In a Summer of Fire, Is Warming a Cause?

By the beginning of the year, analysts at the National Interagency Fire Center in Boise, Idaho, were nervous. All over the inter-mountain West, a slew of warm, wet El Nino winters had fed the growth of new trees and brush. Then came last winter’s La Nina--cold and dry, with howling winds. The forest under-story was becoming the kind of fuel that can turn a small fire into a devastating inferno. The Fire Danger Index, which the Interagency Fire Center uses to estimate the likelihood of a fire being triggered by anything from a cigarette flipped out a car window to a lightning storm, was veering upward of 80%.

The first major blaze went off in February, scorching 40,000 acres of southeast New Mexico ranchlands. Three months, more than 70,000 fires and 5.8 million acres later, an area larger than New Hampshire has been seared, with the fire season still a long way from over. The epicenter of the fires has shifted north, to the Idaho-Montana border, where more than a million acres is burning out of control, and a pall of smoke has blanketed the land for a month. Backed by 65 air tankers, 225 helicopters and mountains of gigantic earthmoving machinery, 25,000 firefighters, augmented by crews from Australia, New Zealand, Canada and Mexico, have joined the battle, alongside elements of the U.S. Army, the National Guard and the Marines. By every measure, this is the worst fire season in 50 years.

Many factors add up to a forest or range fire: dry weather, strong winds, high temperatures. Ecologists would add industrial forestry and the rapidly growing urban population concentrated in our most arid states. Now scientists have begun to study another, more ominous possibility: global warming.

Everybody cautions that computer models aren’t good enough yet to make predictions. “It’s very difficult to say with a great degree of certainty,” warns UC Berkeley Professor John Harte, who studies climate and its effect on the ecosystem, “but what you can say is that this summer’s events are very typical of the kinds of phenomenon we would see more of during global warming.” Ecosystem ecologist Janine Bloomfield of Environmental Defense in New York concurs. “This type of year is the kind of year you’d expect from climate change,” she says.

Computer models project that a sustained increase of only 1 degree Celsius in global mean temperatures will cause changes in regional climates that will affect the ability of forests to grow and regenerate. The United Nations Intergovernmental Panel on Climate Change estimates that a 1 degree to 3.5 degree C heat increase over the next 100 years would adversely affect one-third of the world’s remaining forests. According to a series of papers presented at a 1997 climate forum hosted by the White House Office of Science and Technology Policy, plants from the desert Southwest appear to be migrating north. Current biogeographical models indicate arid-land species are expanding into the Great Basin. By 2050, the Hadley Center for Climate Prediction and Research at the United Kingdom Meteorological Office predicts that huge blocks of Earth’s remaining forest will die back, becoming grassland, steppe or desert.

Most forest fires are ignited by lightning. At any given moment, some 2,000 thunderstorms are in progress across the Earth’s surface, and lightning strikes about 100 times each second--typically at higher altitudes. Timothy Ingalsbee, a PhD at the Western Fire Ecology Center in Eugene, Ore. states, “Global warming unsettles the weather.” Unsettled weather, particularly in warm climates, is the ultimate cause of lightning.

When warm air bubbles up from the Earth’s surface, it collides with cooler air from the upper atmosphere to produce the towering cumulonimbus clouds that become thunderstorms. But because this has been such a dry year, the clouds contain so little moisture they produce lightning without rain.

As the planet heats up, warmer climates expand north and south. Harte, speaking from a field station at the Rocky Mountains Biological Laboratory 10,000 feet up the western slope of the Rockies, explains that the differential between the earth’s warmer surface and the cooler upper air will increase, leading to more violent weather. Earle R. Williams of the Massachusetts Institute of Technology suggests a relatively small increase in local temperatures produces a large increase in thunderstorm activity. A 1 degree C rise in the Earth’s average temperature, according to researcher Colin Pierce of Columbia University, increases the potential for lighting by 10 kilovolts.

For a decade, ecologists have warned that industrial forest practices were a factor in global warming. Big trees store enormous amounts of water in their trunks. When the trees are cut down, the forest stores less moisture. Chad Hanson of the John Muir Project, a Pasadena nonprofit dedicated to ecological restoration in the national forests, says forests have lower temperatures than tree farms, which often lack the under-story shade that cools the earth and prevents winds from ripping through the woods, drying them out. Now, science is beginning to establish a link between timber harvesting--especially clear-cutting--and lightning production.

Trees breathe in carbon and breathe out oxygen. When they’re cut, they release huge concentrations of carbon dioxide into the air. Carbon dioxide is the primary component of greenhouse gases, now at their highest atmospheric concentrations in more than 400,000 years. The Hadley Center predicts that by 2050 forests globally will become a net source of carbon-dioxide emission. A recent study from UC Berkeley’s Lawrence Livermore National Laboratory predicts that if the carbon dioxide in the atmosphere doubles, the annual mean number of lightning-strike fires in the U.S. and the area burned would increase, respectively, by 44% and 78%.

The National Weather Service’s Rick Ochoa works out of the Interagency Fire Center. Our ability to detect lightning strikes has grown rapidly, he says, but reliable lightning statistics only go back a decade. He says nearly all the fires that have blackened 8,900 square miles of the West this summer were caused by lightning, but insists that’s the case nearly every year, “It’s just that this year the land is so dry.” He says he’s seen no firm evidence that lightning strikes are on the rise.

A forest or range fire is an awesome thing to experience. You can hear it roaring toward you, sometimes before you can see it. As the conflagration advances, its glowing columns of smoke shoot off giant sparks that can be 15 feet wide. In the crowns of the great forest trees, flames can easily reach 200 feet into the air. Firefighters can’t even work near flames much higher than three feet.

For the men and women on the ground grubbing out fire lines with chain saws, shovels and “Pulaskis,” a combination ax and hoe--while Chinook and Sikorsky helicopters swoop in with 1,000-gallon water buckets and C-130s and DC-7s dump 3,000-gallon payloads of water and fire retardant from their bellies--forest-fire fighting must have the adrenaline charge of war. But, in reality, firefighters need help from nature--a drop in wind speed, an increase in the humidity--to stop a big fire. The only thing that will stop many of this season’s blazes, particularly those in rugged back-country, will be the autumn’s rains or snows. When they come. If they come. Meanwhile, lightning is sparking new fires every day.