Declining carbon dioxide may have triggered Cenozoic cooling, but increasing high latitude cloudiness may have been the main driver of climatic change. Dust-cloud interactions may have amplified ice age climate changes initiated by orbital variations. Global warming by industrial and agricultural greenhouse gases may be competing with global cooling by pollutant haze and cloud modification. The comfortable assumption of paleoclimatic theorists, that clouds do not change, is probably wrong [ Henderson-Sellers, 1979].
This story reverses the usual progression from fundamental research to applications. In the case of clouds in climate change, the progress is coming in the applied field of global change. The beneficiary is the fundamental field of paleoclimate on geological time scales.
Acknowledgments. I am grateful to Henning Rodhe and Tom Wigley for calling my attention to pertinent work. My thinking on clouds and climate change has benefited from discussions with Brian Davis. This research was supported in part by the U. S. National Aeronautics and Space Administration under Grant NAGW-176.