Vapor pressure deficit: Ever heard of it? Didn't think so. But this wonky atmospheric term is one you might want to study up on, especially if you're a Westerner who likes trees and cares about whether they live or die.
Why? For one, it might be fun. One gardening website attests that once you understand vapor pressure deficit "you start to think and feel like a plant." Who could resist that? Also, it's actually pretty easy to understand. And it's important. High vapor pressure deficits appear to have played an instrumental role in killing off vast numbers of trees in the Southwest over the last decade and a half, whether they died by drought, beetle or wildfire. And that's a big deal because vapor pressure deficits that seem extreme now are likely to become the norm as the world heats up.
Park Williams, a bioclimatologist with Columbia University's Lamont-Doherty Earth Observatory, is one scientist who has helped reveal the role of vapor pressure deficit in forest mortality in the Southwest. His most recent contributions are two studies published last month considering its role in wildfire, and specifically in the big, hot fires of 2011.
But before we get to that, a bit more on vapor pressure deficit, or VPD. Essentially, it is the difference between the moisture the atmosphere is capable of holding and the moisture it is actually holding. In the Southwest, high VPD is usually a function of high temperatures. Here's why both matter to plants, as I explained in a 2013 HCN feature:
Minor temperature increases have an outsized effect on the amount of water the atmosphere can hold: When the temperature goes up, the atmosphere gets a lot spongier. The relationship is exponential. Stubbornly set on fulfilling its potential, warm air sucks water more greedily from both plants and soil. If the water supply it's drawing on becomes depleted, the tension begins to strain a tree's water columns. Picture an eager child sucking the last drops of a milkshake from a straw: The water columns, like the straw, collapse.
During the 2000 drought, when pinon died in seemingly greater numbers than they had during the 1950s drought, ecologist Dave Breshears suspected the reason was that the recent drought was warmer. He and others dubbed it a "global change type drought." VPD, he told me recently, explains why a small rise in temperature could result in a lot more dead trees. "From the plant's perspective, it's more directly experiencing that atmospheric demand (for water) than the temperature itself," Breshears said. "That really gives us insight into why a little bit of temperature goes a long way in having negative effects for a plant."
Previous work by scientists including Breshears and Williams has demonstrated how high VPD can stress trees, making them vulnerable to bark beetles, or even killing them outright. Williams' new work considers its relationship to the West's other primary tree killer: wildfire.
His big picture conclusions echo those of previous studies: High vapor pressure deficits result in more dead trees. One study showed that fluctuations in VPD year-to-year correlated very closely to fluctuations in the number of acres burned. In fact, among climate variables, VPD has been the best predictor of area burned for the last 30 years -- better than temperature or precipitation. Jim Randerson, a scientist at the University of California, Irvine, reached a similar conclusion studying the factors driving growth of wildfires in Alaska's boreal forest. "VPD was more important than temperature or humidity by itself," he said.
Williams' second study considered the climate conditions and wildfires of just one year, 2011, when two of the Southwest's biggest blazes in recorded history -- the Wallow and Las Conchas fires -- scorched Arizona and New Mexico. Interestingly, 2011 was not exceptionally hot in the Southwest, but owing to an extremely unusual alignment of atmospheric anomalies, vapor pressure deficit was off the charts. The air was mind-bogglingly dry, sponging up water from trees and plants like mad.
"2011 gave us this really neat opportunity to look at how forests respond to really high VPD," Williams said. Even though the spike wasn't caused by temperature, forests responded in the same way, affirming, Williams said, the idea that the influence of VPD on fire is especially pronounced. If climate is the Oz of wildfire risk, you might say, VPD is the man behind the curtain. (Although in this version of the story, he has co-workers.)
"When we think about the future," Williams explained, "and VPD is supposed to go up as temperature goes up, this study solidifies that we should really expect burned areas to increase in step with VPD."
Another finding from the study has even more startling implications for the future. No climate models predict that the anomalies that zapped the atmosphere of water vapor over the Southwest in 2011 would ever converge. "Climate models just don't think the climate can be as extreme as it was in the Southwest in 2011," Williams said. Obviously, it can. "If a 2011 event would occur in the 2050s, the results would be very similar, except they'd all be imposed on this warmer background temperature." Which means even higher VPD.
Moreover, projections of changes in VPD based on temperature increases alone show that even in average years, extremes on par with 2011 are probable. "If we still had as much forest, we should expect 2011-like forest fires," Williams said. "But then there's a chance we have this atmospheric craziness. If that happened in the 2050s, the VPDs would be amplified into a realm that is really incomprehensible to me right now."
Williams fears that climate-driven forest loss in the Southwest will not occur gradually, but in catastrophic pulses similar to 2011, and often as a result of wildfire. "Preparing forests for climate change isn't necessarily just figuring out which forests should die and which ones shouldn't," he said. "It's protecting the ones that shouldn't from catastrophic wildfires."
Cally Carswell is a High Country News contributing editor and freelance science and environmental journalist based in Santa Fe, New Mexico. Follow @callycarswell