There is an interesting article in the July 10 2010 issue of the Economist titled
A mammoth effect: Hunting large herbivores may have (slightly) contributed to global warming [subscription required].
It includes the text
Christopher Field is probably best known as the co-chair of the Intergovermental Panel on Climate Change’s working group on impacts, adaptation and vulnerability. In his day-job, however, he still engages in some serious boffinry. Take the paper soon to be published in Geophysical Research Letters which he wrote with two colleagues from the Carnegie Institution for Science in Stanford. They argue that mankind’s prehistoric penchant for mammoth may have had a discernible effect on climate.
From all this the researchers conclude that the extra area invaded by birch would have been 23% smaller, on average, had mammoths continued grazing. Plugged into a climate model this translates into a 0.13°C temperature rise in Siberian climes over the course of several centuries following the extinction (though some places would have warmed by as much as 1°C).
I am glad that this further documnetation of the significant role of landscape change on the global and regional climate system is being published. In his capacity as a co-Chair of the IPCC, I encourage Chris to finally give land over/land use change the proper place in the upcoming IPCC assessment, as this was certainly not true in the 2007 assessment.
With request to their specific study (of the effect of grazing changes on weather and climate), we have also published on this topic; i.e.
Eastman, J.L., M.B. Coughenour, and R.A. Pielke, 2001: Does grazing affect regional climate? J. Hydrometeorology, 2, 243-253.
The abstract reads
Before European settlement, the Great Plains of the United States contained vast herds of bison. These bison altered the landscape through their grazing. Measurement data of the disturbance that such grazing could produce, when scaled for the large population of bison, were used with a coupled atmospheric–ecosystem model to evaluate the likely effect that this grazing had on the growing season weather in the Great Plains. A dynamically coupled meteorological and plant growth model was used to investigate the regional atmospheric conditions over a single growing season. A 50-km horizontal mesh was implemented, covering the central plains of the United States. The modeling system was then integrated, with a time step of 90 s, for a period covering 1 April 1989 through 31 August 1989 using boundary conditions obtained from an objective analysis of gridded archive data. This integration was performed with and without grazing to assess the effects on regional atmospheric and biological processes. The grazing algorithm was employed to represent presettlement North American bison and was switched on and off for different simulations. The results indicated a cooling response in daily maximum temperatures to removal of grazing. The opposite trends were found for the minimum daily temperature. It was also found that grazing produced significant perturbations in the hydrological cycle.