Limits to Growth

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.

Artifacts in the Solar System

One way that astronomers and astrobiologists search for life in the galaxy is observation of rocky planets orbiting other stars. Such planets may contain an atmosphere, liquid water, and other ingredients that are required for biological life on Earth. Once a number of these potentially inhabited planets have been identified, the next logical step in exploration is to send remote exploratory probes to make direct observations of these planets. Present-day study of other planetary systems is so far limited to remote observation with telescopes, but future plans for exploration include the design and deployment of small robotic exploratory spacecraft toward other star systems.

If intelligent, technological extraterrestrial life exists in the galaxy, then it is conceivable that such a civilization might embark on a similar exploration strategy. Extraterrestrial intelligent (ETI) civilizations may choose to pursue astronomy and search for planets orbiting other star systems and may also choose to follow-up on some of these targets by deploying their own remote exploratory spacecraft. If nearby ETI have observed the Solar System and decided to pursue further exploration, then evidence of ETI technology may be present in the form of such exploratory probes. We refer to this ETI technology as “non-terrestrial artifacts”, in part to distinguish these plausible exploratory spacecraft from the flying saucers of science fiction.

In a recent paper titled “On the likelihood of non-terrestrial artifacts in the Solar System”, published in the journal Acta Astronautica, myself and co-author Ravi Kopparapu discuss the likelihood that human exploration of the Solar System would have uncovered any non-terrestrial artifacts. Exploratory probes destined for another star system are likely to be relatively small (less than ten meters in diameter), so any non-terrestrial artifacts present in the Solar System have probably remained undetected. The surface and atmosphere of Earth are probably the most comprehensively searched volumes in the Solar System and can probably be considered absent of non-terrestrial artifacts. Likewise, the surface of the moon and portions of Mars have been searched at a sufficient resolution to have uncovered any non-terrestrial artifacts that could have been present. However, the deep oceans of Earth and the subsurface of the Moon are largely unexplored territory, while regions such as the asteroid belt, the Kuiper belt, and stable orbits around other Solar System planets could also contain non-terrestrial artifacts that have so far escaped human observation. Because of this plenitude of nearby unexplored territory, it would be premature to conclude that the Solar System is absent of non-terrestrial artifacts.

Although the chances of finding non-terrestrial artifacts might be low, the discovery of ETI technology, even if broken and non-functioning, would provide evidence that ETI exist elsewhere in the galaxy and have a profound impact on humankind. We do not argue that the search for non-terrestrial technology should be given priority over other astronomical missions; however, as human exploration into the Solar System continues, we may as well keep our eyes open for ETI technology, just in case.

Sky & Telescope article: Where have all the aliens gone?

If intelligent life is common in the galaxy, then the fact that we have not yet observed any extraterrestrials raises the question: where are they? The upcoming March issue of Sky & Telescope magazine features an article written by myself and Seth Baum where we discuss how the conspicuous absence of extraterrestrials may relate to the problem of sustainable development on Earth.

Funny story, we were initially slated for the January issue (we even made the table of contents), but a printing error lost the last 10 pages which sadly included our article. Nevertheless, the Sky & Telescope staff are excellent people to work with and made a fast turnaround. Be sure to check out the March issue!

Planetary Messenger

At long last my philosophical novel is complete! A journey through space, time, and dreams, Planetary Messenger explores the social, scientific, and spiritual consequences of discovering another planet in the galaxy just like our Earth. I began this project as a NaNoWriMo entry in 2007 and continued editing and revising for a year and a half.

From the back cover:

Since the dawn of humanity we have gazed at the stars to ponder our existence. To the naked eye the skies are dark and lifeless, but what if, through a glass, we looked to the heavens and saw our mirror image, a twin Earth from afar? If we found our uniqueness shattered in the vast cosmic arena, then what, if anything, could we still hold sacred?

Planetary Messenger is now available either directly from Createspace or through Amazon. Thanks to all of you who have been part of my life so far and helped make this possible. Happy reading!