Cathedral has weathered London's acid rain.

Where St. Paul's Cathedral now stands in London, previous cathedrals had fallen to fire, Viking raiders, and war. The current structure, designed by Christopher Wren in the late 1600s, withstood direct hits by German bombers during World War II, but it's been succumbing to a more insidious challenge--acid rain.

That environmental attack is abating, however. On Sept. 13, scientists at the annual conference of the British Geomorphological Research Group will report that St. Paul's stony exterior now weathers only about half as fast as it did in the 1980s. The likely reason? London's atmospheric sulfur dioxide concentrations, which contribute to acid rain, have been dropping.

To monitor the rate of erosion, researchers 20 years ago placed metal reference bolts at six points around the cathedral. They subsequently measured that the surface of the stone building lost nearly half a millimeter between 1980 and 1990 but only a quarter of a millimeter between 1990 and 2000.

"Rocks weather slowly, so we're talking about the thickness of a piece of paper," says Stephen T. Trudgill of the University of Cambridge in England. Yet half a millimeter of weathering over 10 years is about 10 times what's expected in an unpolluted environment, he says.

With the massive reduction of heavy industry in London, atmospheric sulfur dioxide concentrations have plunged from concentrations as high as 100 parts per billion in the 1970s to 10 parts per billion today, says Trudgill.

Beyond the cathedral's overall deterioration from acid rain, Trudgill adds, St. Paul's southwest corner had eroded in the early 1980s at a rate 10 times the average for the rest of the cathedral.

During that time, the Battersea Power Station faced the southwest corner of St. Paul's from across the Thames River. Smokestack emissions traveling downwind likely assaulted the corner, says Trudgill. The power station closed in 1983, and the troubled corner now weathers at the same rate as the rest of the structure, he says.