Comment by Jweb_Guru

The sun is also measurably brighter and hotter than it was when CO2 levels were significantly higher, meaning we should expect greenhouse effects to be more severe. CO2 levels aren't the only thing that contribute to climate. Additionally, while in the past the levels have been much higher, the rate of change in CO2 levels was never nearly this swift in the geological record--it's the sudden change that has many people worried.