Heat waves out West are getting worse as the climate changes, a new study finds.

One example: From mid July to early August 2006, a heat wave swept through the southwestern United States. Temperature records were broken at many locations and unusually high humidity levels were recorded.

The event included extreme muggy heat that is part of a trend of increasing nighttime heat wave activity observed over the last six decades, the researchers said in a statement today. This trend has accelerated since the 1980s and has become especially prevalent in this decade, they conclude.

The results are not isolated, and they fit with predictions that a warmer world will produce greater extremes.

A study in 2007 found European heat waves are nearly twice as long as they were a century ago and the number of hot summer days there have tripled.

Why it matters: Other studies show heat waves are deadlier than hurricanes or tornadoes, and they have been so throughout modern history. Climate experts have warned that the sort of serious heat wave that is now possible given current climate conditions, but which has not struck yet, could kill thousands of U.S. residents.

And there's some irony in the problem:

As heat waves worsen, more energy is used to run air conditioners. If the electricity is generated using fossil fuels, this could also mean even more emissions of heat-trapping gases that cause climate change, scientists wrote in the Journal of Applied Meteorology and Climatology last year.

"Electricity demand for industrial and home cooling increases near linearly with temperature," said the leader of that study, Norman Miller, an earth scientist at Lawrence Berkeley National Laboratory and geography professor at the University of California, Berkeley.

"In the future, widespread climate warming across the western U.S. could further strain the electricity grid, making brownouts or even rolling blackouts more frequent," Miller said.

The nighttime heat waves of 2001, 2003 and 2006 were each unprecedented on record when they occurred. The source of the moisture that propelled the heat waves was an area of the eastern Pacific Ocean where a strong increase in sea surface temperatures has been observed and linked to global-scale trends of human-induced warming of the upper oceans, the new study found.

"Water vapor is the main greenhouse gas. During the night in humid environments, air doesn't cool nearly as much as it does in dry conditions," said study leader Alexander Gershunov of the Scripps Institution of Oceanography at UC San Diego.

"Elevated humidity also causes heat waves to last longer," Gershunov explained. "Hotter nights pre-condition hotter days and the cycle feeds on itself until the winds change. 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 2006 event in particular stressed the state's emergency services programs and killed so many dairy cows that milk production fell 10 percent.

A preliminary version of the study was published July 27 in the online edition of the American Meteorological Society's Journal of Climate.

Miller's team, in last year's study, also looked at the increase in heat-wave severity.

In 2008, California experienced an unusually early heat wave in May, they noted: Some 119 new daily high temperature records were set during the May heat wave, including the earliest day in the year in which Death Valley temperatures reached 120 degrees Fahrenheit (on May 19, beating the old record of May 25 set in 1913).

There's been no letup in the Southwest this year. In Phoenix, this July was the hottest on record, with the average of all highs and lows for the month being 98.3 degrees. One reason, meteorologists said: Overnight lows were much warmer than normal.