September 1, 2009

Southwest heat waves likely to worsen

U.S. scientists studying a mid-July to early August 2006 heat wave that struck the southwestern United States say such extremes are becoming more likely.

The 2006 heat wave broke many high temperature and humidity records, scientists said, leading to the deaths of more than 600 people, 25,000 cattle and 70,000 poultry in California alone.

An analysis of that episode by researchers at the Scripps Institution of Oceanography at the University of California-San Diego suggests such regional extremes are likely to become more frequent as climate change trends continue.

The researchers said they have already discovered nighttime heat waves in California have become more frequent and intense since 1990, characterized by unusually high humidity.

Elevated humidity also causes heat waves to last longer. Hotter nights pre-condition hotter days and the cycle feeds on itself until the winds change, Alexander Gershunov, who led the study, said. The weather pattern that traditionally causes heat waves in California is tending to bring with it more humidity, changing the character of heat waves from the dry daytime heat and cool nights typical for this region, to the muggy heat around the clock that locals are simply not accustomed to.

The study appeared in the July online edition of the Journal of Climate.