Climate change

September 22, 2011

Why population policy will not solve climate change – Part 1

Filed under: Climate Change, Population growth — buildeco @ 3:23 pm

by Barry Brook

I have given lots of talks on climate change over the last few years. In these presentations, I typically focus on explaining the basis of the anthropogenic climate change problem, how it sits in the context of other human and natural changes, and then, how greenhouse gas emissions could be mitigated with the elimination of fossil fuels and substitution with low-carbon replacement technologies such as nuclear fission, renewables of various flavours, energy efficiency, and so on. When question time follows, I regularly get people standing up and saying something along the following lines:

It is all very well to focus on energy technology, and even to  mention behavioural changes, but the real problem — the elephant in the room that you’ve ignored — is the size of the human population. No one seems to want to talk about that! About population policy. If we concentrated seriously on ways to reduce population pressure, many other issues would be far easier to solve.

On the face of it, it is hard to disagree with such statements. The human population has growth exponentially from ~650 million in the year 1700 AD to almost 7 billion today. When coupled to our increasing economic expansion and concomitant rising demand for natural resources, this rapid expansion of the human enterprise has put a huge burden on the environment and demands an accelerating depletion of fossil fuels and various high-grade ores, etc. (the Anthropocence Epoch).  Obviously, to avoid exhaustion of accessible natural resources, degradation of ecosystems and to counter the need to seek increasingly low-grade mineral resources, large-scale recycling and sustainable use of biotic systems will need to be widely adopted. Of this there is little room for doubt.

So, the huge size of the present-day human population is clearly a major reason why we face so many mounting environmental problems and are now pushing hard against planetary boundaries (see diagram above). But does it also follow that population control via various policies is the answer – the best solution — to solving these global problems? It might surprise you to learn that I say NO (at least over meaningful time scales). But, it will take some time to explain why — to work through the nuances, assumptions, sensitivities and global versus region story. So, in a series of posts, I’ll explain why I’ve reached this conclusion, and, as always, invite feedback!

In part 1, below, I outline some of the basic tools required to come up with some reasonable answers. A huge amount of relevant data on this topic (human demography) is available from the United Nations Population Division, the Human Life-Table Database, the Human Mortality Database, and the U.S. Census Bureau. That data and statistics I cite in these posts come from these sources.

First, let’s look at the global situation. As of 1 July 2011, the human population numbered approximately 6.96 billion people (that’s 6,960 million, give or take a few tens of millions), and is expected to cross 7 billion in March 2012. For historical context, in 1900 it was 1.6 billion, in 1954 it was 3 billion, in 1980 it was 4.5 billion and in 1999 it was 6 billion.

The mid-range forecast is 7.65 billion by 2020, 8.61 billion by 2035, 9.31 billion by 2050 and 10.12 billion by 2100. So, population globally is projected to continue to rise throughout the 21st century, but at a decelerating rate. A summary of a range of scenarios is given in the following table:

The annual growth rate can be calculated as the ratio of one 5-year time period over the previous one, e.g., medium variant 2015 = (7,284,296/6,895,889)^0.2 = 1.1 % per annum. This compares to the peak growth rate of 2.2% in the early 1960s — so it’s clear that population growth is already slowing, but only gradually.

The medium and high variants in the table above indicate no stabilisation of population size until after 2100, while the low variant hits a peak in 2045 with a gradual decline thereafter, reaching the 2001 level once again by the year 2100. The low variant involves assumptions about declining birth rates that are beyond the expectations of most demographers.

In this first post, I want to go beyond the standard UN assumptions to look, in brief, at some more extreme scenarios. I should note here that the model behind these projections is reasonably complex, being based on age-specific mortality and fertility schedules, current cohort-by-cohort inventories (in 5-year-class stages), and the forecast trends in these vital rates over time. In the second post, I’ll explain some of the detail behind this demographic projection model, and explore its sensitivity to various assumptions and parameter estimates. In the third post, I’ll look at the country specific forecasts, from both the developed and developing world. But first, here are some alternative global scenarios, which are not meant to be realistic… merely illustrative.

In this Scenario 1, birth and death rates are locked at those observed during the last 5 years. The 2100 population size is slightly larger than the medium variant from the UN, at 9.4 billion in 2050 and 11.3 billion in 2100. The intrinsic population growth rate (GR) in this model is 0.36% per annum, but due to an unstable initial age structure, the lower equilibrium rate is not reached until 2070. Projecting forward many centuries, to the year 2500, the population size is 47 billion. This is obviously ridiculous — such projections with unchanging vital rates obviously cannot hold over the long term.

Scenario 2 is the UN medium variant, listed in the table above. This assumes that total fertility (TF: number of children a woman would produce over her lifetime if she survived through to menopause) declines from today’s level of 2.5 to 2.03 by the year 2100. There is also a slight decrease in death rates (DR), which I will explain more in the next post.

Scenario 3, illustrated below, is the same as Scenario 2, except total fertility is assumed to decline (linearly) to 1.0 by 2100, rather than to 2.03.

In this case, the 2050 population size is 9.03 billion and 2100 is 7.96 billion. This is still higher than the UN low variant, showing that the low variant requires some fairly heroic fertility assumptions. Looking further into the future, if we assume that TF thereafter stabilises at 1, the global population would eventually decline to less than 1 billion by the year 2210, below 100 million in 2300, and below 1 million in 2460! The underlying GR in this forecast, after 2100, is a decline of 2.73% per annum. So, for effective extinction of the human population to be avoided, either birth rates would once again have to rise at some point after 2100, or else (more likely) death rates would decline substantially to to medical improvements. More on this in the second post on this topic.

Right, let’s go even more extreme, Scenario 4. What if a global TF declines to 1 by the year 2030 — within the next 20 years — which could only be achieved by implementation of a global 1-child per couple policy within a decade or two (I’ll led you judge the likelihood of this…). Here are the results:

In this case, the 2050 population size is 7.03 billion, and 3.79 billion by 2100. The 1 billion mark is passed in 2160, and 100 million in 2245.

For Scenario 5, let’s assume that some virus or hazardous chemical causes global sterilization by 2015. Here’s what the trajectory looks like.

Population is 4.90 billion in 2050 and crosses 1 billion in about 2090. Virtually everyone is dead by 2120, as you might expect. Now to be fair, the reality of a scenario like this would almost certainly be much worse, because as the population aged with no children, society would quickly fall apart. Most people would probably be dead due to societal collapse by mid-century.

Finally, let’s wind back the TF assumption a bit, but ramp up the death rates. Assume for instance that climate change has caused more famines, disease etc. such that death rates double over the course of the 21st century, rather than decline (as expected) due to improved medical treatments, and TF declines to 1 by 2100.

In this case, Scenario 6, the 2050 global population is 6.89 billion, and it has declined to 2.26 billion by 2100. The 1 billion mark is crossed in 2120, and 100 million in 2160. This seems to be the most plausible of the extremely low variant scenarios that I can possibly justify (I don’t say this is probable). But even in this grim outlook, global population is, in 2050, about the same as today!

The TF = 1 by 2030 (Scenario 4) just does not seem in any way achievable or desirable, and anyway, the total population size in 2050 is still larger than today’s. The conclusion is clear — even if the human collective were to pull as hard as possible on the ‘total fertility’ policy lever, the result would NOT constitute an effective policy for addressing climate change, for which we need to have major solutions well under way by 2050 and essentially wrapped up by 2100.

In summary (for part #1), I support policies to encourage global society to achieve the low growth variant UN scenario. (More on that in the next post). But I must underscore the point that population control policy is patently not the ‘elephant in the room’ that many claim — it’s more like a herd of goats that’s eaten down your garden, and is still there, munching away…

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