This is an excerpt from his testimony before the US Senate sub-committee on the Environment, on February 25, 2014. When you take the long view, we are in an ice age.
There is some correlation, but little evidence to support a direct causal relationship between CO2 and global temperature through the millennia. The fact that we had both higher temperatures and an ice age at a time when CO2 emissions were10 times higher than they are today fundamentally contradicts the certainty that human-caused CO2 emissions are the main cause of global warming.
Today we remain locked in what is essentially still the Pleistocene Ice Age, with an average global temperature of 14.5°C. This compares with a low of about 12°C during the periods of maximum glaciation in this Ice Age to an average of 22°C during the Greenhouse Ages, which occurred over longer time periods prior to the most recent Ice Age. During the Greenhouse Ages,there was no ice on either pole and all the land was tropical and sub-tropical, from pole to pole. As recently as 5 million years ago the Canadian Arctic islands were completely forested.
Today, we live in an unusually cold period in the history of life on earth and there is no reason to believe that a warmer climate would be anything but beneficial for humans and the majority of other species. There is ample reason to believe that a sharp cooling of the climate would bring disastrous results for human civilization.
Scientifically, one cannot say that any single episode of extreme weather―no storm, no flood, no drought―was caused by climate change; but the global climate has been so extensively impacted by the human-caused buildup of greenhouse gases that many such events are being influenced by climate change. Effective climate-preparedness efforts will require anticipating and planning for changes in the frequency, intensity, and locations of some kinds of extreme events, as well as for more gradual changes such as the continuing rise of sea level and movement of the geographic ranges of pests and pathogens.