Comparing Tropospheric Warming in Climate Models and Satellite Data

Abstract

Updated and improved satellite retrievals of the temperature of the mid-to-upper troposphere (TMT) are used to address key questions about the size and significance of TMT trends, agreement with model-derived TMT values, and whether models and satellite data show similar vertical profiles of warming. A recent study claimed that TMT trends over 1979 and 2015 are 3 times larger in climate models than in satellite data but did not correct for the contribution TMT trends receive from stratospheric cooling. Here, it is shown that the average ratio of modeled and observed TMT trends is sensitive to both satellite data uncertainties and model–data differences in stratospheric cooling. When the impact of lower-stratospheric cooling on TMT is accounted for, and when the most recent versions of satellite datasets are used, the previously claimed ratio of three between simulated and observed near-global TMT trends is reduced to approximately 1.7. Next, the validity of the statement that satellite data show no significant tropospheric warming over the last 18 years is assessed. This claim is not supported by the current analysis: in five out of six corrected satellite TMT records, significant global-scale tropospheric warming has occurred within the last 18 years. Finally, long-standing concerns are examined regarding discrepancies in modeled and observed vertical profiles of warming in the tropical atmosphere. It is shown that amplification of tropical warming between the lower and mid-to-upper troposphere is now in close agreement in the average of 37 climate models and in one updated satellite record.

Troposphere‐Stratosphere Temperature Trends Derived From Satellite Data Compared With Ensemble Simulations From WACCM

“…MSU2 trends for global means and for the deep tropics (20°N‐S) are included in Table , comparing RSS, STAR, and WACCM ensemble results; while the satellite‐derived trends are systematically smaller than in the model, the statistical uncertainties do overlap at the 2 sigma level, for example, 20°N‐S MSU2 trends for WACCM are 0.23 ± 0.03 K/decade, versus 0.18 ± 0.05 for RSS and 0.21 ± 0.06 for STAR data. We note that our derived observational MSU2 trends for 1979–2014 are slightly larger than results in Mears and Wentz () and Santer et al (), although they overlap within uncertainties; these differences occur because of different details in the regression methodologies. This is discussed further in section 4.…”

“…Global average tropospheric warming, as measured by observed 1979–2014 trends in MSU2, is somewhat smaller than corresponding MSU2 trends simulated by WACCM, although the differences are not statistically significant (Table ). These observed versus model differences for MSU2 are similar to previous results based on comparisons with other climate models (Santer et al, , and references therein), although our comparisons reveal only statistically insignificant trend differences for the forced WACCM simulations. The excellent agreement for WACCM is at least partly attributable to forcing using observed SSTs, which tightly constrain tropospheric temperatures, as compared to free‐running atmosphere‐ocean climate models (e.g., CMIP5).…”

“…Tropospheric (MSU2) warming is larger in WACCM than in the observations, which is a difference seen in similar comparisons of numerous climate models (e.g., Santer et al, ). MSU2 trends for global means and for the deep tropics (20°N‐S) are included in Table , comparing RSS, STAR, and WACCM ensemble results; while the satellite‐derived trends are systematically smaller than in the model, the statistical uncertainties do overlap at the 2 sigma level, for example, 20°N‐S MSU2 trends for WACCM are 0.23 ± 0.03 K/decade, versus 0.18 ± 0.05 for RSS and 0.21 ± 0.06 for STAR data.…”

“…Our work builds on similar evaluations of model simulations from the Coupled Model Intercomparison Project (CMIP5) (Taylor et al, ) and the Stratospheric Processes and their Role in Climate (SPARC) Chemistry‐Climate Model Validation activity (CCMval) (Stratospheric Processes and their Role in Climate, ), such as shown in Forster et al (), Gillett et al (), Mitchell (), Seidel et al (), Thompson et al (), and Zhao et al (). The focus of these latter papers is primarily on the stratospheric temperature behavior, as a complement to detailed studies of tropospheric evolution (such as analyzed in Santer et al, , and references therein). Novel aspects of our study include evaluations spanning both the troposphere and stratosphere, based on recently updated and improved satellite data sets, and focus on the variability within an ensemble of simulations from a single chemistry‐climate model, constrained with observed forcings but free to evolve from slightly different initial conditions.…”

Lower Tropospheric Temperatures 1978-2016: The Role Played By Anthropogenic Global Warming

“…They demonstrated a major improvement in signal to noise going from ten year to 32 years trends, and concluded that a minimum of seventeen years was required for identification of human effects on temperature. Subsequently Santer et al (2017) analysed the middle troposphere satellite record for the time period used here, concluding that the claim of no significant tropospheric warming over the last eighteen years is not correct. What is concluded in those analyses reinforces the conclusions reached here about the inappropriateness, in the face of internal climate variability, of trying to evaluate anthropogenic contributions to global temperature using truncated temperature data sets.…”