The institute's director, Dr Lester Brown, says the HIV epidemic is the first of these threats to spiral out of control in developing countries.

He says it "should be seen for what it is: an international emergency of epic proportions, one that could claim more lives in the early part of the next century than World War II did in this".

Water is becoming increasingly scarce

Between a fifth and a quarter of adults are already infected in Zimbabwe, Botswana, Namibia, Zambia and Swaziland.

India, with four million adults now HIV positive, has more infected individuals than any other country.

The institute says the risk from finite water sources is real, since 40% of the world's food comes from irrigated land.

India again faces trouble, with water tables falling annually over much of the country by between 1 and 3 metres.

The International Water Management Institute estimates that aquifer depletion and the cutbacks in irrigation it would cause could cost India a quarter of its grain harvest.

More than half of all Indian children are already malnourished and underweight.

Half the area available

Worldwatch says dwindling cropland threatens food security in countries like Nigeria, Ethiopia and Pakistan.

The amount of grainland available to each Nigerian is expected, as population grows, to fall from 0.15 hectares today to 0.07 ha by 2050.

In Pakistan the share will shrink from 0.08 ha to 0.03 ha, an area barely the size of a tennis court.

Less land has to feed more mouths

Countries which have already fallen to the 0.03 ha level, including Japan, South Korea and Taiwan, each import about 70% of their grain.

Worldwatch says one of the key ways to help slow population growth is to provide more help for reproductive health and family planning.

It says developing countries have largely honoured the commitments on increased spending which they made at the UN population conference in 1994.

But it says the industrial countries have reneged on theirs.

And late last year the US Congress withdrew all funding for the UN Population Fund.

Worldwatch also says the abolition of poor countries' debts could help to slow population growth.