Record summer heat waves are already happening more often and will get even hotter and more frequent over the next 30 years, predict scientists who have run high-resolution climate simulations of temperatures across the United States.
The researchers were specifically looking at the potential effects of a global 2 degree Celsius (3.6 degrees Fahrenheit) warming and found it leads to increasing extreme summer heat waves -- of a lot more than 2 degrees C -- all over the United States, and especially in the West.
Their concern is that limiting global warming to 2 degrees C, as was discussed at the 2009 Copenhagen climate meeting, may not be enough to avoid dangerous climate change.
"The upshot is that we don't live in the global mean. What seems like a small warming can mean a lot of change," in specific places, said Noah Diffenbaugh of Stanford University.
Diffenbaugh and Moetasim Ashfaq published their results in the latest issue of the journal Geophysical Research Letters.
A single degree of global mean rise in temperature can lead to 2 or 3 degrees rise in summer temperatures in North America, Diffenbaugh explained. And that gives a leg up to extreme heat events, which always spike much higher than any means or averages.
The simulation shows that in the second half of this century there will be an additional five to eight record-breaking heat waves per decade over much of the country. The worst heat will be in the Western United States, with up to nine record-breaking heat events per decade as a result of that global mean warming of 2 degrees C.
That's a miserable, deadly heat wave almost every year.
"It's the rare becoming normal," Diffenbaugh told Discovery News.
The researchers looked at the 2 degree C warming because it is a marker that has been batted around a lot by scientists and policy makers. It's also a rather deceptive number, because it's not about how much warming will happen between now and 2040, but how much warming happens from the time humans started large-scale fossil fuel burning in the 1800s until 2040.
The global mean temperature has already risen 0.8 degrees C since pre-industrial times, nearly halfway to a 2-degree-C mark, explained climate scientist Jerry Meehl of the National Center for Atmospheric Research in Colorado. As a result, the heat waves in Russia, the northeastern United States and Japan this summer can all be attributed to the global warming that has already happened, he said.
"We are already seeing that," Meehl said of extreme heat events.
On the other hand, he said there will also be winter cold spells, though they ought to get less frequent as the summer heat waves ramp up.
"Winter still happens," said Meehl.
Even record-setting cold will happen without canceling out the extreme heat that's in store.
"We've published a prediction of the next three decades and we'll find out if it's correct," said Diffenbaugh.