If loss of the rain forest would deplete oxygen, why didn’t deforestation of North America and Europe do so?

Dear Cecil: 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

Cecil replies:

Good question, Michael; we just need to get it cleaned 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, 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 0.03 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, and judging from the weather reports, so can a lot of people living through yet another scorching summer, in both the new and old worlds. Not cool.

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.