VANCOUVER - When Roy Sasano told his parents he was getting sterilized a few years ago to reduce his carbon footprint, he remembers they weren't surprised.

After all, he had already taken other steps to fight climate change by becoming a vegan, “virtually” eliminating single-use plastic and reducing his consumption as much as is practicable.

“I think there was a brief moment of disappointment, but they just laughed it off,” said Sasano, 39, who lives in Vancouver.

“I know quite a few people, especially men, who have chosen to get sterilized and I know women who've done the same.”

Sasano is part of the Voluntary Human Extinction Movement or Vhemt (pronounced vehement) that believes in “refraining” from reproduction.

“Why should we be creating more people to create more suffering for ourselves and the rest of the planet?” Sasano said in a recent interview.

There are other examples that show the argument against human reproduction isn't happening on the fringes of the climate change debate anymore.

“There's scientific consensus that the lives of children are going to be very difficult. And it does lead young people to have a legitimate question: Is it OK to still have children?” Rep. Alexandria Ocasio-Cortez of the U.S. Congress recently asked in an Instagram video.

Katharine Hayhoe, an atmospheric scientist and professor of political science at Texas Tech University, said individual decisions matter in the fight against climate change as much as collective action on things like carbon pricing and the Paris agreement.

But while each additional child adds more carbon, she said children are also tied to a sense of hope.

“Personally, I can't advocate for people not to have children in order to fix climate change because why am I fixing climate change? I'm doing it because of my child and because of everybody else's children,” she said. “So that's what gives us hope. Again, I'm not having 12 children. I have one child.”

Hayhoe said the behaviour of people in developed countries like Canada is also a factor in climate change, noting that per capita, Canadians add much more carbon than Zambians.

“If the entire world were like Zambia we would not really have much of a problem,” she said. “If the entire world was filled with people with the carbon footprint of the average Canadian though, we would have a huge problem.”

Hayhoe said she knows people who've decided not to have children because of the environment.

“It's very hard to bring a child into this world but if we decide not to then there's no hope in the world,” she said. “It's not black and white. It's a very, very grey issue.”

Alistair Currie, head of campaigns and communications at the U.K.-based charity Population Matters, said choosing to have fewer or no children is essential to ensuring people have a “decent living” on the planet in 50 years' time.

It's hard to estimate how many people are choosing not to have children mainly because they feel judged, Currie said, although the perception is slowly changing.

“Particularly women,” explained Currie, whose organization campaigns to achieve a sustainable human population. “We get this all the time, women feeling judged somehow as being inadequate or selfish.”

The United Nations projects the world population will increase by more than one billion people within the next 15 years, reaching 8.5 billion in 2030, then increasing to 9.7 billion in 2050 and 11.2 billion by 2100.

Sixty per cent of the global population lives in Asia, 16 per cent in Africa, 10 per cent in Europe, nine per cent in Latin America and the Caribbean, and the remaining five per cent in Northern America and Oceania.

“We can keep feeding people in their billions for a long time,” said Currie, who has one son. “But we cannot do it without the planet staying healthy and sustaining its soils ... water and our forests.”

Currie said there are other factors that have an impact on climate, adding that an American produces about 160 times the amount of carbon as someone living in Niger. People in countries like India and China are also generating more carbon as they become more affluent.

“That sounds like a more concerning situation when you are looking at both sides ... each person producing more carbon and there being more people.”