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"We know climate is changing, we know temperatures are rising, and we know they are going to continue to change for the next couple centuries," says Jonathan Gilligan, associate professor of earth and environmental sciences at Vanderbilt University. "As the climate warms up, we expect to get more extreme events."
That expectation is shared by many other scientists, including Noah Diffenbaugh, a lead author of the study "Intensification of Hot Extremes in the United States," which was published this year in the journal Geophysical Research Letters. Examining a variety of climate models, the study predicts "warm-season drying and intensification of hot extremes throughout much of the U.S." Speaking by phone, Diffenbaugh, assistant professor of environmental earth system science at Stanford University, tells the Scene that extreme heat waves used to occur in the Southeast once every 50 years. Soon, they could happen three to five times in a single decade. And as time goes on, the heat will continue to rise — well within a lifetime for many of us. Looking at various climate models, Diffenbaugh says, "For the decade of the 2030s, the season that was the hottest in five decades ... in the Southeastern U.S. occurs somewhere between five and eight times in a single decade."
And as time goes by, not only do heat waves happen more frequently, the extremes get more extreme.
"Looking at the hottest temperatures of the year, year after year — say, the hottest 5 percent of maximum daily temperatures," Diffenbaugh says, climate models show the Southeast can expect "something on the order of a doubling" in how often the mercury hits those record-setting marks.
"Whether it's the extreme event of the five-decade period, or what is now the extremes of each year, they both show more frequent occurrence in the Southeast by the decade of the 2030s," Diffenbaugh says. Talk about cruel summers: What are now the hottest of hot days in Nashville could become the norm in as little as 20 years.
Climatologist Heidi Cullen's recently published book The Weather of the Future arrives at similar conclusions by similar means. "All weather is local," she writes, "and ... in the future, all climate change will be local too."
But (to coin a phrase) is this change we can believe? Perhaps it seems counterintuitive to start planning our lives around the prognostications of a few dozen climate model computers. Yet it's important to note that not only are temperatures already rising, past computer models have been surprisingly accurate. In 1988, James Hansen ran computer simulations trying to predict what would happen in the next 30 years under various greenhouse gas emissions scenarios. Actual temperatures between then and now have been extremely close to his "most likely" scenario, which took into account both greenhouse gas emissions and the cooling effects of other things like airborne pollution.
As Vanderbilt's Gilligan notes, this doesn't mean we can be 100 percent confident in everything the models tell us. Specific storms, hurricanes and tornadoes are very difficult to predict, because of myriad contributing factors. Temperature, however, can be forecast much more reliably.
"Increasing greenhouse gases will cause the planet to warm up," Gilligan says. "This is such basic physics that for the planet not to warm up would mean something is very wrong with the basic laws of thermodynamics from the 1800s. You can trust models very well that temperatures will rise significantly over most of the planet and that there will be big changes to weather and climate patterns."
The biggest change headed our way: new definitions of "extreme."
"For years," former Vice President Al Gore tells the Scene via email, "many scientists used the analogy of 'loaded dice' to describe the increased likelihood of more frequent extreme weather events — like the 1,000-year rainfall event we had here in Nashville at the beginning of May. But some of these same scientists are now saying that the 'loaded dice' analogy misses an important point: The climate crisis is now beginning to cause totally unprecedented events. We are not only loading the dice, we are, in effect, painting more spots on the dice. Instead of rolling 11s and 12s, we are now rolling some 13s and 14s."
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