Cities Change Local Climates, May Affect Global Climate

SAN FRANCISCO, California, December 12, 2003 (ENS) - Cities, with their asphalt, buildings, and aerosols, are impacting local climate processes and possibly the global climate as well. New evidence from satellites, models, and ground observations showing this urban effect on climate was presented by some of the world's top scientists at the Fall Meeting of the American Geophysical Union here on Thursday.

This information is important for understanding climate change as within a few years, for the first time in human history, the majority of the world's more than six billion people will be living in urban areas. Roughly 180,000 people are added to the urban population each day.

To study urban impact on local rainfall, the researchers used the world's first space based rain radar, aboard the Tropical Rainfall Measuring Mission (TRMM) satellite, and dense rain gauge networks on land. They found that there are higher rainfall rates during the summer months downwind of large cities like Houston and Atlanta.

The city of Atlanta, Georgia (Photo courtesy Area Connect)

Burian and Shepherd offer new evidence that rainfall patterns and daily precipitation trends have changed in regions downwind of Houston from a period of pre-urban growth, 1940 to 1958, to a post-urban growth period, 1984 to 1999.

Cities tend to be one to 10 degrees Fahrenheit (0.56 to 5.6 degrees Celsius) warmer than surrounding suburbs and rural areas.

Warming from urban heat islands, the varied heights of urban structures that alter winds, and interactions with sea breezes are believed to be the primary causes for findings of climate alteration in a coastal city like Houston.

In related work, Dr. Daniel Rosenfeld, an atmospheric scientist at Hebrew University, Jerusalem, reveals the increased amount of aerosols, tiny air particles, added by human activity to those naturally occurring, also alter local rainfall rates around cities.

Rosenfeld suggests the particles provide many surfaces upon which water can collect, preventing droplets from condensing into larger drops and slowing conversion of cloud water into precipitation.

In summer, rain and thunder increases downwind of big cities, as rising air from urban heat islands combines with 'delayed' rainfall resulting from the presence of aerosols, creating bigger clouds and heavier rain.

The urbanization of the Earth is taking place at an unprecedented rate. In 1800, only two percent of the world’s population was urbanized, according to the United Nations Habitat Program.

In 1950, only 30 percent of the world population was urban, but by the year 2000, 47 percent of the world population was living in cities.

The UN suggests that more than half of the world’s population will be living in urban areas by 2008. By 2030, that figure will rise to 60 percent.

New York City (Photo courtesy FreeFoto)

By 2015, it is expected that there will be 23 cities with a population over 10 million, 19 of them in developing countries, the UN Habitat Program forecasts.

To help scientists like Shepherd and Rosenfeld improve understanding of links between city landscapes and climate processes like rainfall, NASA's suite of Earth observing satellites provides information about the land cover and land use properties that initiate the urban effects.

The satellites track the aerosols, clouds, water vapor, and temperature that describe atmospheric conditions in urban environments. Their measurements allow scientists to make end-to-end studies of urban impacts on the climate system anywhere on Earth.

"The space borne instruments on Terra, Aqua, TRMM, and Landsat provide a wealth of new observations of aerosol particles near and downwind of cities, the cloud optical properties, and surface reflectance characteristics that can help us understand the effects that urban environments have on our atmosphere and precipitation patterns," said Dr. Michael King, NASA Earth Observing System Senior Project Scientist.

"Aura, to be launched in 2004, will add even more data," he said.

Since asphalt has such a large effect on local heat transfer, water runoff, and how winds behave, characterizing asphalt cover is probably the biggest urban effect to be factored into global models, a task that will be taken on by Georgia Institute of Technology's Dr. Robert Dickinson. He hopes to account for the cumulative effects of urban areas on regional and global climate models.