The key disagreement is when exactly the population will peak, and how much it will fall. Some researchers believe that the world cannot support the 11 billion people that the UNPD predicts. The planet is already straining under human activity, Jane O’Sullivan, a sustainability researcher at the University of Queensland who’s also been critical of the IHME’s methodology, told me. One of her major concerns is climate change, which is driven by greenhouse-gas emissions that tend to rise with population growth. The UN’s Intergovernmental Panel on Climate Change has warned that further warming could lead to more heat waves, droughts, floods, and wildfires. And extreme weather is already threatening to destabilize the food supply, all while demand for food increases. Overpopulation, in short, might lead to more conflict over ever-scarcer resources. Life on this planet could involve not just more unemployment but also its messy knockdown effects—violence from a restless populace, unbalanced migration from rural areas to cities, a lack of housing leading to unsanitary informal settlements, O’Sullivan told me.

Then again, if the IHME is correct, the planet won’t be as crowded as the UNPD predicts—but its people will be much older. Countries tend to spend much more on the elderly than on other age groups: Older people are more prone to illness, and many rely on publicly funded pensions and eventually require caregiving. All of this relies on a system of younger taxpayers, workers, and family members. Many countries, including the U.S., are already struggling to meet the needs of the rapidly growing elderly population. There’s a critical shortage of long-term-care workers in many countries and a rising number of people in the U.S. unable to afford their services. Estimates vary, but according to one literature review, somewhere from one-third to two-thirds of workers in America are already at risk of having inadequate income to maintain their living standards in old age, while our Social Security system relies on funds that are facing insolvency. In Japan, the oldest country in the world, officials are already concerned about the high number of elderly people dying alone in their homes in part because of an aging society and weaker family ties.

These potential outcomes seemingly point to different solutions.