SYDNEY, AUSTRALIA: Parents who have more than two children should be charged a lifelong climate change tax to offset the effect of their extra greenhouse gas emissions, an Australian medical expert has proposed.

They should pay AU$5,000 a head for each extra child and up to AU$800 every year thereafter, according to the plan published in the Medical Journal of Australia.

In contrast, contraceptives and sterilisation procedures would be eligible for carbon credits, suggested Professor Barry Walters at the King Edward Memorial Hospital in Perth.

"Every family choosing to have more than a defined number of children should be charged a carbon tax that would fund the planting of enough trees to offset the carbon cost generated by a new human being," he wrote.

Walters, an obstetrician, made his proposal in a letter in which he criticised the government's payment of a AU$4,000 "baby bonus" in a bid to boost the birth rate in this sparsely-populated country of 21 million people.

Paying parents extra for every baby fuelled more emissions and contributed to global warming, he said, adding that the bonus should be replaced with a "baby levy" in line with the "polluter pays" principle.

And Professor Garry Egger, director of the New South Wales Centre for Health Promotion and Research, agreed.

"Population remains crucial to all environmental considerations," he wrote. "The debate (around population control) needs to be reopened as part of a second ecological revolution."