The different findings may be partly due to the very different environments. The desert southwest has little in common with a delta region in China, except for staggering growth. Shepherd found similar results near another desert megalopolis Riyadh, Saudi Arabia. He notes that arid regions would be particularly sensitive to changes in rainfall "since the water supply is so critical."
But even nonarid cities in the United States, like Atlanta and Nashville, Tenn., have shown changes in precipitation over the last few decades, and Shepherd attributes that to the "heat island effect," the hot-button issue of the impact of cities on the environment, particularly weather. Those findings show change in the patterns of precipitation, delivering more water to areas downwind of the cities, in some cases more than a 100 percent increase.
In all those cases, increasing urbanization has changed the landscape in many ways. Highways and parking lots reflect heat, high buildings can have an impact on the movement of the air across the region and less soil remains exposed to trap and contain moisture. And, of course, in the case of Phoenix, there's all those backyard swimming pools, evaporating tons of moisture into the atmosphere. All that water has to come down somewhere, like downwind.
Sorting all that out, however, is a complex problem that raises many questions. The desert southwest, for example, is in the midst of a drought that has lasted for nearly a decade. Could urbanization in places like Arizona, Nevada and California be shifting weather patterns so that less water drops in areas critical to the reservoirs that supply water to tens of millions of residents? It's not clear at this point.
What is clear is that there is much to be learned about anthropogenic changes in the planet by such things as urbanization.
"Urban land use change has been and will continue to be one of the biggest human impacts on the terrestrial environment," Seto and her colleagues report. "At the start of the 1900s, there were only 16 cities with populations over 1 million. By 2000, there were 417."
"Large areas of the United States have been paved or 'built up' such that they are now considered 'impervious surface areas.' This implies that anthropogenic changes in land use could have significant effects on local precipitation throughout the world."
It's not clear at this point just how serious the consequences will be. But the researchers warn that if we don't understand the problems, we won't have the solutions. So Seto will continue with her frequent pilgrimages to a land that has changed much since her youth. And Shepherd and hopefully many others will try to figure out exactly what we have wrought.
Lee Dye is a former science writer for the Los Angeles Times. He now lives in Juneau, Alaska.