[The follow-up to this essay is here.]
In 2004 the United Nations issued a report titled World Population to 2300. (Large pdf) In it, the demographers who authored the report offer “projections” concerning world population growth through this century and all the way to 2300. The following graph, based on the UN’s, shows the three scenarios or projections for which the report is best known. As you can see, their medium scenario, the one on which most people focus, shows what might appear to be a near peaking of population growth around 2050:
I had occasion last year to sit in on a college class in which the students were discussing the issue of world population growth. One student asked the question, “Why should we worry about population growth when the UN says world population will stabilize around 2050?” That’s an important question, and one the student is not alone in having asked. The UN report is cited from time to time by mainstream economists and others who dismiss concern over population growth. (Sometimes those on the far right voice instead a kind of corporate concern about the slowing of population growth in developed countries. That is a topic for a future post.)
Do they have a point? Should we take comfort in UN report? Should we see it as reason to dismiss, at least to some extent, population growth as a serious concern? I will try here to show why that would be a mistake. To understand why, we have to look closely at the report to understand what it really says and to recognize its limitations.
The report is a large document, attached to which are 12 supplementary essays by an array of authors. I won’t attempt a thorough critique of it here. To do it any justice at all, in fact, would take a long essay, much longer than the already-too-long-for-the-blogosphere essay you’re reading now. (I promise shorter posts to come!) Instead, I’ll address certain aspects of the report in this and one or two subsequent posts (not necessarily appearing consecutively), all aimed at answering the question, “Should the UN report’s projections be of any comfort to those concerned about population growth?”
First, I should note, for the sake of accuracy, that the medium scenario does not, in fact, have world population peaking in 2050. It has it continuing to grow until a peak of 9.22 billion in 2075. That people frequently refer to a peak in 2050 is, I think, a result of the graph above ending at 2050, as well as a fair bit of talk in the report about projections to 2050.
What are those “projections,” really?
To begin forming an answer we must examine the nature of those projections. We need to consider carefully what the authors say about this issue. Let’s look at a few points made in the report’s introduction. There, the authors first say:
This report presents projections of world population, and even of the populations of individual countries, over the next 300 years. Given the inherent impossibility of such an exercise, these projections have a special character. They are not forecasts. They do not say that population is expected to reach the projected levels. Rather, they are extrapolations of current trends. They give what paths population would follow if, and only if, historical trends and trends previously forecast up to 2050 continue. Of course one cannot expect these trends to continue as is… (emphasis added, p. 3)
They are telling us, then, that their “projections” are not the predictions most assume them to be. They explain in several places that they are merely plausible scenarios, given certain fixed assumptions derived from observations of recent history, assumptions the authors make clear will almost certainly not match reality. The medium scenario is not a “best guess.” It is simply one scenario, albeit perhaps more plausible than the high or low scenario.
To emphasize their point, they mention that “constructing long-range projections such as these is a little like predicting the outcome of a basketball game after the first five minutes. No one can do that reliably.” (p. 3)
[That the report looks at long term implications of short term trends] should not be taken to imply that these trends are actually expected to continue. To some extent, the reverse is true. The projected long-range path for population is reported partly to facilitate thinking about how to prepare for it, but also to encourage action to modify this path, to make it more favourable… (p. 3)
If one were to attempt to use the projections as predictions, one would run into problems as “Projected figures for the near term (say up to around 2010) benefit from the accuracy of base data and are unlikely to be off by much. Projected figures for 2050, in contrast, are much less certain.” (emphasis added, p. 4) (Note that such a statement momentarily strays from the recognition that these projections are not intended as predictions. My guess it that even for some associated with the report, maintaining that clarity of purpose was not always easy.)
The supplementary essays shed further light on the nature and limits of the document. We can glean a good deal even from the title of Michael S. Teitelbaum’s, “Projecting the Unknowable: A Professional Effort Sure to be Misinterpreted.” Continuing on to the content we find first the caution that “the core goals of the effort might be described as located somewhere on a continuum between the Implausible and the Impossible. The truth is that none of us has the ability to see very far into the future…” (p. 165) And again, “[The projections] can be understood only as hypothetical scenarios, and not as forecasts.” (p. 166)
To underline his point, Teitelbaum adds:
“The truth is that much about the future of demographic trends a century (or three centuries) from now is unknowable, and quite literally so. None of us has any way of detecting whether fertility rates over the coming century will be lower or higher than at present, nor whether erratic/unstable or fluctuate in some stable and predictable way.” (p. 166)
Teitelbaum offers an illuminating analogy: “Consider what range of ‘plausible’ 300-year demographic projections a sophisticated statistician in 1700 might have developed for the year 2000.” (p. 167) Nor, he adds, could a demographer in 1900 have made a 100 year projection with much greater confidence. Similarly, focusing only on the UN’s projections through 2075 — the year in which world population is projected to peak — we might consider the ability of a demographer in 1933 trying to project demographic trends through 2004. As Teitelbaum suggests, the United States baby boom, alone, would have been completely unforeseeable.
Reminding us of what the projections are, Teitelbaum argues:
[They] can be understood only as hypothetical scenarios, and not as forecasts. The word scenario (from the Latin scaenarium, a place for erecting stages) is defined as “a sequence of events especially when imagined; especially: an account or synopsis of a possible course of action or events” In contrast, “forecast” has a far stronger assurance of plausibility, of predicting future events on the basis of credible information… (p. 166)
Writing before the report was published, Teitelbaum even recommended that its language be carefully calculated to avoid the impression that the projections were forecasts or predictions: “Avoid use of the indicative future tense, such as ‘will be’ or ‘will increase to.’ Also, “Avoid use of the verb ‘to project,’ as in ‘We project that…’ This formulation often is misunderstood by non-technical readers to mean the same as ‘We predict that…'” (p. 167)
Reading through the report reveals that Teitelbaum’s recommendations were essentially ignored, contributing heavily, no doubt, to the frequent misuse of the report’s data.
We are cautioned still further in the essay, “Foretelling the Future,” by John R. Wilmoth:
Even with proper warnings, I think that both projections and forecasts are often interpreted as predictions of the future in the mind of the average listener. Furthermore, most people are unlikely to notice anything more than the “medium variant,” which they see as a professional’s “best guess” concerning future demographic trends. Although some may take note of the uncertainty implied by presenting multiple variants, I believe from experience that even the most sophisticated consumers of such information do not see demographic projections and forecasts as mere illustrations of possible future paths for the world’s population – in other words, at best they view them as predictions with a degree of uncertainty. Perhaps this issue could have been addressed most effectively by choosing a different title for the report and then stating clearly the motivation behind this unusual exercise… (pp. 174-175)
Small differences lead to big differences
Part of the difficulty of predicting future population size, and adding to why the authors did not attempt such prediction, is described in the essay by Tim Dyson: “Small but sustained deviations in total fertility result in big differences in eventual population size – e.g. in the low scenario, in which total fertility is essentially 0.25 of a birth lower than in the medium scenario, the world’s population in 2300 is only 2.3 billion (compared to 9.0 billion)” Demographers have enough uncertainty to deal with in trying only to anticipate widely divergent fertility rates. To attempt to differentiate between possibilities varying by small fractions of a percent would be folly. Yet, as Dyson suggests, those small, unpredictable differences add up in time to huge differences in population numbers.
Then what’s the point of the thing?
What then is the use of the UN report? Simply, it allows us to consider current data, to compare how plausible futures would vary as a function of different assumptions. It enables us to consider “what if” scenarios, to ask, “What if recent trends were to continue? Would they necessitate action on our part? Do they suggest we would be headed for problems? Do they point to anything alarming enough that we might consider taking certain actions now?”
There are far too many social and otherwise complex elements at work in population growth to allow for any sort of reliable predictions at this stage in the demographer’s craft. Joel Cohen, mathematician and author of the book, How Many People Can The Earth Support? sums it up: “demographers are better at forecasting the demographic consequences of choices people might make about fertility and mortality and migration than at predicting what those choices will be.”
Just from this examination of the nature of the UN report’s projections, then, we can see that it would be a mistake to say, “The UN says world population is going to peak around 2075,” much less to add that “Therefore, there’s nothing to worry about.” Simply put, the UN says neither.
There are other reasons why such statements are mistaken. We’ll look at those in the near future.
[The follow-up to this essay is here.]