Soaring population will force millions to flee water shortages in search of refuge - and, according to new figures, Britain will be one of the world's 'lifeboats'. On the eve of a major population conference, Science Editor Robin McKie asks: could the UK cope?

Britain will become one of the world's major destinations for immigrants as the world heats up and populations continue to soar. Statistics from the United Nations show that, on average, every year more than 174,000 people will be added to the numbers in the UK and that this trend will continue for the next four decades.

By then, only the United States and Canada will be receiving more overseas settlers, says the UN. This increase in British numbers is likely to put considerable strain on the country's transport, energy and housing, experts warned last week.

"The US and Canada will be taking in more people than us every year by 2050 but they are huge countries," said demographer Professor Tom Dyson of the London School of Economics. "Britain, by contrast, is a small nation. We will feel the impact of all these people. There will be no getting out of it. Simply controlling our carbon dioxide emissions will become harder and harder as more and more people arrive on our shores. In addition, housing, water supplies and transport will be strained and will need greatly increased investment."

However, other experts say such increases could also produce benefits for the nation, bringing in immigrants who could provide a vital supply of young workers. These demographers point out that, by 2050, more than a third of the UK population will be aged 60 or over. By then there will be a desperate need for bus drivers, care-workers and others to keep the country running and immigrants could fill this gap.

In addition, there is the issue of humanitarian responsibility. Britain is likely to be one of the few nations to survive the worst effects of climate change while other nations, particularly those in the developing world, have their farmland and fishing grounds destroyed. It could be argued that the UK has a moral duty to provide shelter for as many refugees as our shores can support.

But deciding what numbers the country might support is a highly controversial issue and will be the focus of a conference on sustainable populations which will be held this week in London. Organised by the Optimum Population Trust, the meeting will hear that the United Nation expects that by 2050 the world will be inhabited by around 9.2 billion people, compared to its current level of 6.8 billion. Every day, the equivalent of the population of a large city is added to the numbers of humans, a rise that is now straining the planet's resources to breaking point.

At the same time, Britain's population will rise from its current level of 61 million to 72 million by 2050. The nation will then be the most populous in the European Union, outstripping Germany, whose population will slump from 82 million to 71 million people as its immigration figures plummet.

The idea that Britain could one day support such numbers has been questioned by Aubrey Manning, emeritus professor of natural history at Edinburgh University. "There are far too many people living in Britain already," he said. "Once our population passed the 20 million level around 1850, it became too numerous. That is the figure at which we could no longer sustain our population from our own resources. We are now three times over the limit and heading for more. We have long passed the line of sustainability. As for the planet, its maximum sustainable population is no more than 3 billion, I would say."

The rise in population indicates that the country is set for some considerable overcrowding. Britain's land area is only two-thirds that of Germany, yet it will soon support the same number of citizens. "This population rise, brought about by rising immigration, will strain our infrastructures - our housing and water supplies - and bring very little advantage to the nation," said Dyson, who will address the conference. "Nor do I think these extra people will be able to help in looking after our older people."

But these points were disputed by Tim Finch, head of immigration for the Institute of Public Policy Research. "A healthy economy sucks in young, educated people and that is what has happened to this country over the past couple of decades. These young immigrants have helped keep the country running as our population has started to get older and they will become more important as the decades go past and that ageing intensifies. The immigration system picks out the best and the brightest of immigrants and they will be of great service to Britain. That is just a fact."

The problem is that discussions of population numbers in the past have been associated with talk of eugenics and with attempts at controlling ethnic populations. As a result, there is little discussion today of the subject or its impact on the environment, a point stressed by James Lovelock, the distinguished environmental scientist. "The subject has become a taboo, a matter of political correctness," he said last week. "And that is dangerous, for the numbers of humans on Earth are going to be crucial to our survival."

Manning added: "We have stopped worrying about population because other issues - acid rain, climate change and others - have occupied our attention and because past fears of global food shortages were proved unfounded. But the subject will not go away. Our planet is now dangerously overpopulated."

Another conference speaker, Chris Rapley, director of the Science Museum, in London, agreed. "We desperately need to bring down our emissions of greenhouse gases but the truth is we will never get the contribution of each individual down to zero. Only the lack of the individual can bring it to zero, and that is an issue for population control which we need to talk about openly and urgently."

Rapley will tell delegates that the Earth's population is now rising at a rate of around 80 million a year. "That is roughly the same as the number of unwanted pregnancies across the world," he said. "If we can prevent unwanted pregnancies, we can halt this spiral in our numbers."

To do that, contraception will have to become universally available - and political and religious opposition to birth control removed. If that happened, the world's population could be stabilised to around 8 billion by 2050, added Rapley.

But many climatologists believe that by then life on the planet will already have become dangerously unpleasant. Temperature rises will have started to have devastating impacts on farmland, water supplies and sea levels. Humans - increasing both in numbers and dependence on food from devastated landscapes - will then come under increased pressure. The end result will be apocalyptic, said Lovelock. By the end of the century, the world's population will suffer calamitous declines until numbers are reduced to around 1 billion or less. "By 2100, pestilence, war and famine will have dealt with the majority of humans," he said.

One of the few places to survive the worst impacts will be Britain. "Our climate will be one of the least affected by global warming," added Lovelock. "As a result, everyone will want to live here. We will become one of the world's lifeboats. The trouble, of course, will be that, even if we wanted to, we will not be able to pick up everyone. There will be some hard decisions to make."

Many experts predict that disaster will strike long before 2050. Last week, the government's chief scientific adviser, Professor John Beddington, said the planet faced "a perfect storm" of food, energy and water shortages which could strike in less than 20 years. In a speech to the Sustainable Development Commission conference in London, Beddington said that one in three people were already facing water shortages and that by 2030 world water demand would increase by more than 30%; energy demands would increase by 50%. "There are dramatic problems out there, particularly with water and food, but energy also, and they are all intimately connected."

In the long run, however, humanity should benefit, said Lovelock. "If you look at our species over the past million years, there have been a number of major climatic events, some devastating. Between the Ice Ages, sea levels rose by 120 metres and tracts of land were flooded. Yet that period covers the time that early humans emerged and evolved into Homo sapiens

"Often our numbers were brought to catastrophically low levels by climate change and numbers were reduced to only a couple of thousand on a couple of occasions. Every time things got bad, our numbers plummeted and we improved as a species. That is certainly going to happen again over the next 100 years."

The world by numbers

1 million Britain's population in Roman times

6 million Britain's population around the time of the English civil war

47 million Britain's population in 1945

52,000 The number of tonnes of carbon dioxide pumped into the atmosphere every minute

267 The average number of births every minute worldwide; the average number of deaths per minute is 118

78 million The planet's annual population increase, a number roughly equivalent to the population of Germany

1 million The number of chimpanzees in Africa in 1900. Today, thanks to habitat loss and hunting, numbers have dropped to around 15,000

38.4 The median age in the UK rose from 34.1 years in 1971 to 38.4 in 2003 and is projected to reach 43.3 in 2031. (The median is the age that separates the oldest half of the population from the youngest.)

10 billion The number of chickens eaten by man worldwide every year

500 million The number of ducks eaten every year

1.3 billion The population of China

1.2 billion India's population

500 million The population of the EU

74 million The number of barrels of oil pumped daily across the planet; 15 million tonnes of coal are dug every day

9 Between 2010 and 2050, nine countries will account for half of the world's projected population increase: India, Pakistan, Nigeria, Ethiopia, the United States, the Democratic Republic of Congo, China, Bangladesh, Tanzania

• Sources: World Clock; Poodwaddle; UN Population Division