Urban Sprawl Reduces Annual Photosynthetic Production

The study also reveals that urbanization may be creating vast heat islands that can actually lengthen the growing season, but do not improve the productivity of the land.

NASA Goddard Space Flight Center (Greenbelt, Md.) researcher Dr. Marc L. Imhoff presents his findings during a news media briefing at the 2000 American Association for the Advancement of Science (AAAS) Annual Meeting at the Marriott Wardman Park Hotel (Washington, D.C.) on Monday, Feb. 21 at 3 p.m. in the Wilson Room.

According to Imhoff's research, urbanization and industrialization have resulted in the development of mega-cities and urban and suburban sprawl. The environment is altered as a result of replacing land cover with roads, housing, and commercial and industrial structures.

"Human survival depends on the ability of the landscape to produce food," said Imhoff. "Food production can be fundamentally linked to primary production or photosynthesis. If the capacity of the landscape to carryout photosynthesis is substantially reduced - then the ability of the planet to support human life must also be diminished."

Imhoff said data from the mid-1990's from two different satellite systems were combined with land cover maps and census information on population and housing to study the effect of urbanization on photosynthetic production in the United States. Nighttime images from a Department of Defense satellite, which show a dramatic picture of Earth's city lights, were used to determine which areas and how much land have been converted to urban, suburban, or industrial use. Maps showing urban, peri-urban (suburban), and non-urbanized areas were created from the "city-lights" satellite data.

"Using a computer, we combined the city-lights satellite data with another type of satellite data that records a measure of 'greenness' or photosynthetic potential of the landscape over the course of an entire year," Imhoff said. "By merging the satellite data we could examine how urbanization affects the potential of the land surface to carryout photosynthesis by looking at the 'greenness' index inside and outside the urbanized areas for the whole continental United States."

Results show that urbanization can have a measurable but variable impact on photosynthetic productivity. Annual photosynthetic productivity can be reduced by as much as 20 days in areas where housing and commercial land use is very dense.

"However, we also found that in resource limited regions, human activity can increase productivity by altering the environment," he said. "For example, this was the case for arid and semi-arid areas where lawn irrigation and planting changed the ecosystems from shrub lands and desert to deciduous forests."

A most interesting finding according to Imhoff was that urbanization seems to elongate the growing season, yet still reduces the overall productivity of the land. "Vegetation greens up earlier in the spring and takes longer to senesce in the fall, but has lower peak season productivity than similar nearby areas that are not urbanized," he said. "This could be demonstrating a profound urban heat island effect and have implications in climate change, especially in the northern Hemisphere where urban development is most intense."

Analysis of the data also found clear evidence that human beings definitely tend to locate themselves on the most productive land and that those lands are being transformed into less productive types.

"The results of this study should increase our awareness of the importance of land use planning especially in the context of sustainable growth and development," Imhoff stated. "Human survival depends on photosynthesis. If urbanization and industrialization continue, the capacity of the landscape to carry out photosynthesis is substantially reduced. "

For supporting images: http://svs.gsfc.nasa.gov/imagewall/AAAS