Wednesday, November 26, 2014

Feeding 9... er... 11 billion people

Demographers have been telling us for a while that global populations will level off to about 9 billion, and this "9 billion" number has indeed become the conventional wisdom -- so much so that one of your trusty G-FEED bloggers actually teaches a (presumably excellent) class called "Feeding Nine Billion".

With the current global population at just over 7 billion, the belief that population might level off at 9 billion has given some solace to folks worried about the "pile of grain" problem, i.e. the general concern that feeding a bunch of extra mouths around the world might prove difficult.  9 billion people by 2100 implies a much slower population growth rate over the coming century than was observed over the last century, and while a scandalous 800 million people in the world continue to go to bed hungry every night, there has been notable success in reducing the proportion of the world population who don't have enough to eat even as populations have skyrocketed.  This success, if you can call it that, has in part to do with the ability of the world's agricultural producers to so far "keep up" with the growing demand for food induced by growing incomes and populations, as evidenced by the general decline in real food prices over the last half century (the large food price spikes in the last 5-7 years notwithstanding).

But a paper last month by Gerland et al in Science (gated version here), straightforwardly titled "World population stabilization unlikely this century", provides some uncomfortable evidence that the magic 9 billion number might be a substantial underestimate of the population we're likely to see by the end of this century.  Turns out that fertility rates have not fallen as fast in Africa as expected:  while the total fertility rate has fallen, the decline has only been about a quarter as fast as what was observed in the 1970s and 80s in Latin America and Asia.  This is apparently due both to slow declines in African families' desired family sizes, as well as a substantial unmet need for contraception. Here's a plot from this paper showing the relatively slow decline in African fertility:

So run the world forward for 85 years taking these slower-than-expected fertility declines into account, and you get population projections much higher than 9 billion.  In fact, the mean estimate in the Gerland et al paper of population in 2100 is 11 billion, with their 95% confidence interval barely scraping 9 billion on the low end and 13 (!) billion on the high end.  In fact, their 95% confidence interval for 2050 barely contains 9 billion. Here's the relevant plot (R users will appreciate the near-unadulterated use of the ggplot defaults):

Figure 1 from Gerland et al 2014, Science

So perhaps David should retitle his class, "11 is the new 9", or, "Feeding 9 billion in the next 20 years", or, "Feeding 11 billion (95% CI, 9 billion to 13 billion)".  In any case, these 2+ billion extra mouths are not entirely welcome news for those worried about the global pile of grain.  These much larger numbers imply that even greater progress needs to be made on improving agricultural yields if we want to (a) keep prices at reasonable levels and (b) not have to massively expand agricultural land use to do it.  Thanks, Gerland et al!  

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