Human population continues to grow, with recent United Nation projections estimating over 11 billion people by 2100. Likewise, global energy use continues to grow at an exponential rate as we all seek higher standards of living. Technology continues to increase resources and reduce costs for everyone, but can this growth in population and technology continue indefinitely?

One of the first scientists to examine this question was Sebastian von Hoerner, a radio astronomer who conducted most of his research at the Green Bank Observatory in West Virginia. Von Hoerner argued in 1975 that continued growth of energy consumption on Earth would eventually start to contribute direct heating to the planet. (This is a consequence of the conservation of energy and is a separate issue from the emission of fossil fuels.) Even if technology is able to continually lower costs, we will eventually reach a limit to growth where our technology itself starts to warm the planet.

In a paper titled “Population growth, energy use, and the implications for the search for extraterrestrial intelligence,” part of the Futures special issue on the Detectability of Future Earth, Brendan Mullan and I update von Hoerner’s approach to calculate limits to population and energy growth. We demonstrate that Earth could conceivably support up to 20 billion people by optimizing current farmland or up to 100 billion people if all available land were dedicated to agriculture. These limits would require everyone to adopt a strict vegetarian diet and a life of poverty, so increasing the average standard of living would decrease the total carrying capacity. We also show that direct thermal heating of the planet from increased energy use could occur in the 2300’s to 2400’s if energy growth continues at a rate of about 2% per year.

If our civilization ever reaches this point, then our energy consumption as a civilization will equal the total energy Earth receives from the Sun. If such an endpoint is possible and sustainable, then any advanced extraterrestrial civilizations may already have achieved such an energy-intensive state. If we do eventually discover that energy-intensive civilizations are commonplace enough in the galaxy, then we can have greater confidence that our own future will survive any transitions as we approach limits to growth. But if energy-intensive civilizations are rare, or if we are the only ones, then our challenge for the future is even greater. The long-term success of civilization on Earth depends upon how we manage our population and energy growth over subsequent generations.