In 2007, Tom Swetnam, a fire expert and director of the laboratory of Tree-Ring Research at the University of Arizona, gave an interview to CBS’s 60 Minutes. Asked to peer into his crystal ball, he said he thought the Southwest might lose half its existing forests to fire and insects over the several decades to come. He immediately regretted the statement. It wasn’t scientific; he couldn’t back it up; it was a shot from the hip, a WAG, a wild-ass guess.
Swetnam’s subsequent work, however, buttressed that WAG. In 2010, he and several colleagues quantified the loss of southwestern forestland from 1984 to 2008. It was a hefty 18%. They concluded that “only two more recurrences of droughts and die-offs similar or worse than the recent events” might cause total forest loss to exceed 50%. With the colossal fires of 2011 and 2012, including Arizona’s Wallow fire, which consumed more than half-a-million acres, the region is on track to reach that mark by mid-century, or sooner.
But that doesn’t mean we get to keep the other half.
In 2007, the Intergovernmental Panel on Climate Change forecast a temperature increase of 4ºC for the Southwest over the present century. Given a faster than expected build-up of greenhouse gases (and no effective mitigation), that number looks optimistic today. Estimates vary, but let’s say our progress into the sweltering future is an increase of slightly less than 1ºC so far. That means we still have an awful long way to go. If the fires we’re seeing now are a taste of what the century will bring, imagine what the heat stress of a 4ºC increase will produce. And these numbers reflect mean temperatures. The ones to worry about are the extremes, the record highs of future heat waves. In the amped-up climate of the future, it is fair to think that the extremes will increase faster than the means.