In Climate Accounting, Fire’s an Orphan
Fire ecologists have documented how some species have evolved in the presence of recurring fires. Near the top of the mitt-shaped lower peninsula of Michigan, for instance, the now-endangered Kirtland’s warbler, sometimes dubbed the “bird of fire,” successfully nests under low-lying boughs only in forests of recently burned, but regenerating, jack pine—trees that can regenerate only after the intense heat of the frequent fires pries open their cones for seed release.
We, too, are a “fire species,” in Balch’s view, because our own evolution has been intertwined with fire. It appears that it was recurring fire that suppressed the growth of trees on the world’s savannas. It was on the African savanna, in the presence of abundant and accessible game, that humans evolved. Over time, humans learned how to use fire to their own ends, for cooking, warmth, signaling and, eventually, for more dramatic ends, the management of the lands around them.
The Science article points to evidence that humans evolved with some tolerance to smoke. Tolerance appears to be greatest among those who descended from colder-climate ancestors, whose domestic fires burned in more confined spaces. And then there’s the matter of making some space for the very grey matter that defines our species. According to Balch, evidence suggests that when hominids learned to cook, and thereby tenderize food, it allowed the evolution of a smaller jaw, which in turn may have allowed the skull to accommodate a larger brain.
Sometime in the span of tens of millions of years ago, humans began to use fire to control fuel loads and to manage landscapes. For example, long before European colonization, Native Americans periodically set fire to clear land for such purposes as managing hunting grounds for game species, including deer, which tend to thrive in clearings, forest edges or young, early successional forests. Fire abounded as European settlers arrived, then marched westward, clearing forests as they moved.
In the past two decades, satellite imagery has offered a detailed picture of contemporary life on our “fire planet.” Notably, it has shown us that fires, sometimes of tens of thousands of acres, are burning in rainforests, in what Balch terms “a pantropical band of fire.”
“In some ways, what’s happening in the tropics today isn’t that different from what happened in North America after colonization. But it’s happening at breathtaking speed,” she says.
In other ways, though, the tropical fires are something new—the Earth’s wet forests are burning. “The idea of frequent large-scale fires in the tropical rainforest used to be unthinkable,” says Balch, “Now we’re seeing several large fires every decade.”
In both tropical and temperate zones, climate change appears to be playing a role. In the American West, larger and more intense fires dovetail with measurable increases in temperatures and earlier snowmelts.
But there’s at least one additional major factor—the unintended consequences of years of fire suppression in forests that evolved in the presence of periodic fires, a phenomenon Bowman calls “Smokey the Bear blowback.”
Fire suppression can lead not only to a buildup of fuel in a forest, but to dramatic changes in forest structure. Recurring, cooler fires might once have been restricted to periodically burning off and suppressing the growth of low-lying vegetation. Today, with these natural fire patterns themselves suppressed, taller undergrowth becomes not only fuel for hotter fires, but functional ladders that allow flames to leap into canopies, killing mature trees that would have easily survived the lower, cooler fires.
Fire, meanwhile, is interacting with climate in a complex, and still poorly understood, dance. A forest fire releases carbon dioxide into the atmosphere, but a regenerating forest can take the carbon back up. But if a farm field replaces the forest, the carbon uptake calculus changes. Fires can also alter surface reflectance (or albedo). A scorched, blackened landscape absorbs rather than reflects heat. A young, greened-up landscape that shortly follows a burn might reflect more heat than the original forest.
To arrive at their rough estimate of the sort of influence that fire could be having on climate, the paper’s authors focused solely on intentionally set deforestation fires. They estimated that these fires have been responsible for 19 percent of the human-caused global warming to date. That’s a piece of the equation, the author’s contend, that’s missing from existing climate models.
“We’re arguing,” says Balch, “that a new generation of climate models needs to do a much better job of accounting for fire.”
In a more sweeping sense, the 2008 conference and the 2009 article that followed might signal a coming day when science does a much better job of integrating the study of fire on Earth. Whether “pyrogeography” becomes a formal new discipline is an open question. But perhaps it is at least an early hint that the study of fire may no longer be what Bowman calls “an intellectual orphan.”
As he puts it, “How could something be so obvious and yet be so long overlooked?”