COMPARISON OF LONG-TERM GREENHOUSE PROJECTIONS WITH THE GEOLOGIC RECORD

Previous climate modeling studies suggest that temperature increases beyond 2100 AD due to greenhouse gas changes could be comparable to the Cretaceous. However, these projections have not been tested against a range of emission scenarios, nor have estimates of past temperature change been well restrained quantitatively. Herein we use a 1D energy balance model (EBM) with an upwelling-diffusion ocean model to explore the temperature response to extreme cases of unrestricted and severely restricted greenhouse gas increases. Results are compared against revised estimates of global temperature change over the last 100 million years. For four of the six scenarios, global temperature projections by the years 2400-2700 equal or exceed levels of the Eocene or Cretaceous warm periods. The two exceptions involve scenarios with relatively low climate sensitivity and stringent conservation measures, but even in these projections temperatures are as warm as any time in the last 30 million years. These calculations suggest that regardless of emission scenario or system sensitivity, future greenhouse warming will be large even on a geological scale.