We humans alter our climate. That has been true since we cleared areas of forest or grasslands to grow crops or build shelter. The manner in which we alter our natural landscape changes the amount of light that reflects off the surface, the amount of water that evaporates into the air (or percolates into the groundwater), and the amount of sunlight that plant leaves intercept.
But for a long time, land use changes were thought to be primarily local, microclimatic effects. Only recently has it been proposed that our land use changes are affecting global temperatures.
To estimate the extent of the impact of land cover changes on temperatures, Australian researchers Andrew Pitman and Mei Zhao ran a general circulation model (the CCM3 from the National Center for Atmospheric Research) linked to a biosphere model. They ran the GCM for background (280 ppm) and current (355 ppm) levels of CO2, and simultaneously compared natural (historic) vegetation to current vegetation at each grid cell.
Using those methods, which were very conservative, Pitman and Zhao found that only 8 percent of the Earth’s land surface had been altered from the natural state (based on changes in a quantity called the “leaf area index”; other researchers have estimated this number to be 45 percent).
Pitman and Zhao found that, in regions where the land cover has changed significantly (mostly Europe, North America, India, and China), temperature changes arising from changing land use account for more than 50 percent of the combined CO2 and land use effect. Surprisingly, in areas where no major land use changes occurred (such as the large area of warming in northern Eurasia), land use nevertheless accounts for as much warming as CO2.
According to the authors, land cover change, or LCC,
“contributes 30 percent to 40 percent of the total warming contributed by a combination of LCC and change in CO2 at [high northern] latitudes. There were no prescribed modifications to land use this far north, hence these changes are the result of natural model variability or teleconnections. . . . [P]ossible mechanisms to explain these changes, involving the position and strength of key elements of the general circulation, have been suggested.”
In other words, according to this model, land cover changes have regional or global as well as local effects, possibly by altering global wind and pressure fields. And in some regions, those changes influence temperatures as much as do increases in CO2.
If the real influence of land cover change is in fact closer to 45 percent than 8 percent, it’s easy to imagine that CO2 is getting blamed for a temperature increase that may well be related to other factors.
Robert E. Davis, Ph.D., is an associate professor of environmental science at the University of Virginia.
Pitman, A.J. and M. Zhao, 2000. The Relative Impact of Observed Change in Land Cover and Carbon Dioxide as Simulated by a Climate Model. Geophysical Research Letters, 27, 1267-1270.