Dr. Feng He

University of Wisconsin-Madison, Center for Climatic Research

Nonlinear Climate Sensitivity Caused by the Ocean Dynamics: Implications for the Early Anthropogenic Hypothesis

Room 811 AO&SS, February 22, 2012, 2:30 PM

Abstract

Ruddiman proposed that greenhouse-gas emissions associated with the early agriculture several thousand years ago have been keeping Earth's climate anomalously warm during the Holocene and preventing glacial inception. In this study, we aim to quantify the role of ocean dynamics in simulating the northern hemisphere permanent snow in CCSM4.

Two sets of the CCSM4 experiments were conducted in either fully coupled or slab ocean configuration with greenhouse gas concentrations from the present-day (PD, CO2=355 ppm), pre-industrial (PI, CO2=280 ppm) and hypothetical non-anthropogenic (NA, CO2=245 ppm) scenarios. Our results show that the sensitivities of Northern Hemisphere permanent snow, Northern Hemisphere sea ice and global mean temperature are only slightly non-linear in the slab ocean runs, but highly non-linear in the fully coupled simulations, with larger sensitivity during the transition of colder climates between PI and NA. The comparison between the fully coupled and the slab ocean simulations show that ocean dynamics causes significantly larger increase of North Atlantic sea ice during the PI/NA transition that the PD/PI transition, suggesting a threshold might have been crossed during the PI/NA transition. Our results suggest that a small increase of greenhouse gas concentration from the early agriculture several thousand years ago might have prevented the Earth from crossing this threshold that leads to glacial inceptions.