Consider this a working paper of sorts. It adds to the last post here which discussed the relationship between population and consumption. But it’s only a snapshot of an initial bit of online and library research. I hope to flesh out the topic more fully in the future.
At the end of that post I mentioned two issues I had barely touched on, which deserved more attention. They were (a) the question of whether, even hypothetically, we could ignore population growth and count solely on advances in clean energy technologies to escape ecological catastrophe, and (b) the implications of the observation that over the last century global energy consumption has increased more than population numbers. In my view, the former question is the simpler one, and I’ll get to in the near future. In this post I’ll provide some of what I’ve found concerning the latter issue.
The consumption argument
It’s a common observation that, over the last half century or more, resource consumption rates have increased at a faster pace than population size. I’ve seen this observation used to support the view that population growth isn’t as serious an environmental problem as our growing rates of consumption. Sometimes a proponent of this argument presents data showing that the magnitude of growth of total world energy consumption, or of total consumption of a specific resource, is considerably larger than that of population.
As an example, I found a page on the United Nations Earthwatch website where the writer points out that “materials use has grown far faster than population: in the US, total consumption of virgin raw materials was 17 times greater in 1989 than it was in 1900, compared with a threefold increase in population.” The implication is that the rate of consumption is the more important factor to consider. Is the writer’s argument justified? A closer look reveals it’s not that simple.
Let’s dissect this a little
E = P x e
That is, total energy use equals population size times energy use per capita. Likewise, the total use of a particular resource or resource sector (such as “materials use”) would equal population size times the per capita use of that resource. Comparing population growth to growth in total energy or resource use is, therefore, to compare one factor in the equation with the product. Naturally, we would expect the product generally to be larger!
For example, imagine a population of 2. Assume the per capita rate of use of some resource is 1. (i.e., one unit of some kind, applicable to that resource) This population’s total use of that resource use will be 2 x 1 = 2. Now assume the population grows 100% to 4, and the per capita resource use also grows 100% to 2. Now total resource use becomes 4 x 2 = 8. Total energy use has thus grown from 2 to 8, or 300%. This, while population only grew 100%. Can we say then, that resource use is the more important problem in this scenario? Definitely not. Notice that per capita resource use and population each grew by exactly the same amount. To determine which variable is the greater problem, we can’t compare one factor in the equation to the product; we must compare the two factors. The product, after all, is the problem. 
The more valid comparison
Let’s turn then to the more valid comparison of population growth to energy use per capita. Not surprisingly, here the data do not always show consumption growth to outpace population growth. For instance, per capita oil consumption was roughly stable from 1982 to 2005 (pdf), while population grew by about 40%. From 1950 to 2000, per capita CO2 emissions (a reflection of fossil fuel consumption) did not quite double. (And they’ve been roughly level since the late 1970s.) During the same period world population grew about 2.4-fold.
On the other hand, if we look from 1850 to 1990, per capita energy use of industrial forms of energy grew almost 22-fold, while world population grew about 4.7-fold. But per capita energy use of traditional forms of energy decreased by 42%. (Holdren, 1991. p. 245)
Holdren (1991) provides the math to reveal how much of the growth in total world energy use can be attributed to population growth versus growth in energy use per capita. (The basic equation is: population share of growth = annual average population growth rate / annual average energy growth rate. I refer you to the article for the details.) He concludes that with regard to industrial energy consumption from 1890 to 1990, population is responsible for 40% of the growth. For total energy consumption population accounts for 49% of the growth. The contribution of population growth to total energy consumption in the United States is even greater. Other authors’ analyses have suggested still larger contributions from population growth.
To be fair, these conclusions have been the subject of some debate in the Scientific literature. Prominent participants were John Holdren and Paul Ehrlich emphasizing the joint contributions of population, affluence, and technology (affluence being analogous to per capita energy use in the equation above), and Barry Commoner arguing for the primacy of the technology factor. Robert Kates provides a summary of this debate and his view of its current status.
No minor players
Given the evidence, it seems reasonable to conclude, much as Kates points out, that the consensus tends toward emphasizing the combined contributions of population and consumption. My impression, merely from perusing a lot of sources, is that with regard to energy related resources, per capita consumption may currently be the slightly more potent driver of total consumption.  But clearly, as I emphasized in the previous post here, we ignore either factor in the equation at our peril.
As I said at the opening of this post, this is just a preliminary look at this topic. I’ll return to it in the future, and will soon get to the remaining question from the previous post: Would population growth be a problem if we could develop and make widespread use of extremely clean energy technologies?
 In fact, the product, “E,” almost is the environmental impact in this scenario. To get the actual impact, we can plug “E” (or P x e) into a related equation in which it is multiplied by “i” which represents the environmental impact of the technologies used to provide each unit of consumption: I = P x e x i. This can be generalized to the better known I = PAT, in which “I” is environmental impact, “P” is population, “A” is affluence (analogous to “e”), and “T” is the environmental impact of the technologies used to provide each unit of consumption. That equation was published by Ehrlich and Holdren in 1971.
 I have not said much in this discussion about non-energy-related resources. I believe, in many of those cases, Population may be the greater driver of total consumption or environmental impact. I will of course devote some future space to investigating that question further.
Holdren, J. (1991). Population and the Energy Problem. Population and Environment, 12 (3), 231-255.
Ehrlich, P.R. & Holdren, J.P. (1971). Impact of Population Growth. Science, 171, 1212-1217.
Image source: The Sustainable Scale Project which encourages viewers to use its materials with attribution.