|Misleading: CO2 Has Been Much Higher
in the Past and It Was Colder
Misleading: CO2 Has Been Much Higher in the Past and It Was Colder
Figure 1 shows the radiative forcing due to solar irradiance and carbon dioxide as documented by Royer (2005). The dark shaded bands correspond to periods with strong evidence for geographically widespread ice.
So, yes, CO2 was much higher many millions of years ago, and yes, there was widespread ice at these very high levels. Because the sun was much weaker many millions of years ago, the claim that "CO2 has been much higher in the past and it was colder" is essentially comparing apples to oranges.
A much better comparison would have to include an ice-free time period closer to today. As discussed in Greenhouse Gases, Hanson et al. (2008) showed that the last time the earth was essentially ice-free was approximately 50 million years ago. CO2 concentrations then were 425 ±75 ppm which means the atmosphere today is already over the lower end of that range!
Last updated: 11/23/09