Warning against the increasing growth of sprawling cities

Urban sprawl worldwide is gobbling up rich agricultural lands and reducing productivity, with some of the "mega-cities" even able to affect local climate. The speed with which urbanisation is swallowing up land has been graphically illustrated at the American Association meeting using photographs taken by the orbiting Landsat satellites over the past 21 years.

The rapidly growing city of Atlanta, Georgia, was used as an example, but urban expansion was seen right around the world, Dr Jeffrey Masek of the University of Maryland said.

Washington DC's urban areas had grown by about 22 sq km a year since 1973, he said, too rapidly for proper land use development.

There were 400 sq. m of urban space per capita of population there, compared with just 100 sq. m per capita in Portland, Oregon, which had strict controls on land use.

Dr Marc Imhoff of NASA described his detailed study of Atlanta and its environs where 55 acres of trees had been cleared each day for the past 19 years to make way for housing and city development.

The satellite pictures had shown the "dramatic change in the amount of urbanisation that has occurred", he said. Reduced vegetative cover meant that urban areas were much warmer than lands away from the city.

This "heat island effect" was seen in temperature records with central Atlanta reaching 120 Fahrenheit compared with just 80 F away from the city on a typical summer's day. At night temperatures would typically remain between 20-25 F warmer over the city, which affected the weather, according to Dr Imhoff.

The heat island can spark precipitation and even thunder showers just beyond the city in the direction of the prevailing weather patterns.

"The style of land use is such that it tends to occur on the better soils," Dr Imhoff said. This changed the "biological productivity of the landscape".

Reduced land availability around Atlanta had reduced productivity, the equivalent of taking 20 days from an average growing season - "like turning the lights out in a greenhouse for 20 days", according to Dr Imhoff's figures.

He also found that while the heat island tended to extend the growing season, reduced overall productivity of the land was also noted. "If urbanisation and industrialisation continue, the capacity of the landscape to carry out photosynthesis is substantially reduced," he said.