U.S. 'brightening' stronger than thought

BOULDER, Colo., May 7 (UPI) -- Scientists say solar radiation reaching the Earth's surface in the United States occurred at higher rates than thought during the past 12 years.

Several studies have found evidence of an increase in solar radiation -- called "brightening" -- reaching the Earth during the late 1990s.

Past efforts correlated brightening trends to decreases in atmospheric aerosol concentrations and recovery from the Mount Pinatubo eruption.

Researchers from the National Oceanic and Atmospheric Administration, the Pacific Northwest and Brookhaven National Laboratories, and Switzerland's Eidgenossi Technical University studied measurements of shortwave radiation collected at 12 surface sites across the United States. That data, they said, show widespread brightening occurred over the continental United States during the past 12 years at rates higher than previously thought.

The authors said they determined changes in aerosol concentrations and other direct effects cannot fully explain the shortwave radiation changes. They suggest the causes of global brightening are complex and are best studied locally or regionally, rather than on a global or continental scale.

The study appears in the Journal of Geophysical Research-Atmospheres.