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Climate Model Problems: I. Temperature and Humidity
Volume 10, Number 52: 26 December 2007

In the introduction to their recent study of the subject, John and Soden (2007) write that "atmospheric water vapor is widely recognized to be a key climate variable," primarily because "it is the dominant greenhouse gas and provides a key feedback for amplifying the sensitivity of the climate to external forcings." As a result, they say "there has been a considerable effort to assess the credibility of model simulations of atmospheric water vapor," which is what they do for vertical profiles of both water vapor and temperature, as generated by 16 fully coupled ocean-atmosphere climate models, against which they compare observational temperature and humidity fields obtained from ECMWR (Uppala et al., 2005) and NCEP (Kalnay et al., 1996) reanalysis data for the period 1990-1999, and from the Atmospheric Infrared Sounder (AIRS), a high spectral resolution radiometer with 2378 bands in the thermal infrared and 4 bands in the visible, for the period August 2002-July 2006.

So what did they learn?

The researchers report they "identified significant biases in the ability of current climate models to simulate the zonal, annual mean distribution of water vapor and temperature." More specifically, they discovered that the models exhibit a major moist bias in the free troposphere," which for the mean of all the models they studied "approaches 75% in the upper troposphere," although they say "it can exceed 200% for individual models." Likewise, they note that "model simulated temperatures are systematically colder by 1-4C throughout the troposphere," and that the bias increases with altitude, "with maxima located near 200 hPa in the extra-tropics where the bias exceeds 6C compared to all three observational data sets."

In discussing these results in their concluding paragraph, John and Soden write that "to investigate the consequences of biases in [the] models' mean state on the utility of these models for climate change studies," they conducted analyses that "find" that "the model-simulated response of tropospheric temperature and water vapor to change in surface temperature is insensitive to biases in the mean state and show that water vapor and lapse-rate feedbacks in these models are uncorrelated to the base-state biases in water vapor and temperature," implying we can rest assured that what the models suggest about earth's climate in a CO2-enriched world of the future is reasonably accurate.

We find this implication extremely difficult to swallow. To state that "water vapor and lapse-rate feedbacks in these models [our italics] are uncorrelated to the base-state biases in water vapor and temperature," tells us something about the models alright, but not about the real world. If these wonderful models cannot correctly determine the annual mean vertical profiles of temperature and water vapor in our current atmosphere, why should we believe anything about what they suggest about the future?

Sherwood, Keith and Craig Idso

References
John, V.O. and Soden, B.J. 2007. Temperature and humidity biases in global climate models and their impact on climate feedbacks. Geophysical Research Letters 34: 10.1029/2007GL030429.

Kalnay, E. et al. 1996. The NMC/NCAR 40-year reanalysis project. Bulletin of the American Meteorological Society 77: 437-471.

Uppala, S.M. et al. 2005. The ERA-40 reanalysis. Quarterly Journal of the Royal Meteorological Society 131: 2961-3012.