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Testing the Tropical 200-300 mbar Warming Rate in Climate Models

A paper by Dr. Ross McKitrick and Dr. John Christy published in Earth and Space Science tests a major component of climate models, being the warming of the tropical mid-troposphere at 200 to 300 mbar pressure corresponding to 9 to 12 km altitude, and finds that the models used in the last IPCC report show a large, statistically significant warming bias. This layer is where models exhibit the clearest and strongest response to greenhouse warming. The authors show that a failure of the models to reproduce the actual warming of the tropical mid-troposphere falsifies the models.

Actual temperature measurements of 1958-2017 from three weather balloon datasets were compared to all 102 model runs. Trends and confidence intervals were generated for a simple trend and one with a break at 1997 corresponding to the Pacific climate shift. The trend is 0.33 +/- 0.13 °C/decade in the models and 0.17 +/- 0.06 °C/decade in the observations. With a break term included they are, respectively, 0.39 +/- 0.17 °C/decade (models) and 0.14 +/- 0.12 °C/decade (observed).

The authors constructed divergence terms consisting of each model run minus the average balloon record. The histograms of trends in these measures ought to be centered on zero if the model errors were mere noise. Instead the distributions are entirely positive, indicating a systematic positive bias. The histogram without the break term is here;

model observatios histogram M&C2018

The descrepancy is even larger with the break term.

 

The mid-tropospheric warming trends in the models are caused by increasing water vapour in response to rising CO2 levels due to flawed moist convection thermodynamic parameterization. Correcting the parameterization to match the temperature and water vapour amounts would greatly reduce the model’s climate sensitivities by reducing the water vapour feedback net of the smaller lapse rate feedback. McKitrick wrote a description of the paper at the Climate Etc blog. Robert Lyman wrote a summary on the FoS blog.


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