August 8, 2011
An abnormally warm decade is part of the new ‘normal’
It turns out that, even when youre talking about the weather, “normal” is a relative term.
On July 1, normal temperatures all across the United States became slightly warmer. Thats because the 30-year averages used to determine “normal” changed, dropping the decade of the 1970s and adding the decade of 2001 through 2010.
“Definitely on a national scale and globally, 2001 through 2010 was the warmest decade in history. On a regional basis, it gets a little thornier to say its the warmest, but it is up there,” said Karin Bumbaco, a University of Washington research scientist with the Joint Institute for the Study of the Atmosphere and Ocean in the College of the Environment.
Normal weather conditions are revisited at the end of each decade, when the National Climatic Data Center, an arm of the National Oceanic and Atmospheric Administration, adds the figures for the most recent decade and eliminates those from the decade that ended 30 years earlier.
In the past the process didnt make much of a splash because it had little practical effect. But in this case, Bumbaco said, the change could be more noticeable, particularly in some parts of the country that have had significantly warmer temperatures in the last 10 years.
Some places could now have a normal high temperature for a given date that is a degree or two higher than it was last year. So a warm day might not seem quite so warm by comparison to the new normal, though in fact it might be well above the old normal. Likewise, a cold day might sound even colder when compared with the new normal.
“The average person isnt going to think about the time period that goes into determining the normal, so shifting the reference could seem confusing because people arent taking into consideration all of the climate variables that go into ‘normal,” said Bumbaco, who is the assistant Washington state climatologist.
Those variables include climate change triggered by the buildup of greenhouse gases in the atmosphere. But they also include a variety of natural factors such as the El Niño Southern Oscillation, the seasonal phenomenon that produces El Niño and La Niña events, and the Pacific Decadal Oscillation, which can produce warmer or cooler temperatures over a large area spanning a period of decades.
The new normals could get an interesting test this winter. Last winter was relatively rainy and chilly in the Northwest, thanks to a La Niña in the South Pacific. Those conditions waned as spring progressed, but now scientists see the same conditions re-emerging for the coming winter, though probably less pronounced than last winter. If the region does experience some of the same conditions as last winter, it could sound a bit colder and gloomier when compared to the new normals.
But Bumbaco noted that the differences might appear starker on paper than most people will notice in their everyday experience.
“It will be interesting to see how it plays out. Will some meteorologists compare temperatures to both normals for a while until people get used to the idea? Thats something we might look at for our newsletter,” she said.
Current and archived copies of the state climatologist newsletter are available.
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For more information, contact Bumbaco at 206-543-3145 or kbumbaco@uw.edu, or state climatologist Nick Bond at 206-526-6459 or nab3met@uw.edu.
