Park Williams likes to compare the dry air around our mountain forests to a thirsty sponge, eager to soak up moisture that drought-stressed plants give up only grudgingly.
The drier the sponge, the worse for the trees. “It yanks water out of the plant until the plant can no longer afford to give water away,” Williams said. Under drought stress, the trees shut down the tiny holes on their leaves where they exhale water as part of their process of making the food on which they live. Instead, they consume their own tissues. “They start feeding on themselves,” he said.
That is bad, but fire can be worse. With the right source of ignition, dry forests can burn with unusual fury. And never in more than a century of weather record keeping, the Columbia University scientist has concluded, has the sponge been as dry and the fury therefore as great as in the Southwest as the summer of 2011.
In May of that year, the Wallow Fire in Arizona lit up, eventually burning 841 square miles. Then in June, a power line in the Jemez Mountains set off the Las Conchas Fire, which at its harrowing peak was swallowing an acre of forest per second.
The hotter the air, the drier the sponge. It would be easy (and correct) to draw an important global warming message from Williams’ work. Higher temperatures in coming years are likely to push us to a climate regime by midcentury in which the extremes of 2011 become the average year, Williams and his colleagues found.
“Warming temperatures are going to matter,” U.S. Geological Survey fire ecologist Craig Allen, who has worked with Williams but was not directly involved in this research, said in an interview.
But Williams’ analysis of the conditions of the year of the Las Conchas Fire offers a more subtle point. Although 2011 was warmer than the average, it “wasn’t spectacularly warm,” in Williams’ words. Separate from the warming, climate patterns driven by large-scale changes in the oceans left us with little precipitation in the months leading up to the Las Conchas Fire, and a long spate of unusually dry air. The high temperatures made things worse.
There’s a tendency in our political discourse about climate change to look for simple yes/no answers to the question, “Did climate change cause that?”
Williams’ research, done with a team that included scientists at Los Alamos National Laboratory and five universities, points to the need for a more nuanced discussion about the future of forests and fire in the Southwest.
Variability – natural swings between warm and cool, wet and dry – has always been a part of our climate. The impact of greenhouse warming from fossil fuel emissions is then layered on top of that topsy-turvy climate.
Williams’ research makes two important points about this. The first is that greenhouse gases, by driving up temperatures, are likely to make the average year in the 2050s as bad as the worst-on-record 2011.
But even scarier, Williams and his colleagues show that the natural variability, the swings between good and bad years, can be more extreme than scientists thought even without global warming. Add the two together, and bad years in the 2050s could be very bad for New Mexico’s forests.
The team’s research shows, as you might expect, that acreage burned goes up in direct proportion to the dry-sponge air. That does not mean the acreage burned will keep going up forever, however, Allen said. Eventually, the amount of forest available to burn will decline. “The fuels become self-limiting,” he said.
But Williams hopes that, with a careful approach to forest management, this does not mean the complete end of mountain forests in New Mexico. The research shows there are pockets on the landscape – protected mountain drainages, places that are high enough and cool enough to stay wetter even as the forests around them are drying out – that could be saved.
“There’s going to be forests,” Allen said.
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