ScienceDaily: The team demonstrated that only a relatively small rise in atmospheric carbon dioxide (CO2) was associated with a period of substantial warming in the mid- and early-Pliocene era, between three to five million years ago, when temperatures were approximately 3 to 4 degrees Celsius warmer than they are today. Climate sensitivity — the mean global temperature response to a doubling of the concentration of atmospheric CO2 — is estimated to be 1.5 to 4.5 degrees Celsius, using current …