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.

Labour unveiled their Climate Change Bill promising to cut emissions of carbon dioxide and other greenhouse gases blamed for global warming by 60 per cent from 1990 levels by 2050. Suggested policies to achieve this ranged from banning standby buttons on electrical equipment and old-fashioned, inefficient light bulbs to 'capture and storage' of pollution from coal-fired power stations. Conservatives grabbed headlines with a plan to limit air travel - a small but fast-growing source of greenhouse gases.

These have been well-intentioned, if not always convincing, ideas. At an Oxford conference, scientists argued against the 'Hollywoodisation' of the problem, that it is being promoted beyond the science. And still, everybody is talking only about one half of the equation: the emissions we generate, not how we generate them. All the standby buttons and low-energy light bulbs are dwarfed by the pressure of a global population rising by the equivalent of Britain every year.

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. Some question whether there is a problem at all. Blair says Britain doesn't need a population policy, and he has a point: Britain's population grows only because of immigration. But greenhouse gas emissions are a global problem, so it should not matter which countries people live in (some say developed countries have higher standards of living so moving people into them increases overall emissions, but it is hard to argue we should deny others our quality of life).

At a global level, optimists say advances in science and technology will provide the solution; more aggressive estimates suggest we could double consumption and halve our impact on the planet. But other evidence suggests it is too soon to relax. Even if huge advances can be made on slashing greenhouse gases, there is an argument that densely populated countries cannot cope with local environmental stresses such as home-building, fresh water use, waste, traffic, light pollution and noise. More worryingly, the evidence that technology can solve the problem is not yet convincing: the recent failure of European car-makers to meet voluntary emissions reductions is a reminder that a decade after the international community made a serious pledge to tackle global warming, emissions are still rising. Another deterrent to discussing population is the uncomfortable suspicion that environmentalism is a soft cover for more objectionable population agendas to stop or reduce immigration or growth in developing countries. Sometimes it might be. But that doesn't take away the underlying fact: that more people use more resources and create more pollution. This is why some braver voices - Sir David Attenborough, Jonathan Porritt and Professor Chris Rapley, director of the British Antarctic Survey, to name a few - have begun to raise the issue.

The biggest obstacle to debate is the matter of possible solutions. Propositions such as ignoring disease or limiting life-saving medical treatment can be ruled out as unacceptable, and birth control is objectionable to many on moral, religious and libertarian grounds. It is not surprising that green groups and politicians, worried about offending supporters, stay silent. There remains a fourth barrier to raising the population issue: even when people acknowledge the problem and brave the debate, it seems too big to solve. But there are things that can be done at least to reduce population growth. Last week the UN Population Fund said its latest projections 'underline the urgency of family planning needs'. It says 200 million women in the world don't have access to 'safe and effective' contraceptive services, and calls for a big increase in funding for family planning, especially in developing nations.

Britain's Optimum Population Trust also calls for 45 countries to drop policies to increase birthrates - mostly because of worries about paying pensions for an aging population. Is this enough to tackle such a big issue? Even with the most optimistic assumptions about falling birth rates, the UN forecasts a population increase to 7.8 billion by 2050. But that is still considerably less than a population of 9.2 billion. And the OPT says the success of campaigns in countries such as Iran and Thailand suggests the best family planning services, especially combined with women's education and human rights, could go even further. It is understandable then that people are worried about discussing population, but fear of misrepresentation, offence or failure are not good enough reasons to ignore one half of the world's biggest problem: the population effect on climate change.