Forests … are in fact the world’s air-conditioning system—the very lungs of the planet—and help to store the largest body of freshwater on the planet … essential to produce food for our planet’s growing population. The rainforests of the world also provide the livelihoods of more than a billion of the poorest people on this Earth… In simple terms, the rainforests, which encircle the world, are our very life-support system—and we are on the verge of switching it off.

In the early days of dealing with climate change, I wouldn’t go out on a limb one way or another, because I don’t have the qualifications there. But I do have the qualifications to measure the scientific community and see what the consensus is about climate change. I remember the moment when I suddenly thought it was incontrovertible. There was a lecture given by a distinguished American expert in atmospheric science and he showed a series of graphs about the temperature changes in the upper atmosphere. He plotted time against population growth and industrialisation. It was incontrovertible, and once you think it’s really totally incontrovertible, then you have a responsibility to say so.

Most discussions of the population crisis lead logically to zero population growth as the ultimate goal, because any growth rate, if continued, will eventually use up the earth... Turning to the actual measures taken we see that the very use of family planning as the means for implementing population policy poses serious but unacknowledged limits the intended reduction in fertility. The family-planning movement, clearly devoted to the improvement and dissemination of contraceptive devices, states again and again that its purpose is that of enabling couples to have the number of children they want.
With the publication of this article 'zero population growth' and the acronym 'ZPG' came into general use.

Our contemporary culture, primed by population growth and driven by technology, has created problems of environmental degradation that directly affect all of our senses: noise, odors and toxins which bring physical pain and suffering, and ugliness, barrenness, and homogeneity of experience which bring emotional and psychological suffering and emptiness. In short, we are jeopardizing our human qualities by pursuing technology as an end rather than a means. Too often we have failed to ask two necessary questions: First, what human purpose will a given technology or development serve? Second, what human and environmental effects will it have?

The future of mankind is going to be decided within the next two generations, and there are two absolute requisites: We must aim at a stable-state society [with limited population growth] and the destruction of nuclear stockpiles. … Otherwise I don't see how we can survive much later than 2050.

The late Alan Gregg pointed out that human population growth within the ecosystem was closely analogous to the growth of malignant tumor cells within an organism: that man was acting like a cancer on the biosphere. The multiplication of human numbers certainly seems wild and uncontrolled… Four million a month—the equivalent of the population of Chicago… We seem to be doing all right at the moment; but if you could ask cancer cells, I suspect they would think they were doing fine. But when the organism dies, so do they; and for our own, selfish, practical, utilitarian reasons, I think we should be careful about how we influence the rest of the ecosystem.

The solutions put forth by imperialism are the quintessence of simplicity...When they speak of the problems of population and birth, they are in no way moved by concepts related to the interests of the family or of society...Just when science and technology are making incredible advances in all fields, they resort to technology to suppress revolutions and ask the help of science to prevent population growth. In short, the peoples are not to make revolutions, and women are not to give birth. This sums up the philosophy of imperialism.

There is no question that climate change is happening; the only arguable point is what part humans are playing in it. … Personally I would be absolutely astounded if population growth and industrialisation and all the stuff we are pumping into the atmosphere hadn’t changed the climatic balance. Of course it has. There is no valid argument for denial.

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.