The Sky is Falling: It's Rain

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.

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."