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John Sulston is leading a study into the future and sustainability of global human population

, which you are heading. Why now?

The topic of population is moving up the agenda again. It was very much discussed 40 years ago. Then, with the green revolution, people felt things would be fine because the world population was increasing and everyone wasn鈥檛 starving to death as predicted. But now we are facing a whole series of resource limitations. We are also facing the results of our own emissions 鈥� it is only in the last 10 years that we鈥檝e had the hard evidence to say that rising levels of carbon dioxide really are leading to rising levels of global warming.

Population is a contentious topic. How can you pick apart the issues in a scientific way?

Some people don鈥檛 want to talk about population, and get a little antagonistic. But it would be silly to ignore the issue of population. We have to look at it in a solid, scientifically evidential way and find out the likelihood of possible outcomes. The working party I鈥檓 heading includes scientists from many places and disciplines, in order to produce a report that is well informed by all parts of the world.

Doesn鈥檛 this evidence-based approach become problematic when you鈥檙e talking about humanitarian and family issues?

Well, the rationality is to ask what will happen under certain circumstances, and this is absolutely necessary for giving people guidance. It doesn鈥檛 mean we are going to prescribe people鈥檚 morality, but what we can do is to say: 鈥渋f we continue to do such and such then these events will follow鈥�. If we have a certain number of people in the world then we need to have resources for that number of people. These are the hard facts.

Surely calculating what resources we need is not just about numbers but also about the way that people live their lives?

Absolutely. It is a product of per capita consumption and the number of people. If we have fewer people, we can consume more. What we cannot do is push the product of those two beyond what the world can sustain, and the report will look at both.

Some people argue that technology will save us. How can you predict outcomes if you don鈥檛 know what solutions lie around the corner?

Nobody can predict what the future holds but we do have some sense of the capabilities and limits. I think it鈥檚 a mistake to look at technology in the absence of demography and social patterns. I have colleagues who are passionate about what we can achieve with modifying crops and so on, and they are right; we need to be doing all those things. But that alone will not be enough.

If nobody really knows what will happen, won鈥檛 your findings be just a matter of opinion?

No. We aren鈥檛 going to come up with a complete guaranteed answer, but what we can do is improve and refine the probabilities. The real contention is whether policy should influence population planning.