Human influence on local climate

By the Science and Environmental Policy Project
September 26, 1998

Human beings can and do effect local climate, and scientists are taking more and more of an interest in the various ways that this occurs. Airline contrails may create thin cirrus clouds that create regional warming over areas of heavy airline traffic. Irrigation and swimming pools in desert areas raise humidity levels, which can affect rainfall patterns. But one of the strongest influences on local temperatures is the "urban heat island effect," which is especially noticeable in higher nighttime and wintertime temperatures. Since higher "lows" are also thought to be a symptom of global warming from CO2, this creates some difficulty in assessing the surface temperature data. It also underscores the need to put greater reliance on the satellite data, which are truly global and diffuse the distortions created by these local effects.

The following article from the Arizona Republic newspaper gives a good explanation of the urban heat island effect and the startling impact it can have on local temperature readings. Although top officials of the U.S. EPA and NASA tend to pooh-pooh the effect of heat islands in their public discussions of global warming, this article indicates that a serious study is now underway in 10 cities to fully assess this phenomenon.

Copyright 1998 Phoenix Newspapers, Inc.
September 25, 1998 Friday, Final Chaser


You think the Valley of the Sun is hot now? Stick around another generation and feel the heat with twice as many people.

Phoenix area temperatures could continue to increase - perhaps as much as 15 to 20 degrees above historic averages - as buildings, roads and parking lots march across former croplands and virgin desert.

Phoenix is becoming the quintessential example of what scientists call an urban heat island. Imagine a huge dome of air over the city - hottest at its center and cooler at its edges. As development spreads, increasing amounts of asphalt and concrete soak up the sun's radiation during the day. Those artificial surfaces then release heat at night more slowly than irrigated farmland or natural desert soils.

This retained heat means higher electric bills, more smog, higher wind velocities, more water usage - and more deaths. Yet experts predict it may be decades before the increased heat and its effects discourage most people from moving here.

"As long as Phoenix grows, there's little doubt that it will get warmer," said Robert Balling, director of the Office of Climatology at Arizona State University.

It was Balling who a decade ago documented a curious trend, one that explains why people here don't keep their windows open at night as often as they once did.

Since World War II, when many Phoenix residents still slept on porches outdoors, average summertime lows have broken through the comfort zone. They've risen more than seven degrees over the past half century, from a balmy 73 degrees to more than 80 degrees.

What happened over 40 years was a tenfold increase in the Valley's population, from 150,000 residents to more than 1.8 million people when Balling completed his study in 1984.

Since then, the Valley's population has grown half again to nearly 2.8 million. It is expected to pass 4.6 million by 2020, and 7.3 million by 2050, according to Tom Rex, research manager for ASU's Center for Business Research.

Interestingly, average daytime summer highs in Balling's study remained about the same - hovering between 102 and 104 degrees. This relative constant is due to the wind's dispersal of the sun's energy during the day. Is this still the case? Balling isn't sure, having switched his study to global warming.

A compilation of National Weather Service data by The Arizona Republic shows that both the average summertime highs and lows in the past decade have increased slightly.

But Balling said the data cannot be accurately compared, since the location of the Weather Service monitoring station at Sky Harbor International Airport was switched in 1994. The new location records temperatures 2 degrees cooler than the old location, and still the averages are higher, said Craig Ellis, Weather Service meteorologist.

To find out more precisely what is happening, a team of federal investigators from the Environmental Protection Agency and the National Aeronautic and Space Administration recently began a study of the heat island effect in 10 U.S. cities, including Phoenix.

"What we want to do is provide information to decisionmakers - city councilmen, mayors, city planners, even health officials," said Dale Quattrochi, senior research scientist at NASA's Global Hydrology and Climate Center in Huntsville, Ala.

"We're trying to understand how we as people living in cities can actually modify the urban environment ... the overall heat island effect," said Quattrochi, who will be a featured speaker Oct. 14 at a Tucson forum, Hot Topics-Cool Solutions, aimed at finding ways to keep cities cool.

Quattrochi estimated that Phoenix temperatures likely will increase as much as 15 degrees, and possibly up to 20 degrees, over historic averages the next several decades.

In Atlanta, one of the most studied heat islands, researchers found that storms skipped over the city and left downwind rural areas with more rain. In Sacramento, scientists have detected swings in temperature of more than 50 degrees between buildings and nearby wooded areas.

Besides Phoenix, Atlanta and Sacramento, the federal research project involves Tucson, Salt Lake City, Houston, Chicago, Los Angeles and Nashville, and Baton Rouge, La.

Wil Orr, director of the Sustainability and Global Change Program at Prescott College, called human-caused climate change "civilization's defining issue."

"If we want this region to be economically competitive in the future, we need to do everything we can to reduce the impacts on the environment," said Orr, who has identified heat island impacts in the simplest of actions.

Flipping a light switch, for example, creates more heat inside the home, which must be cooled by an outside air-conditioning unit. The more the air-conditioner works to keep the inside cool, the more hot air it dumps outside, Orr said. Likewise, a car takes in relatively cooler air and releases it as hot exhaust.

Some of the solutions, therefore, also involve simple actions, scientists said, such as buying cooler compact fluorescent light bulbs, driving less, planting trees (and in the right places), using lighter colors on walls, roofs and pavements that reflect heat, rather than retain it.

One computer study by the University of California-Berkeley showed that summertime temperatures in Los Angeles could drop up to nine degrees, simply by covering buildings and roads with light-colored surfaces. Other scientists, however, question if covering surfaces in all light colors may prove too glaring or ugly for some tastes.

That same study showed that, by reducing heat, the amount of smog also could be reduced by up to 10 percent - the equivalent of removing more than 3 million cars from the road.

Here in the Valley, the Salt River Project estimates that for every degree increase in temperature, the utility's 610,000 residential customers pay $3.2 million to $3.8 million extra per month in cooling costs, or about $5 to $7 per customer per month.

How hot can it get before the Valley's quality of life is compromised? Maybe there is no limit, said ASU's Balling, who is among the world's most skeptical of scientists when it comes to human influence on global warming.

"I'm sure that in the next century, the technology curve will stay out ahead of the warming curve, and the majority of people in the city will live quite comfortably. We should withstand most heat stress incidents, because the car is air-conditioned, the house is air-conditioned and the office is air-conditioned," he said.

"I don't think it's going to be a limiting factor at all. I do not believe people are going to stop moving to Phoenix because it's too darn hot here."

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