The phrase “first order climate forcing” has been used in our papers and in my posts. I want to make sure this terminology is clearly defined; thus the reason for this post.
I offer this definition
A first-order human climate forcing is one that results in alterations in the climate system which have significant effects on societally and environmentally important resources. These alterations would include changes, as examples, in drought, flood and hurrricane patterns.
Using this definition, added CO2 is clearly a first order climate forcing as the alteration in the atmospheric and ocean concentration of CO2 is a biogeochemical change which results in alterations of the physiology of plants and other organisms. It also is a positive radiative forcing as the 2007 IPCC reports.
However, in contrast to the narrow perspective presented in the 2007 IPCC report, there is a diverse set of other first order climate forcings. The 2007 IPCC view seems to be that only changes in the global annual radiative forcing matters in terms of multi-decadal climate forcing, and this is their implicit definition of a first order climate forcing. This is the dominant (i.e. first order climate forcing) in their view.
However, this is a flawed incomplete perspective as we have written on in our paper
Pielke Sr., R., K. Beven, G. Brasseur, J. Calvert, M. Chahine, R. Dickerson, D. Entekhabi, E. Foufoula-Georgiou, H. Gupta, V. Gupta, W. Krajewski, E. Philip Krider, W. K.M. Lau, J. McDonnell, W. Rossow, J. Schaake, J. Smith, S. Sorooshian, and E. Wood, 2009: Climate change: The need to consider human forcings besides greenhouse gases. Eos, Vol. 90, No. 45, 10 November 2009, 413. Copyright (2009) American Geophysical Union.
In that paper the hypothesis
Although the natural causes of climate variations and changes are undoubtedly important, the human influences are significant and involve a diverse range of first- order climate forcings, including, but not limited to, the human input of carbon dioxide (CO2). Most, if not all, of these human influences on regional and global climate will continue to be of concern during the coming decades
is the only view that is supported by the peer reviewed literature. These other climate forcings alter atmospheric and ocean circulation features away from what they would be in the natural climate system [NRC, 2005], and, moreover, as with CO2, the lengths of time that they affect the climate are on multidecadal time scales and longer.
presents insightful comments on how this hypothesis can be fine tuned. However, a clear message is that, in addition to added CO2, deliberate and inadvertant land use/land cover change, and a diverse ramge of influences from aerosols are also first order climate forcings. The next IPCC assessment must include this broader view in their assessment of climate science.