A popular argument of climate change deniers is that the Earth's warming and cooling over millions of years is not connected with the amount of carbon dioxide in the atmosphere. A closely argued statement by the Geological Society of London, citing an immense body of research, refutes such claims, and concludes (with classic understatement) "emitting further large amounts of CO 2 into the atmosphere over time is likely to be unwise, uncomfortable though that fact may be."

The report says that natural events such as changes in the tilt of the Earth did cause global warming in the past. They triggered reactions from the oceans leading to releases into the atmosphere of carbon dioxide and water vapour, trapping heat and accelerating the warming process. This cycle has caused abrupt climate change at least three times. The report says that now there is no natural event causing warming, only humans burning fossil fuels, making cement, and cutting forests.

What happened last time carbon dioxide levels were at the current level? The geologists report the global temperature was 2C to 3C higher, but, more worrying, seas levels rose 10 to 25 metres. The temperature at the poles was about 10C higher, causing vast ice loss. In this distant past, the report says, humans were so few that they could migrate to higher ground and cooler climes, but for billions of the current human population this is not an option.