Jim Hansen has graciously agreed to write an invited response to the Climate Science weblog Comments On “Air Pollutant Climate Forcings Within The Big Climate Picture” By Hansen et al. 2009.
Guest Weblog By Dr. Hansen
We include land-use changes as one of the climate forcings in our climate modeling, as discussed in the papers “Efficacy of climate forcings” (J. Geophys. Res. 110, D18104 doi:10.1029/2005JD005776, 2005) and “Dangerous human-made interference with climate: a GISS modelE study” (Atmos. Chem. Phys. 7, 2287-2312.
Those simulations used a global data set of Ramankutty and Foley for land use changes over the past two centuries. The effect of the land use changes was found to be quite large, even dominant, on local regions where a large fraction of the gridbox was affected by land use change. But on global average the effects of greenhouse gases and aerosols were much larger than the effects of land use changes.
As noted in those papers, we did not include irrigation, which can also have large regional effects, because of the absence of a good global data set for irrigation fluxes. Nevertheless, from the simulations that have been made, and from comparison of the climate forcings, these papers make it clear that the largest global climate forcings are changes of greenhouse gases and aerosols.