In the quarter century the Wilson Center’s Environmental Change and Security Program has been pondering the issues for which it’s named, the world’s demographic future has been wobbling. A key concern of analysts: How many people will farmers need to feed in 2050? Mainstream projections have teetered between 8.9 billion and 9.8 billion, amounting to an increase of between 13 and 21 percent over today’s 7.7 billion. This significant variation in projections is rarely acknowledged by prognosticators. Many simply round up today’s latest guess and state confidently that there will be 10 billion people in 2050—though just a few years ago, the number most confidently stated was 9 billion.

Humility is an essential nutrient for forecasting. Demographers imbibe enough to offer not forecasts of population, but projections—calculations of the sum of births, deaths, and migration based on assumptions about the future that mostly reflect a continuation of current trends. The main assumption is that there will be no surprises—or at least no surprises that affect human numbers and their distribution.

It makes some sense to assume a no-surprise future. Surprise—civilization collapse, for example—doesn’t exactly lend itself to probability calculation. But this makes population projections unreliable guides to the future. Moreover, it leads to lazy thinking—that population’s future is inevitable, well understood, and not subject to our influence.

That’s dangerous. It implies that the unprecedented nature and scale of this century’s challenges are mere irritants, unlikely to affect births, deaths, or migration. Yet we are already seeing such effects. They stem at least indirectly from destructive climatic and environmental trends that are actually (and perhaps surprisingly) more predictable than demographic ones.

We now know, for example, that sea-level rise will continue as glaciers melt and oceans expand from added heat. Continued atmospheric warming seems almost as certain, as does the continued depletion of aquifers and ongoing loss of biodiversity. All of this threatens human life, directly and indirectly. Amidst this destined change, the most reasonable assumption about the future of population is that no assumptions are safe.

This sounds grim, but it offers some clarity to those who care about population, environment, development, and health. The challenge is to pursue paths that lead to lower rates of both birth and death, now and in the future. That has been the demographic success story of the last century—population growth rates have slowed even as we’ve seen greater child survival and longer lives. Will we be able to maintain and ideally accelerate this historic global triumph into the coming decades? That is among the most important questions to track in the years ahead.

Recent trends are encouraging. One, in many places across the globe, the status of women has improved. All over the world, girls and women are less accepting than ever of second-class status. They are standing up, forcefully and with inspiring dignity, for equality. Empowered women, on average and through their own autonomy and choice, have fewer children, later in their lives.

Moreover, as outspoken individuals from Congresswoman Alexandria Ocasio-Cortez to Greta Thunberg have demonstrated, women and girls are influential leaders in advocating for the planet. Some women—illustrated most dramatically by the BirthStrike movement—are even suggesting that the future of the planet may affect their own decisions about childbearing. All of these are trends worth watching.