I have no doubt that the fundamental problem the planet faces is the enormous increase in the human population. You see it overrunning everywhere. Places that were very remote when I went there 50 years ago are now overrun.

Man must at all costs overcome the Earth’s gravity and have, in reserve, the space at least of the Solar System. All kinds of danger wait for him on the Earth… We are talking of disaster that can destroy the whole of mankind or a large part of it… For instance, a cloud of bolides [meteors] or a small planet a few dozen kilometers in diameter could fall on the Earth, with such an impact that the solid, liquid or gaseous blast produced by it could wipe off the face of the Earth all traces of man and his buildings. The rise of temperature accompanying it could alone scorch or kill all living beings… We are further compelled to take up the struggle against gravity, and for the utilization of celestial space and all its wealth, because of the overpopulation of our planet. Numerous other terrible dangers await mankind on the Earth, all of which suggest that man should look for a way into the Cosmos. We have said a great deal about the advantages of migration into space, but not all can be said or even imagined.

Our lifetime may be the last that will be lived out in a technological society. If the world continues to behave as stupidly as it has behaved in the past, we are going to have an increase in population, an increase in violence. We will try to support the population by ripping up earth’s resources, producing pollution at a greater and greater rate, ending, perhaps, in a nuclear war. The earth will have its oil burnt up, most of its most easily available coal used up, its metals distributed thinly over the entire world. We simply won’t have the material basis to build up another technological civilization. The greater the population, the greater the pressure on technology to produce things. Also, there is a great deal of pressure to produce things that don’t directly relate to the quantity of people in the world, but are useless, energy wasting. Socrates is reported to have looked over a bazaar in great wonder and said, “How very many things there are that I do not need.” There are a great many things that we don’t need.

The century of biology upon which we are now well embarked is no matter of trivialities. It is a movement of really heroic dimensions, one of the great episodes in man’s intellectual history. The scientists who are carrying the movement forward talk in terms of nucleo-proteins, of ultracentrifuges, of biochemical genetics, of electrophoresis, of the electron microscope, of molecular morphology, of radioactive isotopes. But do not be misled by these horrendous terms, and above all do not be fooled into thinking this is mere gadgetry. This is the dependable way to seek a solution of the cancer and polio problems, the problems of rheumatism and of the heart. This is the knowledge on which we must base our solution of the population and food problems. This is the understanding of life.

The key to understanding overpopulation is not population density but the numbers of people in an area relative to its resources and the capacity of the environment to sustain human activities; that is, to the area’s carrying capacity. When is an area overpopulated? When its population can’t be maintained without rapidly depleting nonrenewable resources…. By this standard, the entire planet and virtually every nation is already vastly overpopulated.

We are a plague on the Earth. It’s coming home to roost over the next 50 years or so. It’s not just climate change; it’s sheer space, places to grow food for this enormous horde. Either we limit our population growth or the natural world will do it for us, and the natural world is doing it for us right now.