• Analysts at Morgan Stanley said in a note to investors last month that the "movement to not have children owing to fears over climate change is growing and impacting fertility rates quicker than any preceding trend in the field of fertility decline."
• Some people are choosing not to have children because they fear that that doing so will amplify global warming while others are concerned about extreme weather events their children may have to endure and the knock-on effects.
• "Over the last few years, the climate has definitely been a major contributor to us not wanting children," Daniel told CNBC, requesting that his surname be left out of the story over fears that he may be targeted online by people who disagree with him.

A growing number of people are reluctant to bring a child into a world that's set to be ravaged by climate change in the coming decades.

This week, the United Nations issued a "code red for humanity" as the world's leading climate scientists delivered their starkest warning yet about the deepening climate emergency. The Intergovernmental Panel on Climate Change's report on Monday said global temperatures are likely to rise by 1.5 degrees Celsius in the next two decades, exceeding a key target of the Paris Agreement — a landmark accord considered critically important to reduce the risk of a climate catastrophe.

Scientists' increasingly bleak outlook for the future of the planet is putting more and more people off having children.

Analysts at Morgan Stanley said in a note to investors last month that the "movement to not have children owing to fears over climate change is growing and impacting fertility rates quicker than any preceding trend in the field of fertility decline."

To support their argument, they pointed to surveys, academic research and Google data that shows climate change is directly and indirectly accelerating the decline in fertility rates. UCLA researchers showed that the number of births in the U.S. fell in the nine months after an extreme heat event while a study of 18,000 couples in China last year showed that climate change, and particulate pollution in particular, was associated with a 20% increased likelihood of infertility.

Some people are choosing not to have children because they fear that that doing so will amplify global warming.

"Having a child is 7-times worse for the climate in CO2 emissions annually than the next 10 most discussed mitigants that individuals can do," analysts at Morgan Stanley said.

A Swedish study, published in IOPscience in 2017, found that having one fewer child per family could save approximately 58.6 metric tons of carbon each year in developed countries.

However, Kimberley Nicholas, one of the study's authors, said in an interview with Vox this year that reducing the population is not the way to solve the climate crisis. "It is true that more people will consume more resources and cause more greenhouse gas emissions," she said. "But that's not really the relevant timeframe for actually stabilizing the climate, given that we have this decade to cut emissions in half."

Enduring extreme weather

Others are concerned about extreme weather events their children may have to endure and the likely knock-on-effects. Crops could fail in some parts of the world, for example.

Daniel, a 35-year-old Brit who currently lives in Dubai, United Arab Emirates, has been married to his partner for almost 12 years. They were open to the idea of having children earlier on in their relationship but now they're less keen.

"Over the last few years, the climate has definitely been a major contributor to us not wanting children," Daniel told CNBC, requesting that his surname be left out of the story over fears that he may be targeted online by people who disagree with him.

The couple, who rely on air conditioning most of the year and like to travel, have been looking for ways to significantly offset their carbon footprint. "We thought about it quite a lot and quickly realized that adding another human being to the world would have a huge environmental impact," Daniel said.

Prince Harry said in 2019 that he and his wife Meghan were planning to have a maximum of two children, citing environmental concerns.

The issue of bringing more people into a warming world is being discussed by people on social media with big followings.

In a 2019 Instagram live stream to her 1.5 million followers, 31-year-old New York Rep. Alexandria Ocasio-Cortez said: "Basically, there's a scientific consensus that the lives of children are going to be very difficult. And it does lead, I think, young people to have a legitimate question: Is it okay to still have children?"

'Existential angst'

Jessica Combes, a 39-year-old English teacher, told CNBC: "I refuse to bring children into the burning hellscape we call a planet."

Combes said she has always been unsure about having children of her own. "Now, as I look at the state of the economy, shoddy global healthcare and climate change, I just feel like all my trepidation was well justified," she said. 

Some of those who already have children are also worried. Thom James, 39, a managing partner at advertising and public relations firm Havas U.K., told CNBC: "I had a major depressive episode last year based on existential angst over the world my children would be growing up in."

James has two girls aged three and six. "Worrying about their future is a frequent trigger for me," he said. "I'm constantly thinking about when it's going to be appropriate to dissuade them from having children of their own, as I think we're really past the point of no return."

Of course, if everyone stopped having children then humanity would eventually cease to exist. A fringe group of anti-natalists believe that's exactly what should happen, but most people don't share this view.

Indeed, many people see having children as a fundamental human right and one that can bring happiness and joy to families.

However, the climate emergency is a result of an increase in greenhouse gas emissions from burning fossil fuels, not population growth.

The IPCC's report warned that some of the climate changes researchers observed — such as continued sea level rise — were projected to be "irreversible over hundreds to thousands of years."

The report also reaffirmed the urgent need for "strong and sustained" reductions of carbon emissions and other greenhouses gases to limit climate change.

U.N. Secretary-General, António Guterres said the findings were "a code red for humanity."

He added: "This report must sound a death knell for coal and fossil fuels, before they destroy our planet."

At present, even as policymakers publicly acknowledge the necessity of transitioning to a low-carbon society, the world's dependency on fossil fuels is expected to get even worse in the coming decades.

— CNBC's Sam Meredith contributed to this report.