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Woodland Lab
May 17, 2002

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Right now, as you are reading this, you are giving off greenhouse gases. Yes, you are responsible for some of those heat-trapping emissions that have scientists worldwide concerned about future rapid warming of the earth’s climate.

How are you doing this? Mostly, just by breathing.

pollution
image: UCAR

When you breathe out, you exhale a combination of carbon dioxide and water vapor. Carbon dioxide, or CO2, is considered by many scientists to be the most damaging greenhouse gas. The water vapor you exhale is also considered to be a greenhouse gas.

The fact is that lots of things in nature generate these gasses, and for scientists to accurately assess the impact of industry on climate change, they must also consider what sources occur naturally.

Carbon dioxide normally exists in the atmosphere in relatively small amounts, less than 1 percent of the Earth’s total. Industrial pollution and car emissions have increased that by almost one third.

Scientists are still measuring the impact of such a change, but many worry that the impact could significantly change our climate in the next hundred years.

But one large, often natural source of carbon dioxide is forest fires. All living things are made of carbon, and when they burn the carbon combines with oxygen to form carbon dioxide. Although many fires are man-caused, others are started by lightning and are part of a process that has been happening long before people were around.

While industry and pollution may be the largest sources of all the greenhouse gasses, fires can have a significant impact. Fire consultant William Small estimates that the large number of forest fires in North America in the year 2000 was responsible for 5 percent of all the greenhouse gasses emitted that year, a total of about 75 tons out of the 1500 tons emitted from all sources.

big forest fire
image: WGBH/NOVA

Nature has a way of fighting back and removing that carbon dioxide from the air. Plants, through photosynthesis, take in carbon dioxide, keep the carbon atoms for growth and return oxygen into the air. The more plants and trees there are, the more carbon is trapped inside those plants and kept out of the air. This is all part of the carbon cycle.

This is known as carbon sequestration.

Without interference from man, there is a natural cycle of growth trapping carbon, and decay or fire releasing carbon. It’s called the carbon cycle.

Some scientists worry this could actually amplify the impact of industrial pollution. It’s what they call a "positive feedback" system—where one event triggers a second event that makes the first event bigger. That, in turn, makes the second event bigger still, which then makes the first event even bigger still.

In this case, they worry that greenhouse gasses from industrial pollution will warm the planet, making conditions in the forests warmer and drier. The positive feedback happens because that would improve the chances that small fires would become large ones, releasing still more gases and eliminating plants that, before the fire, had been helping rid the atmosphere of greenhouse gasses. More gasses mean more heat is trapped, making conditions warmer and drier in other forests. And on and on.

Many scientists are working together to try to measure the impact of forest fires on global warming. "Frostfire" was a fire set as an experiment so that scientists could accurately measure how much greenhouse gases were created. In the experiment, they set fire to nearly 2200 acres of Alaskan forest. Nine-hundred actually burned, producing about 24,000 tons of greenhouse gases. That’s roughly equal to burning 2.4 million gallons of gasoline.

The scientists in ’Frostfire" are now watching the area’s hydrology or water flow. The burned out part of the forest handles water differently, and water is, of course, essential toward determining what plants grow back and where. The scientists are also observing and measuring how the forest grows back. This is important because the re-growth—part of the carbon cycle—will again trap carbon dioxide.

Will the new growth, over time, trap as much greenhouse gas as the fire released? Could it be more? Does any long-term benefit outweigh short-term problems? These and similar questions will keep scientists studying the impact of forest fires on global warming for years to come.

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by Jack Penland


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