An excellent new student reading for the science section of climate courses is a piece in Science looking at observational evidence from Earth’s past to project potential warming over the course of this century and beyond, Kiehl, Lessons from the Past, 331 Science 158-59 (2011).

Among the key take-aways:

1. Atmospheric carbon dioxide concentrations have reached 390ppmv;
2. The study reconstructed atmospheric concentrations of carbon dioxide throughout history and concluded the following the last time that concentrations reached levels projected by the end of this century under business as usual projections (900-1100 ppmv) would produce the following results:
   • When carbon dioxide concentrations reached 1000ppmv 35 million years ago, tropical to subtropical sea surface temperatures were in the range of 35-40C, vs. present day temperatures of 30C, while sea surface temperatures at polar latitudes in the South Pacific were 20-25C vs. modern temperatures of 5C;
   • Net radiative forcing during this period was 6.5-10 Wm-2
   • Global annual mean temperature 35 million years ago was 31C vs. 15C during pre-industrial times. This indicates that the climate feedback factor (the ratio of change in surface temperature to radiative forcing) may be as much as 4x greater than predicted by climate models
3. Reduction of carbon dioxide to lower levels of the recent past took tens of millions of years; by contrast, the burning of fossil fuels will return Earth to higher levels within centuries;
4. Earth’s concentration of carbon dioxide is rapidly rising to levels not seen in 30-100 million years, with potentially very large amplification of warming via climatic feedbacks.