During the years when the Book of Revelations was being laid down, some time apparently in the first century AD, human populations were likely, with some exceptions, to be small, imperilled, and surrounded by a seemingly infinite planet. Officially at least, on October the 31st this year the population of the earth reached seven billion. And there are times when, psychologically at least, the planet seems to have shrunk to the size of a smartphone. As I blogged here following a recent BMJ conference, climate change looks set to release all four horseman of the apocalypse. Population growth might well be the fifth.

According to an article in this month’s Prospect, the United Nations Population Division (UNPD) has two projections for global population in 2100. The low one is 6.2 bn; the high one a surely catastrophic 15.8bn. As Bronwen Maddox writes, “Where the figure falls between these poles will help shape the 21st century: which countries are rich, which powerful, which have revolutions and wars, what happens to migration, pollution, and climate change.” Population growth is by no means uniform and the overall rate of growth is slowing. Although old Europe has for some time been having only enough children to replenish itself, according to Maddox, they have also been joined by Brazil, China, Indonesia and southern India. But much of the rest of the world is not, certainly not yet, following suit and it is growth here that could push global population toward the upper end of the UNPD’s projection.

Predicting future population is of course a complex science, subject to almost meteorological uncertainty. It is also incendiary. Given the loose inverse correlation between per capita income and family size, more sanguine commentators argue that economic growth will quite naturally take care of it. But population growth can also have a toxic effect on economic development. And attempts to control it – Maddox cites China’s notorious one-child policy and Indira Ghandi’s programme of enforced sterilisation – can be brutally coercive. They can also enrol health professionals in violations of fundamental human rights. Attempts to limit population growth by less coercive methods – by family planning, by supporting women’s reproductive rights – can also run into powerful religious opposition. The population problem also divides environmentalists. If levels of consumption are the problem, then concentrate on the bloated appetites of the west, not the family size of those who consume so little. As Robin Stott points out in this week’s BMJ, countries with high development, low fertility, and presumably better overall health emit more than 10 tonnes of carbon per person per year; less developed countries with higher fertility consume about one and a half. The links between health, population, and the environment are therefore complex.

But although there is sense in all this, I find it difficult to avoid Maddox’s conclusion. Although she cheerleads the extraordinary ingenuity of our kind – our ability to make inroads into the most seemingly intractable of problems – it seems at least intuitively right that “the greater the numbers, the more migration there will be, the more carbon emissions, and the more pressure on states to fail. It is fanciful to think that these are not easier to manage if there are fewer people.” I have no idea whether the predictions of the sanguine or the melancholic will prevail. But if population levels do continue to rise, pressure will return piecemeal to engage more draconian measures. And once again health professionals may find themselves pressurised to participate in coercive measures of population control, pressures that must, it goes without saying, at all costs be resisted.

Julian Sheather is ethics manager, BMA. The views he expresses in his blog posts are entirely his own.