HAVING large families should be frowned upon as an environmental misdemeanour in the same way as frequent long-haul flights, driving a 4x4 car and failing to reuse plastic bags, according to a report to be published tomorrow by a green think tank.

The paper by the Optimum Population Trust (OPT) will say that if couples had two children instead of three they could cut their family’s carbon dioxide output by the equivalent of 620 return flights a year between London and New York.

John Guillebaud, co-chairman of OPT and emeritus professor of family planning at University College London, said: “The effect on the planet of having one child less is an order of magnitude greater than all these other things we might do, such as switching off lights. An extra child is the equivalent of a lot of flights across the planet.

“The greatest thing anyone in Britain could do to help the future of the planet would be to have one less child.”

In his latest comments the academic says that when couples are planning a family they should be encouraged to think about the environmental consequences. “The decision to have children should be seen as a very big one and one that should take the environment into account,” he added.

Guillebaud says that, as a general guideline, couples should produce no more than two offspring.

The world’s population is expected to increase by 2.5 billion to 9.2 billion by 2050. Almost all the population growth will take place in developing countries. The population of developed nations is expected to remain unchanged and would have declined but for migration.

The British fertility rate is 1.7. The EU average is 1.5. In some countries, such as France, the government is so concerned it has introduced financial incentives for women to have more than two children.

Despite this, Guillebaud says rich countries should be the most concerned about family size as their children have higher per capita carbon dioxide emissions.

The suggestion has been criticised by family rights campaigners. Eileen McCloy, a geography graduate from Glasgow with 10 children, said: “How dare they suggest how many children we should have. Who do they think are going to look after our elderly?

“According to this I would have five couples’ quota of children. I believe my children will be productive members of society.”

For more on the environment, read the Eco-Worrier blog

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