|PIC1|We do it about 215 million times a day, so humans need to stop shying away from talking about sex - and the babies it makes - to help avert the global climate crisis, environmentalist and author Robert Engelman says.

With 78 million new homo sapiens arriving every year, the human race urgently needs to address population growth through debate, Engleman says in his new book, "More: Population, Nature and What Women Want".

Engleman, a programme director at Washington's Worldwatch Institute, spoke to Reuters about how to get the conversation going, and why we need to hurry, but be calm, about it.

Q: Condoms for climate change. Is that what you're saying?

A: At the risk of oversimplifying, it's possible to boil it down to say condoms to help prevent further climate change. The book is very much about relations between human numbers and the environment. And it's very much suggesting that better access to contraception, not just condoms but the whole range, is important in thinking about the environment.

Q: Why is it so hard to talk about this topic?

A: In some ways it is an ultimate taboo. People feel that there is no safe ground - even to bring it up, as an issue, it sounds as though you are telling other people how many children to have, and that is unforgivable as reproduction and having children is so sensitive and so personal.

Q: What are the taboos in this discussion, exactly?

A: Several topics come up that are unpleasant or difficult to deal with, one is abortion, one is immigration, one is racial or ethnic differences in fertility. Mostly northern Caucasian peoples are on average having fewer children than other ethnic groups, or other nationalities. That's not universally true but there's a tendency to perceive it that way. So that makes it even more sensitive. You combine sexuality, abortion, immigration and racial and ethnic differences. What sensitive issue haven't we not thrown into this mix?

Q: You suggest there simply too many humans running around emitting global-warming related carbon to be sustainable?

A: We have succeeded too well at expanding the species into every nook and cranny of the planet. In some ways we are now suffering some of the negative impacts of that very success. We discovered that we could burn coal when we ran out of forest wood. Coal launched an industrial revolution that allowed us to have the larger population we have. On one hand it's a triumph. On the other hand we wouldn't be facing a potentially catastrophically changing climate if we hadn't had to feed and care for an unprecedentedly large human population.

Q: And now we need to slow down?

A: I can't prove it, I'm not going to predict it, but we could be facing an era when we find that population doesn't grow forever, and we find that it is a finite planet. Eventually food, disease do have an effect on population by affecting death rates.

At this point, I think we would be well-advised not to keep growing. To have some population decline, not too much, not too long, would be just what the doctor ordered.

Q: You suggest that's what most women would choose?

A: There's a term that I think I invented called the demostat, like a thermostat. The idea is that women's childbearing decisions respond to their environment - when conditions are really bad they tend to have fewer children and then when conditions improve they tend to have more.

I don't think it's quite as simple as women just put their finger into the wind and say it's a good time or a bad time but I think that it's the best way to think about population; that we're most likely to arrive at a sustainable population simply by allowing women to make the decision.

Q: Meanwhile, governments worried about having too many, or too few people, should be calm, you say?

A: I call it "Zen and art of population management". Many governments have become fairly successful with this Zen approach - not telling women how many children to have, not taxing children or somehow disincentivising family, but simply making sure women had good health services by which they could make their own decisions about reproduction.

When that happened, women spontaneously grasped it, and fertility rates went down. Not because governments were telling women to have fewer children, but because they were setting up the conditions under which women could achieve the goal they had always had, which was to be able to time their pregnancy.