The Sky is Falling: It's Rain

Big Cities are Changing Rainfall Patterns Around the World


July 18, 2007 —

If you think global climate change is something that only happens far away, you may want to take a look at your nearest megalopolis. Two very different studies have found evidence that rainfall patterns near major cities have changed over the last few decades, and not because of greenhouse gases.

But the changes have some things in common with the greenhouse effect: Both result from human activities that are having a significant impact on the entire planet. And both demonstrate that we are in for some major changes in the years ahead, many of which will probably catch us by surprise.

One study focused on the Pearl River Delta in China, which has rapidly turned from a rural, agricultural region to one of the fastest-growing cities in the world. Researchers found that rainfall during the dry winter season decreased considerably there over the last nine years, when the rainfall in the urban area increased 300 percent.

The other study focused on changes in rainfall patterns — not necessarily overall precipitation — near "urban heat islands," those huge cities like Phoenix and Atlanta, where rainfall has actually increased downwind of the metropolitan area over the last century.

Both studies indicate that the changes are the result of urbanization. They also suggest that the human footprint can leave a conflicting legacy in different environments.

'Urban Precipitation Deficit'

Karen Seto, assistant professor of geological and environmental sciences at Stanford University, returned to her family's homeland near Hong Kong, where she was born, for her research. Seto and Robert K. Kaufmann of Boston University reported their findings in a recent issue of the Journal of Climate. They analyzed Landsat satellite images to determine the explosive growth in the region, and then compared that to monthly climate data from 16 meteorological stations.

The results, they report, "constitute the first statistically meaningful empirical evidence for an 'urban precipitation deficit.' This effect may be generated by changes in the surface hydrology that reduce the flow of water from the land to the atmosphere."

"We found that as the cities get bigger, there is a negative impact on precipitation patterns, such that in the winter season there is a reduction in rainfall as an effect of urbanization," Seto said. "Primarily it is caused by the conversion of vegetated land to asphalt, roads and buildings. As a result, the soils have significantly less ability to absorb water, so in the winter months there is less moisture in the atmosphere and therefore a reduction in precipitation. We don't see the same impact in summer months, in part because the effect of the Asian monsoon masks the effect of urbanization."

Meanwhile, on the other side of the planet, J. Marshall Shepherd, a climatologist at the University of Georgia, has teamed up with colleagues at NASA's Goddard Space Flight Center, and they have found a different effect in this country. They found, for example, a 12 percent to 14 percent increase in rainfall in the area northeast of Phoenix from the pre-urban period (1895-1949) to the post-urban era (1950-2003.) Phoenix has experienced explosive growth during that period, and continues to do so today.

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.