Climate Hot Spot Cleaner Air Means a Warmer Europe
The 1970s were a hazy time: Cars ran on sulfur-rich gasoline, power plants and heavy industry burned sulfur-rich coal. Europe lay under a blanket of fumes filled with sulphate particles. Acid rain brought the particles back to earth, ravaging the continent's forests.
That was then. The situation today is considerably different. Auto emissions are low in sulfur, power plants only run with smoke filters and acid rain is no longer an issue. But the success of efforts to restore Europe's air quality have had an unintended side effect that is just now coming to light. Because the atmosphere over Europe is increasingly clean, global warming is impacting the continent more quickly than other regions of the world.
The dwindling clouds of pollution are apparently the reason that Europe is heating faster than other mid-latitude regions. Since 1980, the average surface air temperature between the Bosporus and the Bay of Biscay has risen by almost an entire degree Celsius -- twice as much as expected. The reasons for this were until recently a matter of heated dispute. Greenhouse gases could explain half that increase, at best. But now climate researchers in Germany, Switzerland and the United States, using data and computer simulations, claim that the rise in temperatures has been caused most directly by a decline in sulfate aerosols in the atmosphere.
Sulfates work like a filter on sunlight: They reflect short-wave solar radiation back into space, thereby letting less energy pass into the layer of air closest to the ground. But because the concentration of sulfur particles is declining so rapidly, this unintended cooling effect no longer works the way it once did -- and Europe is getting hotter. To Martin Wild of the Swiss Federal Institute of Technology Zurich (ETH Zurich), it makes perfect sense: "We have less aerosol in the atmosphere, more radiation reaching the surface of the Earth and an exorbitant increase in temperature."
At the annual conference of the European Geosciences Union (EGU), which begins on Tuesday in Vienna, climatologists and atmospheric researchers will discuss whether the trend will continue. As the air in developing regions like India and Central Africa get ever dirtier and the light penetrating the cloud of pollution dims, Europe is rapidly getting brighter.
Together with Joel Norris, an atmospheric scientist at the Scripps Institute of Oceanography in San Diego, Wild has been calculating fluctuations in European radiation levels. In the middle of the 1980s, there appears to have been a major shift. Until then, as the air became its most sulfur-laden, radiation near the surface reached all time lows of around three watts per square meter. Beginning in 1986, when efforts to clean the air began to pay off and the atmosphere became more transparent to sunlight, radiation started to increase about 2 watts per square meter in each year over the next decade.
Wild based his work on an aerosol-tracking computer simulation developed by researchers at the Max Planck Institute for Meteorology in Hamburg. "Extensive air pollution temporarily compensated for the global warming in Europe," said Johann Feichter, leader of the institute's Aerosol, Clouds and Climate working group.
"Our Findings Contradict the IPCC"
Sulfate particles not only minimize the transfer of solar radiation directly, they also encourage the accumulation of water droplets and clouds in the atmosphere -- which in turn creates a second, indirect form of radiation repellent, because the light-colored surface of clouds reflects sunlight. This is where it gets complicated, though. In the landmark report issued last year by the Intergovernmental Panel on Climate Change (IPCC), the United Nations organization stated that questions persist about the effect that sulfate aerosols have on the atmosphere.
"Our findings contradict the IPCC," said Rolf Philipona of MeteoSwiss, Switzerland's national weather service. He and his colleague Christian Ruckstuhl, who now works as a researcher in California, analyzed data from 25 weather stations in northern Germany and eight in Switzerland.
"We found that the increase in radiation on the ground is considerably greater under a cloudless sky than a clouded one," Philipona told SPIEGEL ONLINE. The direct impact of particulate pollution is about five times greater than the indirect impact, he explained. That shows, he claims, that sulfate aerosols play "a truly important, immediate roll" in Europe's climate conditions.
Geert Jan van Oldenborgh agrees. A physicist from the Royal Dutch Meteorological Institute (KNMI), van Oldenborgh sees a direct connection between cleaner air and climate change in Europe. But he does add a qualification: He says the aerosol effect is not notable year-round. It is only apparent in spring and summer, when days are long and solar radiation is naturally stronger. "But the temperatures in autumn and winter have also risen faster than expected," said van Oldenburgh. There must be another explanation for that phenomenon, he believes.
The Dutch weather institute recently conducted a study on the issue -- but the results have yet to be published, and researchers are currently unwilling to discuss preliminary findings. One can assume that changes in air currents during the cold months play a decisive roll. There is an increase in fronts from the West transporting warm air from the Atlantic; and at the same time the cold Siberian High is weakened.
Europe's air is not likely to get much cleaner than it is now -- neither in summer or in winter. "The concentration of aerosol is stabile," said ETH Zurich's Wild. And Philipona of the Swiss weather service, is sure that "this increase in temperature, as we saw in Europe in the 1980s, will not happen again."
But this is by no means an announcement that the danger has passed. Greenhouse gases are still represent a threat, and increasing and unchecked emissions will almost certainly warm the Earth's atmosphere. The German Meteorological Society (DMG) claims that the median temperature in Europe in 2040 will be 1.7 degrees Celsius higher than the median temperature before the Industrial Revolution. Frequent heat waves, severe storms and other extreme weather are a foregone conclusion.