Environmental Engineering Reference
In-Depth Information
Now let's look at CO 2 . It's a greenhouse gas that exists in trace quantities in the atmosphere—just under
.03 percent (270 parts per million, or ppm) before the industrial revolution, a level that we have increased
to .04 percent (396 ppm). 22
How do we know about the greenhouse effect of CO 2 ? The best way: it can be studied in a laboratory.
The temperature difference between a box with a glass ceiling and normal atmospheric gas concentrations
and one with additional CO 2 is measured when sunlight shines into it.
As with any effect, a crucial question is: What is its magnitude—including, at what rate does additional
CO 2 change the effect? Some phenomena are linear, which would mean that every molecule of CO 2 you
add to the system will add a unit of heat the same size as the last one. In some phenomena, the effect is
constantly increasing or accelerating; in this case, every molecule of CO 2 you add to the system would
be more potent than the last (this is the sense that we get from most popular treatments of the greenhouse
effect). Then there are diminishing or decelerating phenomena—every molecule of CO 2 you add to the
system would be less potent than the last.
Anyone discussing this issue should know what kind of function the greenhouse effect follows. While
I've met thousands of students who think the greenhouse effect of CO 2 is a mortal threat, I can't think of
ten who could tell me what kind of effect it is. Even “experts” often don't know, particularly those of us
who focus on the human-impact side of things. One internationally renowned scholar I spoke to recently
was telling me about how disastrous the greenhouse effect was, and I asked her what kind of function it
was. She didn't know. What I told her didn't give her pause, but I think it should have.
As the following illustration shows, the greenhouse effect of CO 2 is an extreme diminishing effect —a
logarithmically decreasing effect. 23 This is how the function looks when measured in a laboratory.
Notice that we are just before 400 ppm (which means CO 2 is .04 percent of the atmosphere), where the
effect really starts tapering off; the warming effect of each new molecule is not much.
This means that the initial parts per million of CO 2 do the vast majority of the warming of our atmo-
sphere.Theimage belowshowshow,allthingsbeingequal,theheating effectofeachadditional increment
of CO 2 is smaller and smaller.
Figure 4.1: The Decelerating, Logarithmic Greenhouse Effect
 
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