London, Dec 5 : A new research into how stone facades will be altered by changes in the atmosphere has suggested that as pollution patterns change, iconic limestone structures like the Empire State Building and the Pentagon will turn green with lichen and moss in the future.
According to a report in New Scientist, cities will become more colorful as pollution patterns change and wind-swept rain washes away the black coal soot typical of the 20th century.
In the 19th and 20th centuries, cities in Europe and the US were dominated by dark grey and black buildings.
These were often made of cream-coloured stones like limestones, covered in a black crust of coal soot.
According to Peter Brimblecombe and Carlotta Grossi of the University of East Anglia in the UK, the era where atmospheric pollution determined the damage to building materials is over.
The pair recently completed an assessment of how damage to buildings in London has varied over the past 900 years and how it is likely to evolve over the coming century, when soot from coal-burning stoves is unlikely to be a concern, but climate change is.
They used historical climate and pollution data, combined with equations that describe how different climates and different types of pollution affect building materials.
For instance, archive tax records show how much fuel was used through the centuries, which can be used to estimate historical pollution levels.
With estimates of how much black carbon soot deposited on buildings at different times, the researchers can calculate the reflectivity of buildings, which indicates how black they were.
It turns out that for most of the 900 years, buildings were clean.
"It seems the past two centuries (of blackened buildings) were a bizarre anomaly," said Brimblecombe. "In a sense, we are now back in medieval England," he added.
A study on a limestone building known as the Cathedral of Learning at the University of Pittsburgh, using pictures taken throughout its 20th century construction, shows that the cream-coloured facade was black even before the structure was completed in the 1930s.
In subsequent decades, however, air-quality regulations meant black crusts of soot became less of a problem, and parts of the building that get most soaked with rain have been naturally cleaned.
According to the researchers, the coming century will see iconic limestone structures like the Empire State Building, the Pentagon, and the gothic cathedrals of Europe and the US turn yellow, reddish-brown, and even green with lichen and moss.