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Hot pavement, hot planet


Add parking lots and wheat fields to the rogues gallery of global warming bad guys.

A landmark study published last week connects the way people use land to the planet’s warming trend. The study was based on data collected by professor Steven Running, director of the Numerical Terradynamic Simulation Group at the University of Montana.

That the planet is heating up and that greenhouse gases are major culprits is an established scientific phenomenon, says Running. That puts most of the blame on industrialization and the carbon dioxide emissions that accompany it. But until this new study, no one had demonstrated that land use—most notably, urbanization and agriculture—had an undeniable effect on climate too.

“That’s because changes in the chemistry of the atmosphere are so well-known,” Running says. “(But) this study says in addition to greenhouse gases, we have to consider our endless change of the surface land cover.”

Running collected information about how people have altered the surface of the Earth using software written by his UM research team. The software is aboard the NASA satellite Terra, which was launched three years ago. Then Running sent the data to researchers at Colorado State University who plugged it into a computer model that mimics global weather.

“Think of all the desert that used to be dry that we now irrigate, think of the corn field that used to be a forest, and think of all that pavement for highways,” Running says. “We’ve proven this is a variable that needs to be in the master equation that wasn’t there before.”

The relationship between land use and local climate change had already been demonstrated, says Running. Cities are often 3 to 4 degrees Fahrenheit warmer than the surrounding countryside because cars and factories produce heat, for example, while concrete and buildings retain heat. Likewise, the dry prairie of Colorado’s Front Range has experienced a measurable rise in humidity during the past 30 years due to irrigation.

The CSU model showed that the climate of the entire planet was affected by such activities. When all other characteristics of the Earth’s atmosphere were held constant, agriculture and urbanization caused global warming within the model. The study only verified that a significant relationship exists, not the magnitude of that relationship.

Running knows that the study has political implications. The Kyoto Protocol—an international agreement on global warming drafted in 1997 (which the U.S. has not signed)—called for a reduction in greenhouse gas emission. The next agreement will need to include land use patterns, which could further complicate the negotiations. The new study, however, is unlikely to change the prescription for a cooler planet.

“The conclusions are the same,” Running says. “The bottom line is reduce fossil fuel consumption—you couldn’t plant enough trees. And improve the land cover energy balance. You want less of the Earth’s surface paved. It’s rather intuitive.”


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