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Climate Change and the Demographic Shift Font Size: 
By Tim Worstall : BIO| 22 Sep 2020
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A great deal of ink and electrons have been wasted in trying to explain what it is or is not that we should do to try and reduce emissions of CO2 in the United States. One fascinating paper just published provides perhaps the simplest answer of all: just wait for the Baby Boomers to get old.

Given that, past a certain age, virility now comes in blue pills rather than by racing muscle cars, that shuffleboard uses rather less gas than water skiing and the general environmental impact of a generation hobbling towards death is rather less than that same group in vigorous youth, the authors, led by Michael Dalton, seem to think that (along with a few other assumptions), the simple passage of time will reduce CO2 emissions in the US by as much as 40%.

Now of course, one single paper isn't quite enough for us to simply go "Phew!" and declare the issue closed but this is, I hope, the beginning of the next stage of research into the whole area of climate change.

Just to reiterate (before the usual trolls arrive) I am largely Lomborgian in my view of climate change. It's happening, we're at least partly to do with it and the important questions are all to do with what we do about it. In common with Bjorn Lomborg my worries about the science of the issue are nothing at all to do with computer models, satellite measurements or anything of that nature. They are about the economic models (in the Special Report on Emissions Scenarios, or SRES) which provide the data which is then plugged into those models.

One area that has always puzzled me deeply about those SRES models is that population was treated as endogenous to the models. In plainer speech, what the population level was going to be was simply a given, not something that came about from the interactions within the model itself. Which, as I say, is puzzling, for the interactions of population size and the economic wealth of that population are the very things which determine the level of emissions (as moderated by the technology used, of course). Further, some of the population estimates for the SRES were especially commissioned from experts in endogenous estimates (that is, ones that do come from the interactions within the models) but they were not asked for such endogenous ones, but for exogenous ones.

Ok, sorry, I'll stop using $10 words for a bit. Why this is important is because we think we know a few things about what drives shifts in population size. At the heart of it is wealth (as defined broadly, not in purely monetary terms). As increasing wealth leads to better sanitation and medicine, as it has over the past century and more, there is a huge fall in the death rate, most especially amongst children and the women giving birth to them. This leads to a surge in the population and along with the increased life spans in general gives us the "demographic shift". What we think moderates this over time (and why we don't end up with ever accelerating population growth, unlike what Paul Ehrlich has been preaching for decades) is that the very wealth that allows this to happen changes people's behavior.

If, as Darwinian theory would have it (if you prefer Genesis "go forth and multiply" is subject to the same constraints), the aim and purpose of life is to have grandchildren who then go on to have more of the same themselves, then when most children die before they reach maturity then you'd better have a lot of them to ensure your lineage. If almost all will survive to breed themselves, as is true now (more accurately, survive to be capable of breeding, if they should so wish) then you need to have far fewer: the opportunity cost of having few children has fallen. Further, our new found wealth offers (women especially) many more choices than just pumping out the next generation. The opportunity costs of having many children have risen, as a large family inevitably chokes off some of those other opportunities.

It might be worth noting here that it is not just the invention of modern contraception that has led to these smaller family sizes. Not only were effective contraceptives known long before the pill (effective if inconvenient) all the surveys done have shown that it is desired family size that has fallen. It is attitudes that have changed, the new technology most certainly helping in reaching the desired goals, but it isn't the contraceptive pills and salves that have changed that desired family size.

So if we, as we think we do, have a direct relationship between the wealth of a society and the growth or shrinkage of the size of the population, if we were trying to find out what the world was going to be like in a century's time, we'd rather like our models to include that relationship, would we not? Which, as noted above, the SRES models do not.

This is why I very much welcome this paper in Energy Economics. It isn't directly to do with the impact of wealth upon population, rather, the impact of an ageing population upon energy demand. The inclusion of demography into our models of the future, something wholly to be desired. As the abstract states:

Changes in the age composition of U.S. households over the next several decades could affect energy use and carbon dioxide (CO2) emissions, the most important greenhouse gas. This article incorporates population age structure into an energy-economic growth model with multiple dynasties of heterogeneous households. The model is used to estimate and compare effects of population aging and technical change on baseline paths of U.S. energy use, and CO2 emissions. Results show that population aging reduces long-term emissions, by almost 40% in a low population scenario, and effects of aging on emissions can be as large, or larger than, effects of technical change in some cases.

You will, if you read around the internet and the newspapers, see repeated protestations that the debate over climate change is now over, that the science is settled. I'm afraid that I don'tthink that's actually true: I agree that the climate models are getting better, that previously noted anomalies are being resolved, but not that science is either over or stopped.

Rather, that we now need more such science as this paper, to refine the numbers we feed into those computer models.

The author is a TCS Daily contributing writer living in Europe.

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