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IPCC AR6: Tropical Troposphere warm bias, unspun edition

22 Sep 2021 | Science Notes

In honour of our video on Big Trouble in the Tropical Troposphere, which is closing in on 30,000 views in its first month online, we turn to the AR6 discussion of the discrepancy between models and observations in that part of the atmosphere. To understand why it matters a great deal, have a look at our video. To see if the problem has gone away or gotten worse, we turn to the latest IPCC report. As always, except for replacing some author lists with (---) and spelling out some short forms to improve readability, what follows is the IPCC report verbatim, this time from Chapter 3 Section 3.3.1.2.

The AR5 assessed with low confidence that most, though not all, CMIP3 (Meehl et al., 2007) and CMIP5 (Taylor et al., 2012) models overestimated the observed warming trend in the tropical troposphere during the satellite period 1979-2012, and that a third to a half of this difference was due to an overestimate of the Sea Surface Temperature trend during this period (Flato et al., 2013). Since the AR5, additional studies based on CMIP5 and CMIP6 models show that this warming bias in tropospheric temperatures remains. Recent studies have investigated the role of observational uncertainty, the model response to external forcings, the influence of the time period considered, and the role of biases in Sea Surface Temperature trends in contributing to this bias.

Several studies since AR5 have continued to demonstrate an inconsistency between simulated and observed temperature trends in the tropical troposphere, with models simulating more warming than observations (---). Santer et al. (2017b) used updated and improved satellite retrievals to investigate model performance in simulating the tropical mid- to upper-troposphere trends, and removed the influence of stratospheric cooling by regression. These factors were found to reduce the size of the discrepancy in mid- to upper- tropospheric temperature trends between models and observations over the satellite era, but a discrepancy remained. Santer et al. (2017a) found that during the late 20th century, the discrepancies between simulated and satellite-derived mid- to upper-tropospheric temperature trends were consistent with internal variability, while during most of the early 21st century, simulated tropospheric warming is significantly larger than observed, which they relate to systematic deficiencies in some of the external forcings used after year 2000 in the CMIP5 models. However, in CMIP6, differences between simulated and observed upper tropospheric temperature trends persist despite updated forcing estimates (Mitchell et al., 2020). ... Several studies using CMIP6 models suggest that differences in climate sensitivity may be an important factor contributing to the discrepancy between the simulated and observed tropospheric temperature trends (---), though it is difficult to deconvolve the influence of climate sensitivity, changes in aerosol forcing and internal variability in contributing to tropospheric warming biases (---). Another study found that the absence of a hypothesized negative tropical cloud feedback could explain half of the upper troposphere warming bias in one model (---).

… In summary, studies continue to find that CMIP5 and CMIP6 model simulations warm more than observations in the tropical mid- and upper-troposphere over the 1979-2014 period (---), and that overestimated surface warming is partially responsible (---). Some studies point to forcing errors in the CMIP5 simulations in the early 21st century as a possible contributor (---), but CMIP6 simulations use updated forcing estimates yet generally still warm more than observations. Although accounting for internal variability and residual observational errors can reconcile models with observations to some extent (---), some studies suggest that climate sensitivity also plays a role (---). Hence, we assess with medium confidence that CMIP5 and CMIP6 models continue to overestimate observed warming in the upper tropical troposphere over the 1979-2014 period by at least 0.1°C per decade, in part because of an overestimate of the tropical Sea Surface Temperature trend pattern over this period.

3 comments on “IPCC AR6: Tropical Troposphere warm bias, unspun edition”

  1. Latest trend from UAH 0.13C per decade. So, the models overestimate by at least 77%! As Supertramp said, Crisis, what Crisis

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