Urbanization has long been considered a factor that may be contributing to observed increases in surface temperatures over the last 100 years. Numerous studies have been reported to exclude from analysis those stations that are known to have experienced a warming due to growth of cities around the observing site.
Kalnay and Cai (2003a) report an evaluation of a second influence of land-use - an influence other than urbanization - on climate, namely the role of agriculture and irrigation. They use the reanalyzed upper-air data to examine trends over the last 50 years, a method independent of surface-based measurements, to separate urbanization from non-urbanization effects. They conclude (in their follow-on report of corrected measurement) that the trend in daily means temperature due to land-use changes is an increase of 0.35°C per century (Kalnay and Cay, 2003b).
References
Kalnay E., and M. Cai, 2003a: Impact of urbanization and land-use change on climate. Nature, 423, 528-531.
Kalnay, E. and M. Cai, 2003b: Impact of urbanization and land-use change on climate - Corrigenda. Nature, 425, 102.