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The Straight Dope



Environmentalists say the loss of the Amazon rain forest would lead to a shortage of oxygen around the world. But Europe was heavily forested until the late Middle Ages and North America until the 19th century. Most of the world was forested a thousand years ago, and now really big forests exist only in parts of South America, Africa, and Siberia. Since we have enough oxygen now, if there is a relationship between forests and oxygen levels, does that mean in ancient times everyone was going around on an oxygen high? --Michael Klossner, Little Rock, Arkansas

Good question you have in there, Michael; we just need to clean it up a little. A couple misconceptions: first, environmentalists don't say loss of the rain forests would lead to a shortage of oxygen. While forests do produce oxygen as a byproduct of photosynthesis, their real value atmosphere-wise is in getting rid of carbon dioxide. (Strictly speaking, in getting rid of carbon; the dioxide part is benign.) Deforestation of the Amazon means that not only will the rain forest stop taking CO2 out of the atmosphere, but the billions of tons of carbon now trapped in the trees will be returned to the air through burning, decay, etc. Second, there isn't an oxygen shortage. Atmospheric oxygen content hasn't changed dramatically in historic times, and the buildup of carbon dioxide isn't making the air unbreathable. The amount of carbon dioxide in the atmosphere is minute, just a little over .03 percent by volume. The problem is that, unlike nitrogen and oxygen, which make up 99-plus percent of the atmosphere, CO2 traps the earth's heat rather than letting it radiate away into space--in short, it acts like the glass in a greenhouse. Too much warming in too short a time and farming is disrupted, the oceans rise . . . you know the drill.

Now then. You're right that deforestation has been going on for hundreds, even thousands of years. According to one estimate, forests covered 90 percent of western Europe in 900 AD; a thousand years later they covered roughly 20 percent. It's estimated that worldwide the "biomass" (all organic matter) has declined about 7 percent since 1850. The amount of carbon dioxide in the atmosphere has increased correspondingly, perhaps as much as 25 percent since 1850. (Fossil-fuel burning is also a big factor.) Some scientists now claim to detect a global temperature rise as well. Not cool.

Upon reading your column on the demise of the Native Americans following European contact [July 29] I immediately recognized myself as an expert and am forwarding the enclosed articles, making your story far more complex. My question for you is from an old Firesign Theatre album: "Why does the porridge bird lay its eggs in the air?" --Thomas Templeton, Laboratory of Malaria Research, NIAID/NIH, Bethesda, Maryland

Who knows? Maybe she's waiting for the electrician or someone like him. Your enclosures raise an interesting point, but one that, far from complicating matters, is in line with the view I presented. Had I not taken that second coffee break I would surely have thought of it myself. The point is this: recent research suggests that Native Americans lacked "genetic diversity"--bluntly put, that they were inbred; virtually the entire indigenous population of the Americas was descended from just four women, or at least four groups of closely related women. We also know that a virus you contract from a family member is far more likely to be fatal than one you get from a total stranger. That's because the family-bred virus has already figured out your clan's genetic code and can evade your natural defenses. Since Native Americans were all close cousins (at least compared to Europeans), a virus that killed one would pretty much kill them all. In short, American Indians were more vulnerable not simply because they'd been exposed to fewer diseases, as I argued, but also because they'd been exposed to fewer humans. They were exposed to plenty of both once European settlers arrived.

Art accompanying story in printed newspaper (not available in this archive): illustration/Slug Signorino.

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