How many people can the Earth support?

Hannah - Hello and welcome to Question of the Week from The Naked Scientists. With 7 billion of us currently living on Earth, and that number is set to rise to 9 billion by 2050, should we all be making a move to relocate our homes to a terraformed Mars? Steve Davis has been pondering this very question. He wrote in with this...

Steve - I frequently wonder where we are heading as a species and see the one big issue we continue to ignore is a sustainable human population. I have heard the Earth can support about two to three billion people comfortably, and sustainably. Is that true?

Hannah- So can us humans happily fit on this lovely green and blue planet. First step, over to Stuart Basten Research Fellow in demography and social policy at Oxford University.

Stuart - Humans in themselves take relatively little space. Indeed, if every human being on Earth stood side by side, we would easily all fit into an area the size of Berkshire.

Hannah - So if we could all fit into a space that's slightly larger than New York City, what's the problem?

Stuart - Humans can, and do, use a tremendous amount of resources. So the question therefore is determined by three elements which loop back and affect each other: namely - 1 how many people will there likely be in the world; 2 how much will each person consume in terms of food water energy etc; and 3 - what is the likely future of the resources which we use. Indeed the critical question is just how we want humans on average to live.

Hannah - And Professor Martin Rees from Cambridge University agrees...

Martin - I think it's very hard to give a proper answer to that question because it depends on the lifestyle of people. Let me take two extremes. If everyone were to live like present day Americans then the world couldn't support sustainably more than 2 billion people. We're already using more resources than we can sustainably; on the other hand, to take it to a silly extreme - supposing everyone was a vegetarian and ate nothing but rice, lived in little tiny homes, didn't travel at all but just lived in virtual reality, then maybe even 20 billion could be accommodated. So the reality lies somewhere in between. I personally think that it should be possible for 9 billion people by 2050 to live comfortably, provided that agriculture becomes more efficient. We also need to ensure that food is not wasted and is transported better; so this needs better engineering; and we also need to make better use of water resources, because water is one of the constraints on agriculture. But I think that we have the technology - and certainly will by 2050 - to ensure that everyone can live even if there are nine billion of us. But I'm more anxious about whether the political will is there to ensure it actually happens. 

Hannah - So if we develop our technology and make agriculture, food transport and water use more efficient, we should be able to support 9 billion living sustainably on Earth; but the individual and political will has to be there to enable that to happen. Thanks Steve for getting in touch with that question and Stuart and Martin for your answers...