Sir David Attenborough, Jonathon Porritt, Jeremy Irons and other "doomsters" are talking "dangerous nonsense" about the threat of overpopulation, according to the environmental writer Fred Pearce.

Speaking at the Hay festival, Pearce said that the global population "timebomb" was being defused by some of the world's poorest women, thanks to the impact of feminism in Muslim, Christian, secular, developed and developing countries across the world.

Irons, the actor who recently told the Sunday Times "there are just too many of us", and other environmentalists such as Attenborough and Porritt, who are patrons of the Optimum Population Trust, are modern-day Malthusians, according to Pearce, who fail to see that overconsumption, not overpopulation, is what really imperils the planet.

"I believe they are talking dangerous nonsense. The population timebomb is being defused, fast," Pearce told the festival.

The United Nations has predicted the world's population could rise from 6.8bn to 9.2bn in 2050, but, according to Pearce, it will only rise by up to 2bn and will then start to fall.

Half the world now has fertility rates below the replacement rate of 2.3 children. Women in Iran were giving birth to eight children in the 1980s, but now give birth to less than two. In Bangladesh, where many mothers are poor and badly educated, women have an average of just three children. Birth rates have fallen to 2.8 in India and two in Brazil, despite the influence of Catholicism.

In China, "the one-child policy is brutal and repressive but it may not be making much difference anymore", said Pearce, citing similarly low birth rates in Taiwan, Hong Kong and Singapore, where the government has been paying couples to have children for two decades.

Only in poverty-stricken parts of Africa, highly patriarchal societies such as Yemen and among "traumatised" people such as the Palestinians and Orthodox Jews in Israel, are birth rates soaring.

Pearce said he had not set out to write "a feminist tract" when researching his book, Peoplequake: Mass Migration, Ageing Nations and the Coming Population Crash, but discovered in all these societies the key factor was women being able to take decisions for themselves. Given this power, if child mortality rates were such that they no longer needed five or six children for two or three to survive, women were choosing to have fewer children and "get on with their lives".

With the time lag between fertility rates and population, Pearce said the population could still rise by 2bn but would then fall back as a smaller future generation of mothers created a "feedback loop" of smaller populations.

While this was "a good news story", Pearce warned that the environmental threat to the world was not over.

"We haven't begun to defuse the consumption boom," he said, arguing that environmentalists should focus not on restricting population growth in the developing world but on curing the developed world of its addiction to consumption.

The richest 500 million people (7% of the world's population) are responsible for 50% of CO2 emissions, while the poorest 3.5bn people (50% of the population) are responsible for just 7% of emissions. One American's emissions are equivalent to those of 250 Ethiopians.

Women having the right to decide how many children they bear may be the key to stabilising the world's population, but Pearce said that in societies where the feminist revolution was incomplete and mothers were still expected to perform the lion's share of child-rearing duties, many women were rebelling against childbirth.

This trend was particularly strong in southern Europe. If Italy continues with its current birth rate, its population will fall by 86% by the end of this century.

Mass migration and an ageing population would be challenges for the future, but Pearce speculated that an older population could be "wiser and greener", living life at a slower pace with less desire for high-speed consumption.

He predicted that by the end of his lifetime, people would be more worried by a population crash than a population explosion.