next. A remarkable exception to this finding is that the so-called ''mean model'' consistently outperforms all other models in nearly every respect. The usefulness, limitations and robustness of the metrics defined here are evaluated 1) by examining whether the information provided by each metric is correlated in any way with the others, and 2) by determining how sensitive the metrics are to such factors as observational uncertainty, spatial scale, and the domain considered (e.g., tropics versus extra-tropics). An index that gauges the fidelity of model variability on interannual time-scales is found to be only weakly correlated with an index of the mean climate performance. This illustrates the importance of evaluating a broad spectrum of climate processes and phenomena since accurate simulation of one aspect of climate does not guarantee accurate representation of other aspects. Once a broad suite of metrics has been developed to characterize model performance it may become possible to identify optimal subsets for various applications.
J. Geophys. Res. 01/2008; 113.
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B D Santer,
T M L Wigley,
C Mears,
F J Wentz,
S A Klein,
D J Seidel,
K E Taylor,
P W Thorne,
M F Wehner,
P J Gleckler, [......],
Q Fu,
J E Hansen,
G S Jones,
R Ruedy,
T R Karl,
J R Lanzante,
G A Meehl,
V Ramaswamy,
G Russell,
G A Schmidt
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ABSTRACT: The month-to-month variability of tropical temperatures is larger in the troposphere than at Earth's surface. This amplification behavior is similar in a range of observations and climate model simulations and is consistent with basic theory. On multidecadal time scales, tropospheric amplification of surface warming is a robust feature of model simulations, but it occurs in only one observational data set. Other observations show weak, or even negative, amplification. These results suggest either that different physical mechanisms control amplification processes on monthly and decadal time scales, and models fail to capture such behavior; or (more plausibly) that residual errors in several observational data sets used here affect their representation of long-term trends.
Science 10/2005; 309(5740):1551-6. · 31.20 Impact Factor
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B D Santer,
T M L Wigley,
G A Meehl,
M F Wehner,
C Mears,
M Schabel,
F J Wentz,
C Ammann,
J Arblaster,
T Bettge,
W M Washington,
K E Taylor,
J S Boyle,
W Brüggemann, C Doutriaux
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ABSTRACT: Two independent analyses of the same satellite-based radiative emissions data yield tropospheric temperature trends that differ by 0.1 degrees C per decade over 1979 to 2001. The troposphere warms appreciably in one satellite data set, while the other data set shows little overall change. These satellite data uncertainties are important in studies seeking to identify human effects on climate. A model-predicted "fingerprint" of combined anthropogenic and natural effects is statistically detectable only in the satellite data set with a warming troposphere. Our findings show that claimed inconsistencies between model predictions and satellite tropospheric temperature data (and between the latter and surface data) may be an artifact of data uncertainties.
Science 06/2003; 300(5623):1280-4. · 31.20 Impact Factor
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ABSTRACT: The apparent warming of Earth's surface during the 20 th century may be biased by large changes in the coverage of surface temperature measurements since 1900. We investigate this issue using climate model simulations. By imposing observed coverage changes on simulated surface temperatures, we obtain estimates of 20 th -century temperature-change for both full global coverage and for actual historical coverage. In 10 of 16 simulations including human climate perturbations, the temperature change from globally complete model output is significantly larger than that derived from historically-masked model output. The remaining 6 simulations show no significant difference between complete and masked model output. Thus, our results do not support the hypothesis that the increase in Earth's surface temperature has been overestimated due to incomplete observational data. Rather, if the simulations we analyzed are realistic, the true temperature increase over the last century is slightly larger than that estimated from available observations. We also analyzed 8 simulations of natural internal climate variability which omit human climate perturbations. In none of these simulations does the temperature change during 100 years---whether obtained from globally complete or masked model output---come close to the observed 20 th century temperature increase.
05/2001;
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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,
M. F. Wehner
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ABSTRACT: Estimated global-scale temperature trends at Earth's surface (as recorded by thermometers) and in the lower troposphere (as
monitored by satellites) diverge by up to 0.14°C per decade over the period 1979 to 1998. Accounting for differences in the
spatial coverage of satellite and surface measurements reduces this differential, but still leaves a statistically significant
residual of roughly 0.1°C per decade. Natural internal climate variability alone, as simulated in three state-of-the-art coupled
atmosphere-ocean models, cannot completely explain this residual trend difference. A model forced by a combination of anthropogenic
factors and volcanic aerosols yields surface-troposphere temperature trend differences closest to those observed.
Science 02/2000; 287(5456):1227-1232. · 31.20 Impact Factor
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B D Santer,
C Mears, C Doutriaux,
P Caldwell,
P J Gleckler,
T M L Wigley,
S Solomon,
N P Gillett,
D Ivanova,
T R Karl,
J R Lanzante,
G A Meehl,
P A Stott,
K E Taylor,
P W Thorne,
M F Wehner,
F J Wentz
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B D Santer,
R. Sausen,
T. M. L. Wigley,
J S Boyle,
K. AchutaRao, C. Doutriaux,
J. E. Hansen,
G A Meehl,
Erich Roeckner,
R. Ruedy,
G. Schmidt,
K E Taylor
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ABSTRACT: We examine changes in tropopause height, a variable that has hitherto been neglected in climate change detection and attribution studies. The pressure of the lapse rate tropopause, p(LRT), is diagnosed from reanalyses and from integrations performed with coupled and uncoupled climate models. In the National Centers for Environmental Prediction (NCEP) reanalysis, global-mean p(LRT) decreases by 2.16 hPa/decade over 1979-2000, indicating an increase in the height of the tropopause. The shorter European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis has a global-mean p(LRT) trend of -1.13 hPa/decade over 1979-1993. Simulated p(LRT) trends over the past several decades are consistent with reanalysis results. Superimposed on the overall increase in tropopause height in models and reanalyses are pronounced height decreases following the eruptions of El Chichon and Pinatubo. Interpreting these p(LRT) results requires knowledge of both T(z), the initial atmospheric temperature profile, and DeltaT(z), the change in this profile in response to external forcing. T( z) has a strong latitudinal dependence, as does DeltaT( z) for forcing by well-mixed greenhouse gases and stratospheric ozone depletion. These dependencies help explain why overall tropopause height increases in reanalyses and observations are amplified toward the poles. The pronounced increases in tropopause height in the climate change integrations considered here indicate that even AGCMs with coarse vertical resolution can resolve relatively small externally forced changes in tropopause height. The simulated decadal-scale changes in p(LRT) are primarily thermally driven and are an integrated measure of the anthropogenically forced warming of the troposphere and cooling of the stratosphere. Our algorithm for estimating p(LRT) (based on a thermal definition of tropopause height) is sufficiently sensitive to resolve these large-scale changes in atmospheric thermal structure. Our results indicate that the simulated increase in tropopause height over 1979-1997 is a robust, zero-order response of the climate system to forcing by well-mixed greenhouse gases and stratospheric ozone depletion. At the global-mean level, we find agreement between the simulated decadal-scale p(LRT) changes and those estimated from reanalyses. While the agreement between simulated p(LRT) changes and those in NCEP is partly fortuitous (due to excessive stratospheric cooling in NCEP), it is also driven by real pattern similarities. Our work illustrates that changes in tropopause height may be a useful "fingerprint'' of human effects on climate and are deserving of further attention.
Journal of Geophysical Research-Atmospheres, v.108 (2003).
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[show abstract]
[hide abstract]
ABSTRACT: The apparent warming of Earth's surface during the 20 th century may be biased by large changes in the coverage of surface temperature measurements since 1900. We investigate this issue using climate model simulations. By imposing observed coverage changes on simulated surface temperatures, we obtain estimates of 20 th -century temperature-change for both full global coverage and for actual historical coverage. In 10 of 16 simulations including human climate perturbations, the temperature change from globally complete model output is significantly larger than that derived from historically-masked model output. The remaining 6 simulations show no significant difference between complete and masked model output. Thus, our results do not support the hypothesis that the increase in Earth's surface temperature has been overestimated due to incomplete observational data. Rather, if the simulations we analyzed are realistic, the true temperature increase over the last century is slightly larger than that estimated from available observations. We also analyzed 8 simulations of natural internal climate variability which omit human climate perturbations. In none of these simulations does the temperature change during 100 years—whether obtained from globally complete or masked model output—come close to the observed 20 th century temperature increase.
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B.D. Santer,
R. Sausen,
T.M. Wigley,
J.S. Boyle,
K. AchutaRao, C. Doutriaux,
J.E. Hansen,
G.A. Meehl,
E. Roeckner,
R. Ruedy,
G. Schmidt,
K.E. Taylor
Journal of Geophysical Research. 108(2003-D1):ACL1-1-ACL1-22.
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B.D. Santer,
T.M.L. Wigley,
A.J. Simmons,
P. Kållberg,
G.A. Kelly,
S. Uppala,
C. Ammann,
J.S. Boyle,
W. Brüggemann, C. Doutriaux,
M. Fiorino,
C. Mears,
G.A. Meehl,
R. Sausen,
K.E. Taylor,
W.M. Washington,
M.F. Wehner,
F.J. Wentz
Journal of Geophysical Research. 109(2004):D21104.
