Scientists have long studied the impact of cities on local weather, such as the heat island effect, but a new study also finds that skyscrapers and all of that asphalt rest on some of most fertile land, affecting the Earth's complex food chain.
The Price of Urbanization
Cities account for just 3 percent of the continental U.S. land area, but the potential for efficient plant growth in these areas is quite significant. NASA researchers Marc Imhoff and Lahouari Bounoua say that throughout history humans have settled in areas with the best lands for growing food.
Agriculture gives rise to urban growth - as it brings food, wealth and trade to an area, all essential to human productivity. But, these advantages could be even more pronounced if urban areas were planned in conjunction with other environmental factors. Studies like this one may lead to smarter urban-growth strategies in the future.
The Culprit: Fewer Plants
The researchers used two satellites offering daytime and nighttime Earth observation data and a biophysical computer model to estimate annual Net Primary Productivity (NPP). NPP measures plant growth by describing the rate at which plants use carbon from the atmosphere to build new organic matter through photosynthesis. NPP fuels Earth's complex food chain and quantifies amounts of carbon dioxide, a greenhouse gas, which plants remove from the atmosphere.
By using nighttime-lights data from the Defense Meteorological Satellite Program, NASA vegetation maps, satellite data and computer models, the researchers found that when compared to the pre-urban landscape, modern cities account for a 1.6 percent annual decline in NPP. This is especially striking given the small relative land area that cities cover.
Image Above: Earth at Night. Courtesy: DMSP and NASA.
It remains unclear how the reduction in NPP affects biological diversity, but it translates to less available energy for the species that make up the Earth's complex food web. The loss of fertile lands for farming also puts pressure on other means to meet the food needs of an increasing population.
The advantages of urbanization, including the transportation of water to arid areas and the creation of urban heat islands that extend the growing season, are minor when compared to overall negative impact of cities on NPP.
Research partners in this study included the University of Maryland's Earth System Science Interdisciplinary Study, the World Wildlife Fund, and the Center for Conservation Biology at Stanford University.
NASA's Earth Science Enterprise is dedicated to understanding the Earth as an integrated system and applying Earth System Science to improve prediction of climate, weather and natural hazards using the unique vantage point of space.
