By John Feeney:
Update #2: For a correct, non-deceptive comparison of population growth and consumption growth, click here for a recent example from former AAAS president, John Holdren.
Sometimes I read something, such as a recent article by George Monbiot (whose work I’ve often admired, by the way), and realize the basics bear repeating.
Environmental and other writers speak often about resource consumption. On occasion they write something about population growth. Once in a while they tackle the whole package – population and consumption. When they do, they often make a simple error and come to the wrong conclusion.
Typically, it goes something like this: “Yes, population growth is a problem. But growth in consumption is occurring faster, so it’s an even bigger problem.”
Total resource consumption (of one or all resources) is the product of population size and average per person consumption. Naturally, we would expect the growth of the product to exceed that of either of two growing factors driving it! As an example, 2*2=4 and 4*4=16. Here the factors have increased by the same amount; they’ve doubled. But the product has quadrupled. A factor and the product are not comparable elements.
The more appropriate comparison is between the factors, population and per person consumption. There the data tell us the differences are not so pronounced, and it’s clear we cannot prioritize and say it is more urgent to address one than the other.
It’s the same error if you see a comparison suggesting economic growth far outweighs population growth as an environmental problem. Economic growth can, after all, be understood (PDF) as closely overlapping total consumption. It’s held to be driven by population multiplied by per person consumption. George Monbiot certainly sees it that way as he equates economic growth and total consumption in his fourth paragraph.
Obviously, I’m the last person to discount the importance of economic growth in ecological degradation. But we need to get our factors and products straight and realize both population and per person consumption require our complete attention. Naturally, I support efforts to address them simultaneously by tackling economic growth as a whole. But it makes no sense to prioritize and say we should address economic growth while dismissing one of the fundamental factors driving it.
Nor do we know to what degree a focus solely on reducing economic growth, as ecologically necessary as that is in much of the world, would reduce global population growth. (In some countries, at certain stages of development, economic growth and population growth seem to be negatively correlated.) At the root of it, the fundamental causes of population growth are basic ecological laws, not economic growth.
It makes the most sense to bring down population, per person consumption, and economic growth (where appropriate), in whatever ways will be sure to reduce them all. 
In a world in which we are already deeply into overshoot of the earth’s carrying capacity for humans, dismissals of the importance of population are absurd.
So if you happen to run into an environmental writer making one of the statements above, just think about factors and products. Oh, and you might suggest he or she not shy away from either factor just because the product is growing faster or just because it’s trendy among environmentalists to do so.
 Notice as well that there are environmental impacts with great relevance to our very survival which can only be mitigated effectively by addressing population growth itself. The current mass extinction (PDF) is one.
Addendum: I sometimes wonder if some environmental writers think the scientists who emphasize the fundamental importance of population haven’t thought about issues such as total consumption or economic growth. Obviously they have. (One would have to be utterly naive, completely uninformed, or quite lacking in analytical thought to overlook such topics.) Wouldn’t it therefore make sense to consider more carefully the reasons why they nevertheless urge more attention to population?