London's Cleaner Air is Causing More Global Warming

Damned if they do, damned if they don't.

London’s fog, a fixture in the U.K. capital that led to the deaths of 4,000 people in 1952, may be on the wane, contributing to warmer temperatures. Why? Cleaner air.

French-led scientists studied three decades of data from 342 weather stations across Europe and found that on average, the number of days where London visibility was lower than 2 kilometers (1.2 miles) has halved since 1980. The trend correlates with a fall in emissions of sulfur dioxide, a gas associated with burning coal and oil, they said yesterday in the Nature Geoscience journal.

“We’ve moved away from using coal and wood in the home and our industries have become much cleaner,” Dave Britton, a meteorologist at the U.K. government forecaster, the Met Office, said in a telephone interview from Exeter. “So we see much less of that particulate matter which allows water to condense.”

Fog and mist form when water droplets are suspended around particles in the air. The decline in fog across Europe may also have contributed to warming of the continent in recent years, said the study’s authors, led by Robert Vautard, a scientist at the French Atomic Energy Commission outside Paris.

“By enabling less energy to be received at the surface during daytime, the low-visibility phenomenon inhibits surface heating and therefore induces a lower local temperature,” the researchers wrote.

The decline in fog has raised temperatures by 0.08 degrees Celsius (0.14 Fahrenheit) per decade across Europe, or up to a fifth of the total warming observed, the scientists calculated. In eastern Europe, the drop in fog may account for half of the total warming, they said. The influence on warming of declining fog will wane in the future because there are fewer fog days to lose as the air becomes cleaner, they said.