"Modern-day levels of carbon dioxide were last reached about 15 million years ago"

Scientific American summarizes two new studies giving evidence that the earth's climate is highly sensitive to the amount of carbon dioxide in it. One is a a Science article by Aradhna K. Tripati, Christopher D. Roberts, and Robert A. Eagle. David Biello writes:

' "Modern-day levels of carbon dioxide were last reached about 15 million years ago," Tripati says, when sea levels were at least 25 meters higher and temperatures were at least 3 degrees C warmer on average. "During the middle Miocene, an [epoch] in Earth's history when carbon dioxide levels were sustained at values similar to what they are today [330 to 500 ppm [parts per million]], the planet was much warmer, sea level was higher, there was substantially less ice at the poles, and the distribution of rainfall was very different."

Further, "at no time in the last 20 million years have levels of carbon dioxide increased as rapidly as at present," Tripati adds; CO2 concentrations have climbed from 280 ppm to 387 ppm in the past 200 years. And "our work indicates that moderate changes in carbon dioxide levels of 100 to 200 parts per million were associated with major climate transitions and large changes in temperature"indicative of a very sensitive climate.'

As I note in Engaging the Muslim World,, James Hansen, director of the Goddard Institute for Space Sciences, testified in 2008 that twenty years earlier he and his colleagues had worried that 450 parts per million of carbon dioxide in the atmosphere would trigger catastrophic climate change. But by the time of his testimony last year, Dr. Hansen and colleagues had concluded that 387 ppm was already catastrophic and that the world had to be taken back to the 280 ppm of the pre-industrial era if global calamities were to be avoided.

Hansen's reasoning assumes that the world's climate is sensitive to the amount of carbon dioxide in the atmosphere and has little "give" in this regard. The research cited by SA supports the "sensitive" argument.