“Among the knowledgeable people there is no more conversation about whether the danger is real,” Ehrlich told the Guardian. “Civilisations have collapsed before: the question is whether we can avoid the first time [an] entire global civilisation has given us the opportunity of having the whole mass collapse.”

The idea sounds melodramatic, but Ehrlich insists his vision only builds on famine, drought, poverty and conflict, which are already prevalent around the world, and would unfold over the “next few decades”. “What it would look like is getting to the situation where more and more people are living in uncertainty about their future, subject to all kinds of disease,” he said. “The really big discontinuity you can’t predict is even a small nuclear war between [say] India and Pakistan.

“Of course a new emerging disease or toxic problem could alone [also] trigger a collapse. My pessimism is deeply tied to the human failure to do anything about these problems, or even recognise or talk about them.”

Ehrlich has become the modern day equivalent of Malthus, the 18th-century English clergyman who popularised the idea that the number of people would eventually outstrip food production. Now Bing professor of population studies at Stanford University in California, Ehrlich reignited the issue in 1968 with his book The Population Bomb – co-written, without acknowledgment, with his wife, Anne Ehrlich – which has sold more than 2m copies.

Central to the argument of the book was the idea that Earth has a finite capacity to provide the resources needed to feed and protect a global population which was growing exponentially in numbers and its demands to consume. The book succeeded, slowly, in getting the issue of overpopulation into political and public consciousness, an idea now acknowledged by calculations of the “ecological footprint” of anything from nappies to nations.

The global population has since doubled and, although growth is slowing, is still on course to rise beyond the two billion maximum Ehrlich believes Earth can sustain without irrevocably destroying its water, earth and air.

“The next two billion people, should we get them, will put more and more pressure on environmental systems that are struggling today,” he said. “Each individual has to have food from more marginal land … materials from poorer ores, we’re going to use more oil so we have to drill deeper: we’re past the point of dimishing returns.