When the effects of global warming are discussed, Europe is often the focus. While many parts of the Earth have seen little or no warming in the past two decades, Europe has seen a rapid temperature increase of one full degree Centigrade. The rise has been a contributing factor in at least one deadly heat wave in recent years.

A new study suggests much of that warming isn't due to global warming at all, but rather a decrease in atmospheric pollution as a result of clean air legislation. The cleaner air has fewer small particles known as aerosols, which tend to block sunlight from reaching the Earth's surface. A reduction in aerosols leads to an effect known as "solar brightening," which increases surface warming.

The research was conducted by a team of 13 scientists, and led by Christian Ruckstuhl of the Institute for Atmospheric and Climate Science, in Zurich, Switzerland. By measuring an atmospheric characteristic known as optical depth, they determined that a substantial amount of solar brightening had taken place, leading to an increase in surface heat of 1 watt per square meter. Such an change is enough to add some some 360,000 megawatts of solar heat to Germany alone.

The measurements were conducted at a series of sites across Germany and Switzerland. Over the 20 year period from 1986-2006, a 60% decline in atmospheric aerosols was detected. The authors attribute this primarily to emission reductions of sulfur dioxide and carbon black particles, both of which were heavily generated in the 1970s and early 1980s by diesel engines and coal power plants. Clean air regulations requiring ULS (ultra-low sulfur) fuel and scrubber systems for coal-fired plants have dramatically reduced these emissions.  The result is the observed solar brightening, which has "strongly contributed" to Europe's warming over the period.

In performing their analysis, the authors had to subtract out the years from 1991 to 1994, as the eruption of Mount Pinatubo in the Philippines caused a spike in aerosol concentrations in Europe that more than doubled readings in some places.

The research appears in the June 26 edition of Geophysical Research Letters.