Can Cities Change Local And Global Climates?

There is new evidence from satellite and ground observations which indicate that all the asphalt, buildings and aerosols around cities are affecting local and possibly global climate.

To study urban impact on local rainfall, Dr. J. Marshall Shepherd of NASA's Goddard Space Flight Center, Greenbelt, Md., and Steve Burian of the University of Utah, Salt Lake City, used satellite data as well as rain monitoring networks on land. They determined that there is higher rainfall rates during the summer months downwind of large cities like Houston and Atlanta.

Houston is an especially good city to study because scientists can study weather data from before and after an intense urban growth period from 1958 to 1999. Using this data, scientists can compare climate patterns before, during and after periods of urban growth.

All large cities are "heat islands." They tend to be one to ten degrees Fahrenheit warmer than surrounding suburbs and rural areas. Asphalt and concrete tend to act like heat sponges. They soak in heat during the day, keeping the temperature higher than it would normally be. This heat tends to radiate out of the city, warming the winds that transfer it. The varying heights of the buildings themselves can alter the winds themselves. Combined, it is enough to affect at least the local climate.

In related work, Dr. Daniel Rosenfeld, an atmospheric scientist at Hebrew University, Jerusalem has been studying the affect aerosols around urban areas have on climate. Aerosols are tiny particles suspended in the air. Some occur naturally, originating from volcanoes, dust storms, forest and grassland fires, living vegetation, and sea spray. But humans, especially around urban areas add their own amount of aerosols. Burning fossil fuels and altering natural ground cover also produce aerosols. There is a definite increase in aerosols downwind of industrial sites.

Data has shown that the increase in aerosols near large cities alter local rainfall rates. What seems to be happening is that these aerosol particles provide surfaces on which water can collect into droplets. This prevents the droplets from condensing into larger drops which in turn slows the conversion of cloud moisture to precipitation. This means larger clouds and more moisture. In summer, rain and thunder increases downwind of big cities, as rising air from urban heat islands combines with these large clouds.

NASA has an entire suite of orbiting Earth observing satellites to provide information to help these scientists. The relationship of land cover and land use is of particular interest in these studies of urban effects. The satellites also track the aerosols, clouds, water vapor, and temperature that describe atmospheric conditions in urban environments. Their measurements allow scientists to study urban impacts on the climate system practically anywhere on Earth.

Now that we know urban areas affect local climate, scientists need to look at the bigger picture. Local climate usually affects global climate patterns so these changes made by urban areas need to be understood. At first, it might seem that studying asphalt cover globally is a strange thing, but asphalt is a big factor in local climate. Asphalt greatly affects local heat transfer, water run-off and even how winds behave. Characterizing asphalt cover is probably the biggest urban effect to be factored into global models.

We have much to learn about local, regional and global climate patterns and how they affect one another. Our knowledge has increased many fold since the deployment of Earth observing satellites. While it isn't necessary for every single person to completely understand climate change, it is important to understand that humans have an impact on local and possibly global climate and to work to minimize that impact.

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