Before she married her husband, Kiersten Little considered him ideal father material. “We were always under the mentality of, ‘Oh yeah, when you get married, you have kids,” she said. “It was this expected thing.”

Expected, that is, until the couple took an eight-month road trip after Ms. Little got her master’s degree in public health at the University of North Carolina in Chapel Hill, N.C.

“When we were out west — California, Oregon, Washington, Idaho — we were driving through areas where the whole forest was dead, trees knocked over,” Ms. Little said. “We went through southern Louisiana, which was hit by two hurricanes last year, and whole towns were leveled, with massive trees pulled up by their roots.”

Now 30 and two years into her marriage, Ms. Little feels “the burden of knowledge,” she said. The couple sees mounting disaster when reading the latest climate change reports and Arctic ice forums. Anxiety about having children has set in.

“Over the last year I thought, ‘Oh my God, I have to make a decision, it’s not that far away,” she said. “But I don’t know how I could change my mind. Over the next 10 years, I feel like there are only going to be more reasons to not want to have a kid, not the other way around.”

Such fears are not necessarily unfounded. Every new human comes with a carbon footprint.

In a note to investors this past summer, Morgan Stanley analysts concluded 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.”

There is much debate, however, over the idea that having fewer children is the best way to address the problem. In an interview with Vox in April, Kimberly Nicholas, a climate scientist and co-author of a 2017 study of the most effective lifestyle changes to reduce climate impact, said that population reduction is not the answer.

“It is true that more people will consume more resources and cause more greenhouse gas emissions,” Ms. Nicholas said. “But that’s not really the relevant time frame for actually stabilizing the climate, given that we have this decade to cut emissions in half.”

Nevertheless, the concern seems to be gaining traction. Among childless adults in the United States surveyed by Morning Consult last year, one in four cited climate change as a factor in why they do not currently have children.

Another poll in 2018 by Morning Consult for The New York Times found that among young adults in the United States who said they had or expected to have fewer children than the number they considered ideal, 33 percent listed climate change, and 27 percent named population growth as a concern.

While economic concerns remained paramount, with 64 percent citing the high cost of child care, 37 percent cited global instability and 36 percent, domestic politics. To some, those issues are all rolled together. In 2020, the birthrate in the United States declined for the sixth straight year, a dip of four percent believed to be accelerated by the pandemic.

The trauma from nearly two years of coronavirus has also given some prospective parents pause. For Marguerite Middaugh, a 41-year-old lawyer in San Diego, Calif., the pandemic, coupled with climate-related devastation, prompted her to hold off on fertility treatments for a first child. “Seeing people not getting vaccinated, not taking care of their community,” she said. “That really made me pause about whether I want to bring a child into this world.”

While spiraling housing costs, college-debt burdens, not to mention the so-called sex recession for millennials (the oldest of whom are now 40) factor into family planning for many, existential threats, too, are now part of the procreation calculus.

A rise in political extremism, at home and abroad. A pandemic that has killed more than five million. Thousand-year floods that wiped out Western Europe towns. West coast wildfires that grow more unimaginable in scale each summer. Faced with such alarming news, some prospective parents wonder: How harmful might it be to bring a child into this (literal and figurative) environment?

To Jenna Ross, 36, a potter who lives near Fredericton in New Brunswick, Canada, her decision to remain childless in a world threatened by climate change springs from a protective instinct. “Harnessing the love I have for my unborn hypothetical kid comforts me in sparing them an inhospitable future,” she said. “In this way, my choice feels like an act of love.”

Such views do not always travel across lines of geography, politics or social class —particularly since climate change is often painted as a partisan, not scientific, issue in the political arena. In the 2018 New York Times survey, the people who cited climate change as a reason to have fewer children were significantly more likely to be college-educated and Democrats, and slightly more likely to be white, non-religious and high-earners.

Educated professionals also have greater access to abortion and birth control, and the economic means to choose either lifestyle course, although recent restrictions on abortion in Texas, for example, also complicate the procreation calculus.

Regardless, such questions are creeping into the cultural dialogue in a manner that recalls the hippie-era “ecology” movement, when “The Population Bomb,” the seismic 1968 best seller by the Stanford University biologist Paul R. Ehrlich, predicted a barren, exhausted planet where hundreds of millions would die in famines during the 1970s.

Senator Bernie Sanders and Representative Alexandria Ocasio-Cortez both have broached the question in recent years, with Ms. Ocasio-Cortez asserting “a scientific consensus that the lives of children are going to be very difficult,” in a 2019 Instagram Live, which leads “young people to have a legitimate question: Is it OK to still have children?’”

Celebrities have also raised the issue. “Until I feel like my kid would live on an earth with fish in the water,” Miley Cyrus told Elle magazine two years ago, “I’m not bringing in another person to deal with that.”

In an interview with Howard Stern in May, Seth Rogen discussed his decision with his wife to remain childless by choice: “There’s enough kids out there. We need more people? Who looks at the planet right now and thinks, ‘You know what we need right now?”

Writers such as The New York Times columnist Paul Krugman and Katha Pollitt, the poet and essayist, have also chimed in recently.

“Does the world need more people?” Ms. Pollitt wrote in an essay in The Nation this past June. “Not if you ask the glaciers, the rain forests, the air, or the more than 37,400 species on the verge of extinction thanks to the relentless expansion of human beings into every corner and cranny of our overheated planet.”

While climate change is hardly a new concern, the worsening crisis has forced the issue for many prospective parents, said Josephine Ferorelli, a founder of Conceivable Future, an organization that hosts house parties for prospective parents to discuss how climate fears are shaping their reproductive lives.

“Something happened this past summer,” Ms. Ferorelli said. “Three months ago, our inbox was empty. But in the past two months, we’ve been hearing from people all over the country who are upset and distraught.”

No wonder some people who put off having children to pursue careers or other interests now wonder if the kindest thing for their unborn is to keep them that way.

“I literally can’t go to a dinner party without the collapse of a civilization being at least mentioned, if not being the main topic of conversation,” said Myka McLaughlin, 40, who runs a company in Boulder, Colo., that helps women build profitable businesses. “Arable land is decreasing around the planet. We might not have enough food. We’ve lost 80 percent of the biomass in the ocean in the last century; the ocean is essentially dying.”

Since college, Ms. McLaughlin has worried that humankind was on an unsustainable path. Even so, “at 27, I decided to have children and get married, in that order,” she said. Her first marriage, however, ended without children at 32. “He was a salt-of-the-earth farmer who wanted to live in the mountains,” she said. “I was a global citizen who wanted to travel and read The New Yorker.”

By the time she entered a serious relationship in her late 30s, she was having grave doubts about bringing children into a troubled world. “His perspective was, we really need children who are well raised and well loved who can be leaders in our future for what is to come, which I think is a totally valid point,” Ms. McLaughlin said. She, however, now struggles to justify bringing a child into a world she fears may be on the brink. The couple broke up this summer.

“When I see a beautiful young baby, my heart melts, just totally melts,” Ms. McLaughlin said. But at this point it might take a major life epiphany to change her mind.

Political strife, both domestically and abroad, is also a factor for some.

“With my past partner, we both decided that if Trump got re-elected in 2020, we were not going to engage in having children, primarily because the climate would be irreparable and probably extremely devastating,” said Hannah Evans, 33, a senior analyst for Population Connection, formerly Zero Population Growth, the prominent population-stabilization organization that Dr. Ehrlich helped found in the 1960s. .

Like many professional women, Ms. Middaugh, the lawyer in San Diego, put off having children through her 20s and 30s as she built her career and grappled with student loans. When she was around 36, however, she decided it was time to act.

She and her husband had a difficult time conceiving, so they began fertility treatments shortly before the pandemic, when she was 39. Then came the lockdown. The fertility clinic closed for months. She had time to consider the world her child might face.

Not only was she horrified by the lack of a unified response to coronavirus, but on a trip home to Alaska, she visited beaches that, as a child, she remembered teeming with starfish, otters, and fat, briny mussels, but now seemed denuded of wildlife.

It was too much. She postponed her plans for IVF, although her husband is still pushing. “I’ve been asking myself, ‘If I don’t have a baby or at least really try, will I be sad or regret it forever?’” she said. “I don’t know, and can’t really know, but I don’t want to let that alone push me into making a choice to go ahead with trying.”

Such questions are not confined to heterosexual women in monogamous relationships. Single women, husbands, gay couples, as well as people found on any point of the gender spectrum, all have the choice to procreate — and the choice not to.

“As I think of it, having a child is like rolling dice with the child’s life in an increasingly uncertain world,” said Michael Ellsberg, 44, a writer in Berkeley, Calif. “Sure we might figure out how to limit global warming to 1.5 or 2 degrees Celsius. We might figure out how to cooperate as a globe to prevent future pandemics. We might figure out how to limit the risks of nuclear war and terrorism. But we might not.”

Mr. Ellsberg follows “The Exploring Antinatalism Podcast” and other such outlets, and after two breakups in which his desire to not have children was a major factor, he had a vasectomy to cement his decision.

Doomsday fears are hardly the only reason that some choose the child-free lifestyle.

“I was raised in a family that did not try to condition me as a girl-mommy-to-be,” said LiLi Roquelin, 41, a married, French-born singer-songwriter who lives in Queens. She counts herself a proud member of the so-called childfree by choice movement, celebrated on social media under hashtags like #childfree and #neverkids, and recently posted a self-penned anthem of sorts called “Childfree.”

Even so, she said that women who choose not to reproduce often face intense social pressure from family, friends, even medical professionals. “Over the years, I have been criticized as inhuman or unloving,” she said. “In my mid-30s, my gynecologist kept telling me that my hormones would run out.”

For her, such pushback is merely the price you pay. Ms. Roquelin said she enjoys a rich and fulfilling life without children, and is now studying for a master’s degree in business administration to capitalize on her music career. “I have many more things to explore on my journey,” she said, “that do not involve raising other suffering human beings on an out-of-supplies planet.”