CO2 in the past
Very good post at Skeptical Science. From the post:
… So we see that comparisons of present day climate to periods 500 million years ago need to take into account the fact that the sun was 4% less active than now. What about times closer to home? The most recent period when CO2 levels were as high as today was around 15 million years ago, during the Middle Miocene. CO2 levels were at about 400 ppm. What was the climate like at the time? Global temperatures were 5 to 10 degrees Fahrenheit higher than they are today. Sea level was approximately 75 to 120 feet higher. There was no permanent sea ice cap in the Arctic and very little ice on Antarctica and Greenland. The close coupling between CO2 and climate led the author to conclude that "geological observations that we now have for the last 20 million years lend strong support to the idea that carbon dioxide is an important agent for driving climate change throughout Earth’s history." (Tripati 2009).
To sum up, Dana Royer says it best: "the geologic record contains a treasure trove of ‘alternative Earths’ that allow scientists to study how the various components of the Earth system respond to a range of climatic forcings." Past periods of higher CO2 do not contradict the notion that CO2 warms global temperatures. On the contrary, they confirm the close coupling between CO2 and climate.