Pervasive Warming Bias in CMIP6 Tropospheric Layers

 Here is a link to a paper by McKitrick and Christy that shows that the climate models continue to be wrong.  In particular, the IPCC climate sensitivity values are too high.

Here are some excerpts.

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PLAIN LANGUAGE SUMMARY

It has long been known that previous generations of climate models exhibit excessive warming rates in the tropical troposphere. With the release of the CMIP6 (Coupled Model Intercomparison Project Version 6) climate model archive we can now update the comparison. We examined historical (hindcast) runs from 38 CMIP6 models in which the models were run using historically observed forcings. We focus on the 1979–2014 interval, the maximum for which all models and observational data are available and for which the models were run with historical forcings. What was previously a tropical bias is now global. All model runs warmed faster than observations in the lower troposphere and midtroposphere, in the tropics, and globally. On average, and in most individual cases, the trend difference is significant. Warming trends in models tend to rise with the model Equilibrium Climate Sensitivity (ECS), and we present evidence that the distribution of ECS values across the model is unrealistically high.

INTRODUCTION

Numerous studies have pointed to a tendency across climate models to project too much contemporary warming in the tropical troposphere (Bengtsson & Hodges, 2009; Douglass et al., 2007; Fu et al., 2011; Karl et al., 2006; McKitrick et al., 2010; McKitrick & Vogelsang, 2014; Po‐Chedley & Fu, 2012; Thorne et al., 2011) with additional evidence pointing to a global tropospheric bias as well (Christy & McNider, 2017). Here we present an updated comparison using the first 38 models made available in the newly released sixth‐generation Coupled Model Intercomparison Project (CMIP6) archive comparing model reconstructions of historical layer‐averaged lower‐troposphere (LT) and midtroposphere (MT) temperature series against observational analogs from satellites, balloon‐borne radiosondes, and reanalysis products. We compare trends over 1979–2014, the longest interval for which all three observational systems are available and for which models were run with historically observed forcings. None of our conclusions would be different if we extended the end date to 2018. We examine four atmospheric regions: the global LT and MT and the tropical LT and MT layers.

In previous studies, although a warm bias was typically present, over large atmospheric regions the model spread at least partly overlapped the observational analogs, especially at the global level. This is no longer the case. Every model overpredicts warming in both the LT and MT layers, in the tropics, and globally. On average the discrepancies are statistically very significant, and the majority of individual model discrepancies are statistically significant as well.

CONCLUSION

The literature drawing attention to an upward bias in climate model warming responses in the tropical troposphere extends back at least 15 years now (Karl et al., 2006). Rather than being resolved, the problem has become worse, since now every member of the CMIP6 generation of climate models exhibits an upward bias in the entire global troposphere as well as in the tropics. The models with lower ECS values have warming rates somewhat closer to observed but are still significantly biased upward and do not overlap observations. Models with higher ECS values also have higher tropospheric warming rates, and applying the emergent constraint concept implies that an ensemble of models with warming rates consistent with observations would likely have to have ECS values at or below the bottom of the CMIP6 range. Our findings mirror recent evidence from inspection of CMIP6 ECSs (Voosen, 2019) and paleoclimate simulations (Zhu et al., 2020), which also reveal a systematic warm bias in the latest generation of climate models.