In the time it takes you to get to the end of this sentence, seven people have been added to the population of the world. At this rate, the United Nations estimates the number of people on the planet will nearly double by the middle of this century. Even with significant reductions in birth rates, the population is expected to increase from 6.7 billion now to 9.2 billion by 2050.

These figures are staggering. Yet there was hardly a mention of them in a major story last week: the announcement by Britain's two main political parties of how they will tackle what is commonly agreed to be the biggest threat facing the planet, global warming and ensuing climate change.

Put simply, if governments want to cut greenhouse gas emissions by 60 per cent, and the world's population rises to the mid-range forecast of 9.2 billion, each person would in fact have to slash their emissions by 72 per cent. More efficient technology, renewable energy and lifestyle changes will help do that, but growing prosperity and consumption in developing countries will also make it harder. That all our low-energy light bulbs, home insulation, efficient cars, boilers and washing machines have so far failed to stop emissions growing illustrates how difficult cutting them will be to achieve.

Some population activists argue the world can only support a population of two to three billion, even as few as 500 million in future. But even if reducing the world's population is unlikely or distasteful, it is incredible that there is not even a debate about limiting and maybe one day reversing growth. There are many understandable reasons for the prevailing reluctance to talk about population.

So if we believe that the size of the human "footprint" is a serious problem (and there is much evidence for this) then a rational view would be that along with a raft of measures to reduce the footprint per person, the issue of population management must be addressed.

Let us assume (reasonably) that an optimum human population level exists, which would provide the physical and intellectual capacity to ensure a rich and fulfilling life for all, but would represent a call upon the services of the planet which would be benign and hence sustainable over the long term.

A scientific analysis can tell us what that optimum number is (perhaps 2-3 billion?).

With that number and a timescale as targets, a path to reach "Utopia" from where we are now is, in principle, a straightforward matter of identifying options, choosing the approach and then planning and navigating the route from source to destination.

World population is expected to increase by 2.6 billion over the next 45 years, from 6.5 billion today to 9.1 billion in 2050. Almost all growth will take place in the less developed regions, where today's 5.3 billion population is expected to swell to 7.8 billion in 2050. By contrast, the population of the more developed regions will remain mostly unchanged, at 1.2 billion.