Eli Rabett has presented two well posted comments on the relative roles of global average and regional climate forcings, which I am also presenting as a weblog since his contribution helps focus a very important climate change issue:
His comments (posted on January 28, 2007) are:
“Again, which is the forcing and which the response, which is the major effect locally, which the major effect globally. If you increase grassland in one area, and decrease it in another the effects balance globally, but not locally, since land use is inherently local. On the other hand greenhouse gas emissions rapidly diffuse throughout the atmosphere.”
“Bryan, while climate science is about much more that greenhouse gas forcing, your and Prof. Pielke’s insisting on ignoring the elephant in the room is curious. As a physicist, my inclination to spherical elephants is built in. First you look at the largest effects.”
Here is my reply
Eli – Thank you for your several constructive contributions to the weblog.
Your comments succinctly place the relative roles of the different human climate forcings in perspective. We differ on this issue. You are focusing on a global average (such that if, for example, large positive and negative excursions in multi-decadal regional troposphere temperature trends sum to near zero, this is inherently a local (regional) issue.
However, we are finding from theory and from models that such regional tropospheric temperature anomalies result in significant changes in atmospheric circulation patterns, that can substantially alter precipitation, temperature and other aspects of the climate system at large distances (i.e. through teleconnections) from where a land-use/land-cover and/or aerosol emission change occur. This is a global scale climate change which has been presented as a finding in the 2005 National Research Council Report “Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties”.
It is these changes that would have a much greater effect on impacts on important social and environmental resources than a global average multi-decadal trend in tropospheric temperatures. These circulation changes are the “elephant” in the room, which has been inadequately discussed in past climate assessments.
The radiative effect of the more spatially homogeneous forcing of CO2 is still important, but it is “A” human climate forcing, not “THE” dominate human climate forcing on the local, region, and global scales.