S. Fred Singer, Ph.D.

Conventional General Circulation Models, quoted by the IPCC, predict a current "best" warming rate of 0.3°C per decade. Satellite and balloon observations, however, show a slight cooling rate since 1979. Surface observations show no warming trend in the last decade.

The explanation for this discrepancy, put forth by the IPCC, has been an assumed cooling from sulfate aerosols. That this is insufficient can readily be seen from graphs published recently by Hasselman (Science, 1997). The calculated warming rate is only slightly reduced--to about 0.25°C per decade. In addition, publications by Tett et al. (Science, 1996) and Hansen et al. (JGR, 1997) show that the negative forcing from aerosols is quite insufficient.

Perhaps the strongest argument against the aerosol model comes from satellite observations; they show a cooling trend everywhere except at northern mid-latitudes. They show a warming trend there--just where the aerosol cooling effect should be most important.

The discrepancy between models and observations must therefore be ascribed to other exogenous factors (solar variations) or to endogenous factors that are poorly treated in climate models, such as details of clouds and the vertical distribution of water vapor. Clearly, climate science is neither "settled" not "compelling"--as often claimed by politicians--but remains a challenging field for research.