Climate change and global pollution cannot be adequately tackled without addressing the neglected issue of the world's booming population, according to two leading scientists.

Professor Chris Rapley, director of the British Antarctic Survey, and Professor John Guillebaud, vented their frustration yesterday at the fact that overpopulation had fallen off the agenda of the many organisations dedicated to saving the planet.

The scientists said dealing with the burgeoning human population of the planet was vital if real progress was to be made on the other enormous problems facing the world.

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"It is the elephant in the room that nobody wants to talk about" Professor Guillebaud said. "Unless we reduce the human population humanely through family planning, nature will do it for us through violence, epidemics or starvation."

Professor Guillebaud said he decided to study the field of human reproduction more than 40 years ago specifically because of the problems he envisaged through overpopulation.

His concerns were echoed by Professor Rapley, an expert on the effects of climate change on the Antarctic, who pointed out that this year an extra 76 million people would be added to the 6.5 billion already living on Earth, which is twice as many as in 1960.

By the middle of the century, the United Nations estimates that the world population is likely to increase to more than nine billion, which is equivalent to an extra 200,000 people each day. Professor Rapley said the extra resources needed to sustain this growth in population would put immense strains on the planet's life-support system even if pollution emissions per head could be dramatically reduced.

"Although reducing human emissions to the atmosphere is undoubtedly of critical importance, as are any and all measures to reduce the human environmental 'footprint', the truth is that the contribution of each individual cannot be reduced to zero. Only the lack of the individual can bring it down to nothing," Professor Rapley says in an article for the BBC website.

"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."

Professor Rapley says the explosive growth in the human population and the concomitant effects on the environment have been largely ignored by many of those concerned with climate change. "It is a bombshell of a topic, with profound and emotive issues of ethics, morality, equity and practicability," he says.

"So controversial is the subject that it has become the Cinderella of the great sustainability debate - rarely visible in public, or even in private.

"In interdisciplinary meetings addressing how the planet functions as an integrated whole, demographers and population specialists are usually notable by their absence.''

Professor Guillebaud, who co-chairs the Optimum Population Trust, said it became politically incorrect about 25 years ago to bring up family planning in discussing the environmental problems of the developing world. The world population needed to be reduced by nearly two-thirds if climate change was to be prevented and everyone on the planet was to enjoy a lifestyle similar to that of Europeans, Professor Guillebaud said.

An environmental assessment by the conservation charity WWF and the Worldwatch Institute in Washington found that humans were now exploiting about 20 per cent more renewable resources than can be replaced each year.

Professor Guillebaud said this meant it would require the natural resources equivalent to four more Planet Earths to sustain the projected 2050 population of nine billion people.

"The figures demonstrate the folly of concentrating exclusively on lifestyles and technology and ignoring human numbers in our attempts to combat global warming," he said. "We need to think about climate changers - human beings and their numbers - as well as climate change."

Some environmentalists have argued that is not human numbers that are important, but the relative use of natural resources and production of waste such as carbon dioxide emissions. They have suggested that the planet can sustain a population of nine billion people or even more provided that everyone adopts a less energy-intensive lifestyle based on renewable sources of energy rather than fossil fuels.

But Professor Guillebaud said: "We urgently need to stabilise and reduce human numbers. There is no way that a population of nine billion - the UN's medium forecast for 2050 - can meet its energy needs without unacceptable damage to the planet and a great deal of human misery."

Crowded Earth

* The human population stands at 6.5 billion and is projected to rise to more than 9 billion by 2050.

* In less than 50 years the human population has more than doubled from its 1960 level of 3 billion.

* China is the most populous country with more than 1.3 billion people. India is second with more than 1.1 billion.

* By about 2030 India is expected to exceed China with nearly 1.5 billion people.

* About one in every three people alive today is under the age of 20, which means that the population will continue to grow as more children reach sexual maturity.

* Britain's population of 60 million is forecast to grow by 7 million over the next 25 years and by at least 10 million over the next 60 years, mainly through immigration.

* This is equivalent to an extra 57 towns the size of Luton (pop 184,000)

* By the time you have finished reading this column, an estimated 100 babies have been born in the world.