Cities tend to be somewhat hotter than rural areas (or more precisely, to have higher minimum temperatures but similar maximum temperatures). And many weather stations are located in areas that became urbanized in recent years.

Thus, there has been a certain ambiguity to ground-based readings of temperature change, explains NOAA scientist Kevin Gallo. "Are we detecting urbanization rather than global warming?"

Hoping to resolve that ambiguity, Gallo and other NOAA scientists have begun using satellite data to monitor urbanization, for example by watching the light emitted from urban areas. Several satellites, including a Defense Department meteorological satellite originally used for monitoring clouds, are providing data relevant to this purpose.

"We're using three different satellites and a number of pieces of information to analyze as thoroughly as we can the environment around the observing stations," says Gallo. By contrast, past measures of urbanization relied heavily on population size, he notes.

The current research will help scientists correct for local effects in measuring global climate change. Whether such corrections will show reduced global warming is unclear, however. "We've looked at what we believe are truly rural temperatures -- and still found an increased temperature signal," says Gallo.