Six Degrees of Climate Separation
During this time there has been a strong correlation between CO2 in the atmosphere and global average temperatures. But also during this time CO2 hasn't approached today's human-influenced level, so we didn't have direct evidence of what happens when levels of the greenhouse gas are this high.
Late last year, a group of scientists at UCLA published a study demonstrating a new method of estimating prehistoric CO2 levels, extending the estimates back about 20 million years.
Not only do these measurements continue to show a strong correlation between CO2 and climate, they also illuminate the most recent time in the Earth's history when CO2 levels were similar to what they are today.