Settled science: New paper questions usefulness of core IPCC radiative forcing concept

A paper published today in the Journal of Climate finds the core IPCC concept of net radiative forcing at the tropopause is "not a useful concept on short timescales because it fails to distinguish between energy absorbed within the [atmosphere itself] and energy absorbed at the [Earth] surface." According to the authors, "This work begs the question: on what timescales and regimes is the radiative forcing at the tropopause a useful concept and when is the response of the system contingent on the vertical structure of the atmospheric forcing?" In other words, the core concept of IPCC climate projections is "not useful" on a short-term basis and it remains unknown if it is useful on a long-term basis. Additionally, the authors find seasonal heating of the atmosphere is primarily due to "top down" direct heating from the Sun of water vapor in the atmosphere, rather than the IPCC concept of "bottom up" heating of the atmosphere from the Earth surface. 

Note: Satellite data has shown that water vapor is decreasing rather than increasing. Thus, the data shows no evidence of a "positive water vapor feedback" as predicted by IPCC climate models. 

From the paper:

Our work demonstrates that the atmospheric response to heating is localized in the vertical and further suggests that the net radiative forcing at the tropopause (i.e. the Solomon et al. 2007, definition of radiative forcing) is not a useful concept on short timescales because it fails to distinguish between energy absorbed within the atmospheric column and energy absorbed at the surface. The vertical structure of atmospheric heating within the troposphere is irrelevant [in the IPCC  provided the surface layer is in energetic equilibrium and the troposphere is well mixed in the vertical. Our results demonstrate that neither of these conditions are satis ed in either the climatological or perturbed (2XCO2) seasonal cycles and the atmospheric temperature response depends critically on the vertical distribution of the heating.  

This work begs the question: on what timescales and regimes is the radiative forcing at the tropopause a useful concept and when is the response of the system contingent on the vertical structure of the atmospheric forcing? We hope to explore the impact of the vertical structure of atmospheric forcing on the atmospheric temperature response across a myriad of spatio-temporal scales in future work.

Journal of Climate 2013 ; e-View

doi: http://dx.doi.org/10.1175/JCLI-D-12-00713.1

The seasonal cycle of atmospheric heating and temperature

Aaron Donohoe*

Massachusetts Institute of Technology, Cambridge, Massachusetts

David S. Battisti

Department of Atmospheric Sciences, University of Washington, Seattle, Washington

Abstract

The seasonal cycle of the heating of the atmosphere is divided into a component due to direct solar absorption in the atmosphere, and a component due to the flux of energy from the surface to the atmosphere via latent, sensible, and radiative heat fluxes. Both observations and coupled climate models are analyzed. The vast majority of the seasonal heating of the Northern extratropics (78% in the observations and 67% in the model average) is due to atmospheric shortwave [solar] absorption. In the southern extratropics, the seasonal heating of the atmosphere is entirely due to atmospheric shortwave absorption in both the observations and the models, and the surface heat flux opposes the seasonal heating of the atmosphere. The seasonal cycle of atmospheric temperature is surface amplified in the northern extratropics and nearly barotropic in the southern hemisphere; in both cases, the vertical profile of temperature reflects the source of the seasonal heating.

We examine the change in the seasonal cycle of atmospheric heating in 11 CMIP3 models due to a doubling of atmospheric carbon dioxide from pre-industrial concentrations. We find the seasonal heating of the troposphere is everywhere enhanced by increased shortwave [solar] absorption by water vapor; it is reduced where sea ice has been replaced by ocean which increases the effective heat storage reservoir of the climate system and thereby reduces the seasonal magnitude of energy fluxes between the surface and the atmosphere. As a result, the seasonal amplitude of temperature increases in the upper troposphere (where atmospheric shortwave absorption increases) and decreases at the surface (where the ice melts).