RE: climate feeback response and the factor put into computer model: - I will go with the empirically based MODTRANS model findings before IPCC data any day.
There is nearly an order of magnitude difference between the relatively small surface temperature response of 0.6oC to a doubling of CO2 concentration calculated above and the projected responses quoted by the IPCC. The latter are based on computer models and range from about 2oC to about 6oC. The key to the difference is in the formulation of the changing rate of latent energy exchange with temperature used in the computer models.
As explained above, here the rate of increase in evaporation (and latent energy exchange) with temperature is equated to the Clausius Clapeyron relationship of 7% per degree C. As noted by Held and Soden (2006)5, the rate of increase of evaporation with temperature rise for the computer models used in the fourth assessment of the IPCC was, on average, only about one-third this value. This low value in computer models was confirmed by Wentz et al (2007)6, who also identified a range of 1-3% K-1 for the global average evaporation increase across the models.
A reduction in the rate of evaporation increase with temperature has significant consequences for temperature projections under CO2 forcing. The reduced rate of change of latent energy exchange with surface temperature means that the rate of increase of surface energy loss with temperature is also reduced and the slope of the curve of Figure 5 is flattened. As a consequence, the surface temperature can vary over a wider range for the same imbalances between the fixed energy inputand the surface energy loss variation. That is, the tendency to return to the steady state temperature is weakened. Of more importance, if the rate of increase of latent energy exchange with temperature is significantly less than the Clausius Clapeyron relationship then the radiation forcing from a doubling of CO2 concentration produces a larger increase in surface temperature to a new steady state. The changing sensitivity of surface temperature to radiative forcing can be readily assessed by way of equation 3 above. At the average temperature of the Earth (15oC) the rate of increase of surface infrared emission with temperature change is given by the Stefan Boltzman Law as 5.4 Wm-2C-1.
The equivalent rate of increase with temperature of downward infrared radiation at the surface can be assessed, for example using the MODTRANS radiation transfer model. With the assumptions that the US Standard Atmosphere approximates the mean profile of the atmosphere, that relative humidity is constant (that is, the atmospheric water vapour increases with temperature in accordance with the Clausius Clapeyron relationship) and ignoring clouds, it is found that the increase in downward infrared radiation at the surface is 4.8 Wm-2C-1.
Table 1 sets out indicative values for the sensitivity of surface temperature to radiative forcing for a range of rates of latent energy exchange with temperature. The value of 6% C-1 is the global average estimate by Wentz et al (2007) based on satellite estimates of changing precipitation during global warming of recent decades. It is less than the Clausius Clapeyron relationship but this is not unexpected given the magnitude of arid and semi-arid land areas. The other values are typical for computer models (GCM) used in the IPCC fourth assessment of 2007.
Table 1: Indicative values of surface temperature response to radiative forcing and of surface temperature increase from a doubling of CO2 concentration. The rates of surface latent energy exchange, dLH/dT correspond to global values assessed from satellite analysis, and values corresponding to computer models (GCM) used in the 2007 IPCC fourth assessment .
It is very clear from Table 1 that any assessment of the surface temperature response to CO2 forcing is very sensitive to the specification of the rate of increase of evaporation, and hence latent energy exchange, with temperature increase. The analysis at Table 1 clearly points to a high likelihood that the computer models used as the basis for the IPCC estimates of anthropogenic global warming are significantly exaggerating the projected global temperature response. A doubling of CO2 concentration even from current level to near 800 ppm by the end of the 21st century is not likely to cause global temperature rise exceeding 1oC. Such a rise is well within the range of natural variability and should not be construed as dangerous.
