The strontium record shows that around the middle Ordovician, weatherability increasedleading to an increased consumption of CO2. However, this was balanced by increased volcanic outgassing addingCO2 to the atmosphere. Around 446 million years ago, volcanic activity dropped while rock weathering remained high. This caused CO2 levels to fall below 3000 ppm, initiating cooling. It turns out falling CO2 levels was the cause of late Ordovician glaciation.
So we see that comparisons of present day climate to periods 500 million years ago need to take into account that the sun was less active than now. What about times closer to home? The last time CO2 was similar to current levels was around 3 million years ago, during the Pliocene. Back then,CO2 levels remained at around 365 to 410 ppm for thousands of years. Arctic temperatures were 11 to 16°C warmer (Csank 2011). Global temperatures over this period is estimated to be 3 to 4°C warmer than pre-industrial temperatures. Sea levels were around 25 metres higher than current sea level (Dwyer 2008).
If climate scientists were claiming CO2 was the only driver of climate, then high CO2during glacial periods would be problematic. But any climatescientist will tell you CO2 is not the only driver ofclimate. Climatologist 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 andclimate.
Last updated on 9 July 2010 by John Cook.