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Seven billion and counting.

That's the stark figure on global population that's concentrating minds at the Royal Society, and has led to the publication of its latest report "People and the Planet".

According to the UN that figure is set to rise to 10 billion before the end of the century, while dramatic shifts in the distribution, age composition and mobility of all those extra people will twist the kaleidoscope of the global population challenge.

Most of the growth - some two billion - will come in Africa, where fertility rates remain high. But according to Professor Sarah Harper, the director of the Centre for Population Aging at the University of Oxford and a co-author of the report, we're also getting older, migrating to cities, and increasing our levels of consumption as developing nations like China close the gap on the west.

"We're far too hung up on the numbers, and that's hidden the subtle but important changes going on behind the scenes," she explains. "We're getting much denser as people move to urban areas, we're getting older as people live longer, and the distribution of people is changing as we become more mobile."

We don't want to go hair shirts, but equally we don't want to end up with pandemics and conflict as a way of limiting our numbers. Nobel Prize winner, Sir John Sulston

It's the link between population growth and unprecedented levels of consumption that's causing most concern. On the one hand the report concludes there's a moral imperative to raise the poorest 1.3 billion out of extreme poverty, but on the other it argues that unsustainable patterns of consumption that deplete resources and damage the environment have to be addressed.

It's a difficult circle to square. The Professor of Environment and Society at Essex University, Jules Pretty, accepts simply telling people to don hair shirts and go without simply won't work.

"The trick will be to encourage more sustainable forms of consumption that don't impact so heavily on the planet's finite resources. At the moment we're moving in the wrong direction so we need to develop different thinking about consumption, different thinking about what a green economy might look like".

Image copyright PA Image caption Could solar panels on UK roofs be a key way to secure future energy?

So what might a world based on more sustainable models of consumption look like? Jules Pretty offers the example of solar power: "If we all had solar panels on our roofs we'd have millions of net contributors to energy generation" he says, "and if we used that power to charge an electric car we'd get a double benefit".

The planet's continuing ability to sustain us is the "elephant in the corner of the room" according to Georgina Mace, the Professor of Conservation Science at Imperial College London. Overconsumption has put enormous pressure on the natural systems that cycle nutrients, regulate water, provide genetic resources for new food crops and medicines and stabilise our climate.

"As we enlarge our footprint on the earth we're gradually eroding away at the earth's support systems. There's lots of evidence now that we're damaging the ecological resilience of those systems, so we're not doing a good job of gardening the planet."

It's a theme that was stressed by the lead author of the Royal Society's report, the Nobel Prize-winning geneticist Sir John Sulston said on the programme this morning "If we don't do something about it things will simply get worse.

"We don't want to go hair shirts, but equally we don't want to end up with pandemics and conflict as a way of limiting our numbers. We must look at population and consumption together and plan to flourish. Very simple".

It may be a simple concept, but getting to a sustainable balance between population growth and maintaining the planet remains a huge challenge.