Background
In a prior study, Kalnay and Cai (2003) determined differences between observed surface air temperature trends in the conterminous United States and trends derived from reanalyses of global weather over the past 50 years - the NCEP-NCAR 50-year Reanalysis (NNR) project - and used the results to estimate the impact of land-use changes on surface warming, which has historically been determined solely from surface air temperature measurements.  The reanalysis-derived temperatures, by contrast, are based on atmospheric vertical soundings derived from satellites and balloons.  Over rural areas, the surface-derived and reanalysis-derived surface air temperature data sets were shown by the two scientists to yield essentially identical trends; and, therefore, they concluded they "could attribute the differences between monthly or annually averaged surface-temperature trends derived from observations and from the NNR primarily to urbanization and other changes in land use."

What was done
Zhang et al. utilized the approach of Kalnay and Cai (2003) to determine the impacts of land-use changes on surface air temperature throughout eastern China (east of 110°E), where rapid urbanization, deforestation, desertification and other changes in land use have occurred over the last quarter-century, focusing on daily mean, maximum and minimum air temperatures from 259 stations over the period 1960 to 1999.

What was learned
The results of Zhang et al.'s analyses indicate that changes in land-use had little to no influence on daily maximum temperatures, but that they explain about 18% of the observed daily mean temperature increase and 29% of the observed daily minimum temperature increase in this region over the past 40 years, yielding decadal warming trends of about 0.12°C and 0.20°C for these two parameters, respectively.

What it means
The results of this study convincingly demonstrate that changes in land-use can have significant impacts on surface air temperatures; and, hence, they raise serious questions about the magnitude of late 20th-century warming.  In light of these results, and those of numerous other pertinent studies (see the many papers we have reviewed on our website under the headings Urban Heat Island and Temperature Trends: Potential Inaccuracies), people who believe that urbanization and other land-use changes either (1) exert minimal influence on surface air temperature trends or (2) have had the precise values of their influence removed from surface air temperature data sets may want to seriously rethink their positions on the issue.

Reference
Kalnay, E. and Cai, M.  2003.  Impact of urbanization and land-use change on climate.  Nature 423: 528-531.