Scientists point to heat wave as future

Early June temps unprecedented in Denver records

Jim Erickson, Rocky Mountain News

Published June 19, 2006 at midnight

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Climate scientists say this month's shockingly early heat wave may provide a glimpse into Denver's future.

The temperature at Denver International Airport reached or surpassed 90 degrees on 12 of the first 14 days in June, according to the National Weather Service.

The hot spell included seven straight days of 90-or-higher temperatures, marking the earliest such streak since recordkeeping began in 1872. The mercury hit 102 last Wednesday, a record for the date and the earliest triple-digit temperature ever for Denver.

"My perception is that I've never seen it this hot at this time of year," said assistant state climatologist Nolan Doesken.

"It's not ridiculously uncommon for late June to early July, but it is very unusual at the beginning of June," said Doesken, who will take over as state climatologist on July 1.

Western heat waves are likely to increase in severity, frequency and duration in the coming decades as the climate warms in response to the ongoing buildup of heat-trapping "greenhouse" gases, said Gerald Meehl, a senior scientist at the National Center for Atmospheric Research in Boulder.

Future warming is likely to intensify the weather patterns responsible for heat waves in some parts of North America, including the West, according to a 2004 NCAR study published in the journal Science. Meehl was lead author of the study, which used computer models to simulate the future climate.

"This one was pretty intense pretty early," Meehl said of the metro area's recent heat spell. "And that's the kind of thing you'd expect to see more frequently in the future, as the average climate warms."

But no single extreme weather event - whether it's a heat wave or a hurricane - can be blamed on global warming, he said Friday.

By the end of this century, heat waves along the Front Range could last an average of two days longer than they do now, and the number of heat waves per summer could double, according to the NCAR study.

Some changes in Denver temperatures are already apparent, said University of Colorado climatologist Klaus Wolter. But it's unclear if human activities, such as the burning of fossil fuels, are to blame, he said.

Springs and summers have warmed in Denver over the past decade, with the biggest temperature increases in March and July, Wolter said.

Eight of the past 10 Denver springs (defined by climatologists as the months of March, April and May) and nine of the past 10 Julys were warmer than the long-term average at the former Stapleton International Airport in Denver, Wolter said.

While the official National Weather Service temperatures for Denver are now measured at Denver International Airport, Wolter prefers to use Stapleton data because they contain an unbroken record extending back more than 50 years.

"If nine of the last 10 Julys are above average, there's definitely something there that is not just noise," he said. "This is a very significant shift. It's just that we don't understand fully what the causes of the shift are."

Denver residents endured the greatest number of 90-or-higher June days - 17 - in 2002. That year, wildfires burned more than 600,000 acres in the state.

The federal Climate Prediction Center's latest three-month temperature outlook, issued Thursday, calls for above-normal temperatures in Colorado through September.

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