While WWIII, plague or other large-scale disasters would result in a humanitarian crisis, none of those calamities would do much to slow down the growth of global population, which is currently headed toward 10 billion by 2100. From Mark Tran at The Guardian:

“The pace of population growth is so quick that even draconian restrictions of childbirth, pandemics or a third world war would still leave the world with too many people for the planet to sustain, according to a study.

Rather than reducing the number of people, cutting the consumption of natural resources and enhanced recycling would have a better chance of achieving effective sustainability gains in the next 85 years, said thereport published in the proceedings of the National Academy of Sciences.

‘We were surprised that a five-year WW3 scenario, mimicking the same proportion of people killed in the first and second world wars combined, barely registered a blip on the human population trajectory this century,’ said Prof Barry Brook, who co-led the study at the University of Adelaide, in Australia. …

Brook, now at the University of Tasmania, said policymakers needed to discuss population growth more, but warned that the inexorable momentum of the global human population ruled out any demographic quick fixes to our sustainability problems.

‘Our work reveals that effective family planning and reproduction education worldwide have great potential to constrain the size of the human population and alleviate pressure on resource availability over the longer term,’ he said. ‘Our great-great-great-great grandchildren might ultimately benefit from such planning, but people alive today will not.'”