Regarding Gwynne Dyer’s Nov. 1 article, “Avalanche of evidence on vanishing ice caps“: As the Copenhagen conference on global warming approaches, Dyer gives an excellent introduction to the sophisticated science applied to this issue. It seems to be well established that high temperatures are associated with high atmospheric carbon dioxide. The Vostok ice cores show a similar association between high temperatures and high CO2 levels in events that occur quite regularly every 100,000 years — so regularly in fact that when I first saw these data I thought the findings were rigged. But then I thought perhaps there is something that occurs precisely every 100,000 years to get the temperatures high enough to release such vast quantities of carbon.

Remarkably our science is now good enough to determine average temperatures, CO2 content, ice thickness and sea-level rises even as far back as 20 million years ago. Moreover, we are able to discern which caused what.

Skeptics of global warming — that is, the few who understand the science and dissent from it rather than those who rely simply on gut feeling — will see data from 20 million years ago as further confirmation of the view that climate change is primarily caused by external forces such as sun spot cycles, or perhaps random events, rather than by the positive amplification of forces caused by high CO2 levels. International climate change experts themselves are very unsure of the extent of this positive feedback, with estimates varying by two to four times.

Without this positive feedback, the climate models forecast little or no temperature increases. To estimate this amplification factor, the models are fitted to the known climate over the past 100 years. Without amplification of the direct effects of CO2, they cannot be made to fit.

But what if there were there other forces at work? No one knows for sure, and the scientific skeptics quite reasonably want more research into other external forces and into the fundamental physics of cloud formation on which the scientific estimation of feedback crucially depends.

Personally, I will remain an agnostic on this issue until I see the physics of cloud formation fully exposed.

antony clark