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B. D. Santer,
K. E. Taylor, P. J. Gleckler,
C. Bonfils,
T. P. Barnett,
D. W. Pierce,
T. M. L. Wigley,
C. Mears,
F. J. Wentz,
W. Brüggemann,
N. P. Gillett,
S. A. Klein,
S. Solomon,
P. A. Stott,
M. F. Wehner
[show abstract]
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ABSTRACT: In a recent multimodel detection and attribution (D&A) study using the pooled results from 22 different climate models, the
simulated “fingerprint” pattern of anthropogenically caused changes in water vapor was identifiable with high statistical
confidence in satellite data. Each model received equal weight in the D&A analysis, despite large differences in the skill
with which they simulate key aspects of observed climate. Here, we examine whether water vapor D&A results are sensitive to
model quality. The “top 10” and “bottom 10” models are selected with three different sets of skill measures and two different
ranking approaches. The entire D&A analysis is then repeated with each of these different sets of more or less skillful models.
Our performance metrics include the ability to simulate the mean state, the annual cycle, and the variability associated with
El Niño. We find that estimates of an anthropogenic water vapor fingerprint are insensitive to current model uncertainties,
and are governed by basic physical processes that are well-represented in climate models. Because the fingerprint is both
robust to current model uncertainties and dissimilar to the dominant noise patterns, our ability to identify an anthropogenic
influence on observed multidecadal changes in water vapor is not affected by “screening” based on model quality.
Proceedings of the National Academy of Sciences 08/2009; 106(35):14778-14783. · 9.68 Impact Factor
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[show abstract]
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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,
C Mears,
F J Wentz,
K E Taylor, P J Gleckler,
T M L Wigley,
T P Barnett,
J S Boyle,
W Brüggemann,
N P Gillett,
S A Klein,
G A Meehl,
T Nozawa,
D W Pierce,
P A Stott,
W M Washington,
M F Wehner
[show abstract]
[hide abstract]
ABSTRACT: Data from the satellite-based Special Sensor Microwave Imager (SSM/I) show that the total atmospheric moisture content over oceans has increased by 0.41 kg/m(2) per decade since 1988. Results from current climate models indicate that water vapor increases of this magnitude cannot be explained by climate noise alone. In a formal detection and attribution analysis using the pooled results from 22 different climate models, the simulated "fingerprint" pattern of anthropogenically caused changes in water vapor is identifiable with high statistical confidence in the SSM/I data. Experiments in which forcing factors are varied individually suggest that this fingerprint "match" is primarily due to human-caused increases in greenhouse gases and not to solar forcing or recovery from the eruption of Mount Pinatubo. Our findings provide preliminary evidence of an emerging anthropogenic signal in the moisture content of earth's atmosphere.
Proceedings of the National Academy of Sciences 10/2007; 104(39):15248-53. · 9.68 Impact Factor
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ABSTRACT: Observations show both a pronounced increase in ocean heat content (OHC) over the second half of the 20th century and substantial OHC variability on interannual-to-decadal time scales. Although climate models are able to simulate overall changes in OHC, they are generally thought to underestimate the amplitude of OHC variability. Using simulations of 20th century climate performed with 13 numerical models, we demonstrate that the apparent discrepancy between modeled and observed variability is largely explained by accounting for changes in observational coverage and instrumentation and by including the effects of volcanic eruptions. Our work does not support the recent claim that the 0- to 700-m layer of the global ocean experienced a substantial OHC decrease over the 2003 to 2005 time period. We show that the 2003-2005 cooling is largely an artifact of a systematic change in the observing system, with the deployment of Argo floats reducing a warm bias in the original observing system.
Proceedings of the National Academy of Sciences 07/2007; 104(26):10768-73. · 9.68 Impact Factor
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B D Santer,
T M L Wigley, P J Gleckler,
C Bonfils,
M F Wehner,
K Achutarao,
T P Barnett,
J S Boyle,
W Brüggemann,
M Fiorino,
N Gillett,
J E Hansen,
P D Jones,
S A Klein,
G A Meehl,
S C B Raper,
R W Reynolds,
K E Taylor,
W M Washington
[show abstract]
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ABSTRACT: Previous research has identified links between changes in sea surface temperature (SST) and hurricane intensity. We use climate models to study the possible causes of SST changes in Atlantic and Pacific tropical cyclogenesis regions. The observed SST increases in these regions range from 0.32 degrees C to 0.67 degrees C over the 20th century. The 22 climate models examined here suggest that century-timescale SST changes of this magnitude cannot be explained solely by unforced variability of the climate system. We employ model simulations of natural internal variability to make probabilistic estimates of the contribution of external forcing to observed SST changes. For the period 1906-2005, we find an 84% chance that external forcing explains at least 67% of observed SST increases in the two tropical cyclogenesis regions. Model "20th-century" simulations, with external forcing by combined anthropogenic and natural factors, are generally capable of replicating observed SST increases. In experiments in which forcing factors are varied individually rather than jointly, human-caused changes in greenhouse gases are the main driver of the 20th-century SST increases in both tropical cyclogenesis regions.
Proceedings of the National Academy of Sciences 10/2006; 103(38):13905-10. · 9.68 Impact Factor
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ABSTRACT: We have analysed a suite of 12 state-of-the-art climate models and show that ocean warming and sea-level rise in the twentieth century were substantially reduced by the colossal eruption in 1883 of the volcano Krakatoa in the Sunda strait, Indonesia. Volcanically induced cooling of the ocean surface penetrated into deeper layers, where it persisted for decades after the event. This remarkable effect on oceanic thermal structure is longer lasting than has previously been suspected and is sufficient to offset a large fraction of ocean warming and sea-level rise caused by anthropogenic influences.
Nature 03/2006; 439(7077):675. · 36.28 Impact Factor
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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
[show abstract]
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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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W L Gates,
J S Boyle,
C Covey,
C G Dease,
C M Doutriaux,
R S Drach,
M Fiorino, P J Gleckler,
J J Hnilo,
S M Marlais,
T J Phillips,
G L Potter,
B D Santer,
K R Sperber,
K E Taylor,
D N William
Bull. Amer, Meteor. Soc. 81:29-55.
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ABSTRACT: A suite of climate model experiments indicates that 20th Century increases in ocean heat content and sea-level ( via thermal expansion) were substantially reduced by the 1883 eruption of Krakatoa. The volcanically-induced cooling of the ocean surface is subducted into deeper ocean layers, where it persists for decades. Temporary reductions in ocean heat content associated with the comparable eruptions of El Chichon ( 1982) and Pinatubo ( 1991) were much shorter lived because they occurred relative to a non-stationary background of large, anthropogenically-forced ocean warming. Our results suggest that inclusion of the effects of Krakatoa ( and perhaps even earlier eruptions) is important for reliable simulation of 20th century ocean heat uptake and thermal expansion. Inter-model differences in the oceanic thermal response to Krakatoa are large and arise from differences in external forcing, model physics, and experimental design. Systematic experimentation is required to quantify the relative importance of these factors. The next generation of historical forcing experiments may require more careful treatment of pre-industrial volcanic aerosol loadings.
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[show abstract]
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ABSTRACT: Observations show the oceans have warmed over the past 40 yr. with appreciable regional variation and more warming at the surface than at depth. Comparing the observations with results from two coupled ocean-atmosphere climate models [the Parallel Climate Model version 1 (PCM) and the Hadley Centre Coupled Climate Model version 3 (HadCM3)] that include anthropogenic forcing shows remarkable agreement between the observed and model-estimated warming. In this comparison the models were sampled at the same locations as gridded yearly observed data. In the top 100 m of the water column the warming is well separated from natural variability, including both variability arising from internal instabilities of the coupled ocean-atmosphere climate system and that arising from volcanism and solar fluctuations. Between 125 and 200 m the agreement is not significant, but then increases again below this level, and remains significant down to 600 m. Analysis of PCM's heat budget indicates that the warming is driven by an increase in net surface heat flux that reaches 0.7 W m(-2) by the 1990s; the downward longwave flux increases bv 3.7 W m(-2). which is not fully compensated by an increase in the upward longwave flux of 2.2 W m(-2). Latent and net solar heat fluxes each decrease by about 0.6 W m(-2). The changes in the individual longwave components are distinguishable from the preindustrial mean by the 1920s, but due to cancellation of components. changes in the net surface heat flux do not become well separated from zero until the 1960s. Changes in advection can also play an important role in local ocean warming due to anthropogenic forcing, depending, on the location. The observed sampling of ocean temperature is highly variable in space and time. but sufficient to detect the anthropogenic warming signal in all basins, at least in the surface layers, bv the 1980s.
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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
