26 December 2014

Variations of CCSM3 Feedbacks with CO2 Forcing


Are equilibrium climate sensitivity and the associated radiative feedbacks a constant property of the climate system, or do they change with forcing magnitude and type? We address the former question by investigating the impact of the magnitude of carbon dioxide forcing varying between twice and eight times present-day levels on individual feedbacks and climate sensitivity in NCAR Community Climate System Model version 3. We also assess the role ocean dynamics play in determining feedbacks in response to different forcings by comparing CCSM3 results with those obtained from the slab ocean CAM3. To this end, we compare feedbacks and climate sensitivity from 1450 years of fully coupled simulations with CCSM3 and 60 years of slab ocean simulations with CAM3, each forced with 3 consecutive doublings of CO2 concentrations: from 355 to 710 ppm, from 710 to 1420 ppm and from 1420 to 2840 ppm. Feedbacks are evaluated using the radiative kernel technique. Climate sensitivity in both model versions increases by >20% between the first and third CO2 doubling due to increases in positive water vapor and cloud feedbacks. This increase is dampened by a decrease in the positive albedo feedback and an increase in the negative lapse rate feedback.

A. K. Jonko
Jonko, AK, KM Shell, BM Sanderson, and G Danabasoglu.  2013.  "Climate Feedbacks in CCSM3 under Changing CO2 Forcing Part II: Variation of Climate Feedbacks and Sensitivity with Forcing."  Journal of Climate 26: 2784-2795.