A very useful dialog on the issue as to what is a first order climate forcing was initiated by Real Climate. The comment can be read on their web site at the URL listed above. My response, which I posted as a comment on their website is,
“Gavin-thank you for commenting on my Climate Science weblog (http://www.climatesci.org//). With respect to what is a first order climate forcing, the perspective of the 2005 National Research Council report (http://www.nap.edu/openbook/0309095069/html/) includes the following
“Regional variations in radiative forcing may have important regional and global climatic implications that are not resolved by the concept of global mean radiative forcing. Tropospheric aerosols and landscape changes have particularly heterogeneous forcings. To date, there have been only limited studies of regional radiative forcing and response. Indeed, it is not clear how best to diagnose a regional forcing and response in the observational record; regional forcings can lead to global climate responses, while global forcings can be associated with regional climate responses. Regional diabatic heating can also cause atmospheric teleconnections that influence regional climate thousands of kilometers away from the point of forcing. Improving societally relevant projections of regional climate impacts will require a better understanding of the magnitudes of regional forcings and the associated climate responses.”
“Several types of forcings-most notably aerosols, land-use and land-cover change, and modifications to biogeochemistry-impact the climate system in nonradiative ways, in particular by modifying the hydrological cycle and vegetation dynamics. Aerosols exert a forcing on the hydrological cycle by modifying cloud condensation nuclei, ice nuclei, precipitation efficiency, and the ratio between solar direct and diffuse radiation received. Other nonradiative forcings modify the biological components of the climate system by changing the fluxes of trace gases and heat between vegetation, soils, and the atmosphere and by modifying the amount and types of vegetation. No metrics for quantifying such nonradiative forcings have been accepted. Nonradiative forcings have eventual radiative impacts, so one option would be to quantify these radiative impacts. However, this approach may not convey appropriately the impacts of nonradiative forcings on societally relevant climate variables such as precipitation or ecosystem function. Any new metrics must also be able to characterize the regional structure in nonradiative forcing and climate response.” (http://www.nap.edu/books/0309095069/html/6.html)
Although these climate forcings may not alter the global mean surface temperature, they are first order climate forcings in terms of their substantial role in influencing the climate system including the planetary atmospheric circulation. We both agree that the radiative effect of carbon dioxide, methane and sulphates are first order climate forcings. What we need to do now is discuss what are the criteria that we use to apply the term “first order”. The title of the National Research Council report “Radiative Forcing of the Climate System: Expanding the Concept and Addressing Uncertainties” clearly indicates that we need to move beyond the current perspective of referring to global averaged temperature as the primary metric to assess human caused climate change, and of “CO2, CH4 and sulphates (the main non-soot aerosol)” as “the only ‘first order’ climate forcings”. I discuss this subject further on my weblog, and welcome further discussion of this issue.
On “the conclusion was made that the ‘balance of evidence’ supported the notion of ongoing human-caused climate change”, the evidence of a human fingerprint on the global and regional climate is incontrovertible as clearly illustrated in the National Research Council report and in our research papers (e.g., see http://blue.atmos.colostate.edu/publications/pdf/R-258.pdf).”