By the end of this century, atmospheric CO2 concentrations will increase to four times pre-industrial levels. Many independent lines of evidence document that the increase of CO2 in the atmosphere since the start of the industrial revolution is anthropogenic. The most compelling evidence is based on ratios of carbon isotopes.
Although there were a number of cases during Earth’s history when CO2 concentrations and average global temperature exceeded present values, it is the rate of release, not the magnitude, that matters. For example, the Paleocene-Eocene Thermal Maximum (PETM) occurred 66 million years ago when the temperature was 8 degrees C warmer than the average global temperature today. The maximum sustained carbon injection (release) rate during the PETM was approximately 1 gigaton (Gt)/yr, making it the most extreme case of rapid global warming and carbon injection during the last 66 million years. However, the present carbon release rate from anthropogenic sources is about 10 Gt/yr or 10 times the PETM rate. With our present unprecedented rate of about 10 Gt/yr, the response of the Earth is difficult to predict but it is likely that atmospheric and ocean warming, ocean acidification and sea level rise will increase at rates never experienced in Earth history.
Terry Maley, Boise