Increasing population and consumption, propelled by new technologies and globalisation, "are altering the planet on an unprecedented scale", according to the UN Population Fund.
In a report released worldwide yesterday, it said the Earth's population had doubled since 1960 to 6.1 billion, with growth mainly in poorer countries. But despite a parallel doubling of consumption, "half the world still exists on less than $2 a day".
The UN's State of the World Population 2001 report forecasts that humanity's numbers will rise by 50 per cent to 9.3 billion by 2050, with all this growth taking place in developing countries; indeed, the 49 least developed countries will nearly triple in size.
Whether the global figure will be higher or lower by mid-century "will depend on success in ensuring women's right to education and health, including reproductive health, and in ending absolute poverty", according to the report's authors.
"If women have only the number of children they want, families will be smaller and population growth slower," they said.
More gender equality in terms of access to health and education would also help defeat poverty and protect the environment.
At present, however, nearly 60 per cent of the 4.4 billion people in developing countries lack basic sanitation, almost a third have no access to clean water, one quarter lack adequate housing and 20 per cent have no access to modern health services.
The report, released here in partnership with the Irish Family Planning Association, points out that a child born today in one of the rich industrialised countries "will add more to consumption and pollution than 30 to 50 children born in developing countries".
Dr Akinyele Dairo, of the UN Population Fund, told a press briefing in Dublin yesterday that the rich OECD countries, with 20 per cent of the population, account for 86 per cent of all private consumption compared to 1.3 per cent for the poorest countries.
At the same time, poverty and the limited availability of land "are causing many poor people to increase their pressure on fragile natural resources".
At current rates of deforestation, the last significant tropical rain forest could be gone within 50 years, he warned.
Though the world's population had tripled over the past 70 years, Dr Dairo said water use had grown sixfold over the same period.
By 2025, some 70 per cent of all available fresh water was likely to be used, mainly for agriculture, to cater for population growth.
To feed the nearly 8 billion people expected on earth by then, food production would have to be doubled, using new higher-yielding crops, the report said, particularly with the current level of malnutrition throughout the world.
At the same time, as many as 60,000 plant species, one quarter of the world's total, could be lost by 2025.
The stresses on developing countries in particular are likely to be exacerbated by the impacts of climate change, including increased storms, flooding and soil erosion.
Already, it estimated, there are 25 million "environmental refugees" worldwide.
Ms Mary Banotti MEP (FG) highlighted the "devastating impact" of HIV and AIDS in sub-Saharan Africa, saying it could be alleviated by focusing on women's health.
Contraception, previously unmentionable in Ireland, was now widely accepted, she said.
But Senator David Norris (Ind), who also attended the briefing, accused the Vatican of perpetrating a "crime against humanity" by not accepting the use of condoms as a prophylactic against HIV infection - a stance mirrored by many in Islamic countries.
Referring to the refugee crisis in Afghanistan, Ms Banotti said Irish aid was needed to provide basic supplies, such as home delivery kits, to save the lives of women giving birth to babies in the refugee camps. Unless this was provided, thousands would die, she warned.