You can see it in photos taken recently by NASA's Terra satellite, orbiting 435 miles above the Earth: Plumes of brown-gray smoke from Southern California's wildfires blowing west over the ocean, extending hundreds of miles out to sea. Where - and how far - these plumes travel ultimately depends on wind currents and weather patterns. Some of the smoke could, in fact, blow back, becoming a health hazard once again. But scientists see in the plumes an even larger and longer-term problem: global air pollution and its effect on climate change. There are growing concerns among scientists that fires - around the world - are part of a spiraling and destructive feedback loop: Hot, dry weather caused by climate change increases the frequency and ferocity of wildfires. These fires release into the atmosphere ever-larger amounts of particulates, pollutants and greenhouse gases that, in turn, result in even hotter, drier weather and more fires. "It's not unreasonable to argue there's a connection," said Veerabhadran Ramanathan, a noted climate scientist at Scripps Institution of Oceanography in San Diego.
To be sure, the connection is extraordinarily complicated and incompletely understood. Scientists cannot declare Southern California's wildfires to be the unambiguous consequence of global warming. The fires may, in fact, be primarily the result of local factors: a years-long drought, abundant fire-prone vegetation and Santa Ana winds, said Anthony Westerling, an assistant professor of environmental engineering at the University of California Merced. How all of these components fit and work together, said Westerling in a statement with colleagues Thomas Swetnam and Gregg Garfin of the University of Arizona, is "not known with sufficient certainty to conclusively link global warming with this disaster."
Nonetheless, diverse sources of data suggest an ill wind is blowing in an alarming direction.
CHEMICAL BREW
Wildfire smoke is a farrago of burned matter (particulates), gases, vapors and chemical compounds, the recipe unique to each blaze.
More than 90 percent of the mass of wildfire smoke consists of carbon dioxide and water, but hundreds of other chemicals can be present, among them: carbon monoxide, mercury, aldehydes, nitrogen oxides, polynuclear aromatic hydrocarbons (PAHs), sulfur dioxide, volatile organic compounds and ozone. Many of these ingredients pose immediate and obvious health hazards. Smoke particles smaller than the diameter of a human hair can be inhaled deeply into the lungs, causing everything from lung irritation to an increased risk of cancer.
Carbon monoxide (CO) is an odorless, tasteless, invisible poisonous gas. Mercury is an extremely toxic element. Aldehydes are gases that irritate the eyes, nose and mouth. Some forms, like formaldehyde, are carcinogenic, as are PAHs. Nitrogen oxides are associated with acid rain. In the presence of sunlight and hydrocarbons (organic compounds readily produced in fires), nitrogen oxides combine to create ozone, a gas that inflames and impairs the lungs and triggers asthma attacks. These compounds can linger in seemingly clear air for days, even weeks.
Perhaps the best cleanser is rain, followed by time and dilution. But the problem is that the increased frequency and severity of wildfires means there are more fires billowing more pollutants into the air more of the time. And they are doing so in enormous amounts. In 2004, for example, massive wildfires in Alaska and Canada's Yukon Territory - the worst on record - scorched more than 11 million acres, an area roughly the size of the states of New Hampshire and Massachusetts combined. Scientists monitoring the resulting smoke plumes estimate that the fires from June through August produced 30 trillion grams of carbon monoxide - an amount equal to all human-generated CO production for the entire continental United States in the same period. Surface levels of ozone increased as much as 25 percent in the northern United States and 10 percent in Europe. Something similar happened a couple of years earlier when widespread fires roared across western Russia and Siberia, leading to the so-called "dirty winter" of 2002-03, when unnaturally high CO and particulate levels hovered over much of the Northern Hemisphere.
"Satellite observations showed the Russian fires had a huge impact on air quality on a global scale," said David Edwards, a researcher at the National Center for Atmospheric Research based in Boulder, Colo.
Wildfires aren't the only source of airborne pollutants. An estimated 8 billion metric tons of dry biomass is burned in vegetation fires each year around the world, much of it in fires intentionally set to clear land or old crops. "Biomass burns are quite common in the tropics, most associated with agriculture," said Daniel Jacob, an atmospheric scientist at Harvard University.
The result is approximately 4 billion metric tons of carbon released into the atmosphere, the equivalent of about 70 percent of all human fossil-fuel emissions (oil, gas, coal). Black carbon particulate in the atmosphere causes complex problems. So-called "brown clouds" both absorb sunlight, warming the air, and block it, dimming the ocean's surface and interfering with photosynthesis. The overall effect is contradictory, but warming seems to be winning. Ocean temperatures have generally risen over the last 50 years. A study published last year found that as seas warmed, growth rates for phytoplankton (microscopic marine plant life) broadly declined, due perhaps to less photosynthetic activity and less nutrient-mixing in the warmer, stratified water. Carbon particulate plays a role on land as well. As it settles out of smoke and other forms of air pollution, it can behave like an electric blanket, reducing the albedo, or reflectivity, of the snow while simultaneously warming it. "When smoke settles on snowy surfaces, it enhances absorption of sunlight, which plays an important role in the retreat of sea ice and melting snowcaps," said Ramanathan at Scripps. This soot can come from far away. For example, more than 75 percent of the atmosphere-warming, snow-melting soot that falls over the West Coast of the United States each spring is delivered by prevailing winds originating in Asia, according to a recent study by Ramanathan.
North America, in turn, sends its air pollution eastward, the jet stream carrying it over the Atlantic Ocean to Europe. Dust plays a similar long-distance role. A study earlier this year by the National Snow and Ice Data Center found that windblown dust from drought-stricken or disturbed lands can shorten mountain snow cover hundreds of miles away by up to one month.
Virtually all scientists agree that the primary culprits in global warming are greenhouse gases, such as carbon dioxide (CO2), methane and nitrous oxide. All three gases are abundantly produced by wildfires.
Tom Bonnicksen, a professor emeritus at Texas A&M University, estimates the Southern California fires in October generated 19 million tons of these gases in just the first three days of burning. Plant life is among the great absorbers of carbon dioxide, removing the greenhouse gas from the atmosphere during photosynthesis. But the increasing loss of vegetation worldwide from fires and pollution may skew the process. "In the past, forests have been an important buffer against climate change because of the way they absorb carbon," said Johann Georg Goldammer, head of the fire ecology research group at the Max Planck Institute for Chemistry in Germany. "But warming, bringing more frequent drought and fires, may affect the balance of the global carbon pool and release extra CO2 into the atmosphere."
As a source of pollutants linked to global air pollution and climate change, human activity is far more problematic than nature. Wildfires tend to be seasonal. Human pollution occurs year-round. The sprawling brown clouds that blow from Asia (covering the distance in just days) are mostly produced by man-made sources, such as factories and auto exhaust. The situation has only gotten worse as the economies of China and other countries have grown rapidly. In 2004, for example, it was estimated that China would surpass the United States in CO2 production in 20 years. Chinese emissions are now expected to exceed American levels this year. Without doubt, controlling and reducing local pollution sources remains the most pressing issue. "You can't really blame China for air pollution in Los Angeles," said Gabriele Pfister, an atmospheric chemist at NCAR.
But the problem of global air pollution - and its implications for climate change - presents a bigger and more burdensome worry for scientists and, arguably, everybody else living on Earth.
The pollutants churned high into the atmosphere by the recent wildfires pose real and potential hazards, seen and unseen, for anybody downwind. The same can be said for the billions of tons of chemicals spewed daily into the air by human activity around the world.
And in this case, we all live downwind.
