Cities absorb more heat than rural areas.

Nasa

Land-use changes such as the expansion of cities and changes in agriculture have more effect on climate change analysis than was previously thought, according to a new study.

Revised calculations published in this week's Nature suggest that land-use changes are causing the Earth's surface temperature to rise by about 0.27°C every century —  about twice as much as previous estimates. 

Cities tend to be warmer than nearby rural areas because buildings absorb and retain heat. This 'urban heat island' effect means that temperatures are higher at urban monitoring stations than rural ones. Agriculture can also introduce biases, such as when irrigation increases heat retention in soil.

Researchers have been aware of these distorting effects on apparent temperature trends for some time, and have tried to correct temperature measurements accordingly. But these new findings suggest that such adjustments may have been insufficient.

Link to research paper in Nature

Link to Nature Science Update news story

Reference: Nature 423, 528 (2003)