When Claire Anterea landed on the Indonesian island of Bali in December 2007 for a meeting of the United Nations Framework Convention on Climate Change, she didn't expect to have any impact on the proceedings. "I'm nothing," she thought at the time. "I'm just Claire." She was there to represent one of the smallest nations on Earth: Kiribati, whose population of roughly 100,000 people is scattered over 32 coral atolls and one reef island in the central Pacific. Anterea, then 29, was just a humble nun who lived in a convent and didn't know much about climate science.

To her surprise, in the midst of the talks, Anterea found a reservoir of courage she didn't know she had. During one of the conference's events, she stood up in a crowded auditorium and said that Kiribati's atolls were only about 3 to 12 feet above sea level, and that their precarious position would likely make them more vulnerable to high tides and violent storms as the globe warmed. "You young people fighting for your future, please, I know you can also fight for my survival and the survival of my people," Anterea implored the audience of more than 1,000. Then she sat down, trembling with nerves, and listened in shock as applause rippled through the room. Journalists, some of them climbing over chairs, rushed to interview her: a woman from a faraway land whose story seemed to illustrate the immediate, physical threat that climate change poses to humanity.

Kiribati's then president, Anote Tong, later became an international celebrity for urging rich countries to reduce their greenhouse gas emissions or risk putting his country underwater. Tong's climate advocacy during his presidency helped many citizens in Kiribati (pronounced "kee-ree-bas") grasp the significance of climate change for the first time. 

For her part, Anterea admits that she isn't an expert on the data that scientists have collected showing links between rising global temperatures and what appear to be worsening floods. "All the science makes my head dizzy," she says. But she has spent many years speaking with villagers on Kiribati's atolls—many of whom complain that problems like flooding, erosion, unpredictable rainfall, and groundwater salinization are worsening—and she can tell that something is amiss. "Before, high tide would be another fun day—we'd go swim," Anterea says. "But now people say, 'When will the high tide be? I don't think we have enough sandbags.'"

Claire Anterea, a former Catholic nun, says her faith gives her the strength to prepare her people for climate chaos. | Photo by Mike Ives

Anterea first traveled extensively around Kiribati as a nun with the Sisters of the Good Samaritans, a Catholic order founded by Australian missionaries. Kiribati's self-described "Good Sams" live in a complex of light-green buildings that lie steps from the Pacific Ocean on the island of North Tarawa, which has a population of about 6,000 and looks more like the country's rural outer islands than the packed neighborhoods of South Tarawa, Kiribati's capital. The only sources of light on North Tarawa are kerosene lanterns and solar-powered LED lights. Most residents make their living by fishing for tuna or selling the coconuts they harvest from the many trees that line the shoreline by their thatched-roof villages.

In the early 2000s, Anterea traveled by boat, sometimes for days on end, from North Tarawa to islands that were even poorer and more isolated. In dozens of villages, the constant laments she heard about ecological threats to coastal communities made a deep impression on her—so deep that, after returning from the Bali conference in 2007, she decided to shed her nun's habit after a decade of monastic life and work on climate change in a secular capacity. Anterea, now 38, went on to cofound the Kiribati Climate Action Network (KiriCAN), the country's first climate change advocacy group.

Anterea, who has a bright smile and an easygoing manner, says her Catholic faith gave her strength to decide that someone from Kiribati—her, if necessary—needed to speak up more forcefully about the risks that the country's poorest people faced from climate-related changes. "Who will be a good Samaritan for my people? That's the call in the vocation that I fell in love with," she tells me. "And I realized that I can share that love outside the congregation—that's what makes me happy and proud that I left."

Recent floods have been unprecedented and alarming. The cost for small island states to adapt to climate change will be enormous. | Photo by Mike Ives

It would be difficult to overstate Kiribati's remoteness. To get there, I flew 10 hours from Hong Kong to Fiji, and from there another three hours northeast into open ocean until I arrived precisely in the middle of nowhere: about 3,800 miles from northeast Australia and nearly 5,000 miles southwest of California. As my Fiji Airways flight approached Tarawa, Kiribati's most populous atoll, a runway seemed to magically appear amid a vast expanse of crystal-blue water. The airport lies on the atoll's meatiest chunk of land, but even that is the size of a stadium parking lot—and, like the rest of the country, it is threatened by coastal erosion.

Despite having the kind of tropical beaches that hotel developers would kill for, Kiribati has no resorts and practically no tourists. Tarawa's airport, which has a brown-and-green exterior and exposed roof beams, is reminiscent of a Vermont dairy barn. The atoll has the country's only electricity grid, and its guest accommodations consist of some dilapidated motels scattered along Kiribati's only paved road, which runs for about 18 miles across South Tarawa. The road clings to narrow bands of coastline on both sides and cuts through dense, low-lying slums that have some of Asia's worst rates of unemployment and child mortality. 

The road also passes a few eerie landmarks—antiaircraft guns that the Japanese military built after seizing Tarawa during World War II. In 1943, U.S. troops retook the atoll in one of the bloodiest battles in the war's Pacific theater. Many of the guns were never scrapped, and they now bear silent witness to another kind of siege: Tidal surges, known as king tides, increasingly leap over Tarawa's flimsy seawalls, flood its homes, and frighten its residents.

Ieremia Tabai, Kiribati's founding president | Photo by Mike Ives

In almost any scenario imagined by scientists, climate change represents an existential threat to Kiribati's people. According to one of the most conservative estimates, if greenhouse gas emissions remain unchecked, the global sea level will rise about three feet by 2100. Some scientists warn of even greater sea level rise—as much as five to six feet by the end of the century. Such a rise, combined with increasingly unpredictable weather patterns and more intense tropical storms, could make it all but impossible to sustain human life in Kiribati. 

Yet even as scientists say that a "perfect storm" of environmental problems may render some of Kiribati's atolls uninhabitable, many in the deeply Christian nation bristle at the notion that they could be wiped off the map by what sounds to them like a modern version of a biblical cataclysm. Climate skepticism radiates all the way from subsistence fishermen to the political leadership. Teburoro Tito, a former president and devout Catholic whose political party, Embracing Kiribati, regained control of parliament in early 2016, says he is offended by scientists who predict what might happen to the country if climate models prove prescient. "I don't go that far, and I'm very strong on that, and to do otherwise I would have to disbelieve in my god," he tells me one March afternoon during an interview at Kiribati's parliament house on South Tarawa. "I would have to say that my god is so ugly and so unfair—putting us on these islands, and then one day, he will dump us into the ocean." With a preacher's cadence, he continues: "I don't believe that man is as bad as that."  

Religion is the social backbone of Kiribati. Missionaries brought Christianity to the islands in the mid-19th century, and a vast majority of the population identifies as Christian. The Catholic Church is the most popular denomination in the country, followed by the Protestant Kiribati Uniting Church, and then the Mormon Church. The dirt roads of Kiribati's skinny atolls are dotted with cement-block and corrugated-roof churches, and church leaders wield significant influence. Reluctance by many churchgoers to accept climate science is fueling an existential debate among the Kiribati people about which force—science, politics, or religion—exerts the most influence over human destiny.

Anterea is reluctant to discuss how climate change divides parliament's rival parties, which have traded control of the presidency a few times since the former British colony became independent in 1979. "That's politics," she says with a shrug. Yet as Anterea transitioned from her convent to climate activism, she grew into a role as an intermediary between local communities, their parliamentary and church leaders, and the president's office, as well as the foreign embassies and international aid groups that support Kiribati's nascent steps toward climate change adaptation. She learned that, in a country where parliament sits on reclaimed land at the edge of an ocean lagoon, climate politics are as inevitable as Pacific Ocean storms.

The threats of rising waters and weirder weather are intertwined with other issues that are already straining the country's ability to sustain human life. Kiribati's population has roughly tripled since 1960, and about half of its people now live on South Tarawa, a landmass of just 7.5 square miles. Scientists note that coastal damages are worsening at least partly because residents are building structures at the edges of coral atolls where waves deposit and withdraw sand, and whose coastlines are naturally prone to dramatic fluctuations.

"One of the big issues that has not gotten much debate here is that we have a fast-growing population," Ieremia Tabai, Kiribati's founding president, told me one afternoon, sitting cross-legged outside his home in a traditional, thatched-roof gazebo known as a maneaba. "It's the major one, in my view, and since Kiribati is a Catholic country, some people are sensitive to raising the matter. But it's a real issue: We can never hope to improve our standards unless we cut the rate we grow our population every year."

The most obvious example of this is the way in which a growing population is squeezing Kiribati's water supply. South Tarawa's primary water source has been polluted by wave overtopping and human waste from squatter settlements. Many residents now boil their well water, and some get their drinking water exclusively from rainwater tanks. Scientists say further population growth could stretch the island's water supply to a breaking point, perhaps forcing the government to import water.

Enter climate change. With significant sea level rise, Kiribati's freshwater supply will be even more imperiled. Its coastlines will also be increasingly vulnerable to extreme weather events. Water shipments and sophisticated seawalls could help mitigate those obstacles, but for how long? The cost for small island states to adapt to sea level rise, according to the Intergovernmental Panel on Climate Change, will likely be "enormous in relation to the size of their economies." The Tarawa atoll alone could lose an annual $210 million to $430 million in capital assets during storm-intensive periods by 2050, the World Bank reports—an astronomical sum for a country whose per-capita GDP is only $1,300.

Flooding events in recent years have been alarming and unprecedented, and changes in weather patterns are an even more urgent threat to Kiribati's survival, says Tong, who stepped down as president in March after three terms in office. "So either we push them away as something that will not happen again and be stupid, or we begin to get worried in light of the science that's coming forward," he tells me. "And I choose to be the wiser."

Anterea's 2007 speech put Kiribati's climate worries on the world's radar. President Tong then lifted the country's international profile higher. As the 2015 climate negotiations in Paris neared, he toured the world asking rich countries to cut their carbon emissions in order to save low-lying islands from ruin. A TED interview he gave just before the Paris conference, which received nearly a million views, was headlined "My Country Will Be Underwater Soon—Unless We Work Together." 

Back in Kiribati, Tong advanced his climate agenda through the $16.6 million Kiribati Adaptation Program, a World Bank-led initiative that has been seeking to climate-change-proof Kiribati's infrastructure. So far KAP has built seawalls, plugged leaks in Tarawa's water supply, and funded grassroots efforts by Anterea's KiriCAN to plant mangroves and install rainwater tanks on outer atolls.

But Tong has always had bolder plans for addressing climate change. In 2014, he convinced parliament to allocate $8.7 million to buy 5,500 acres in Fiji, creating what could become one of the world's first climate change refuges for humans. He also urged Kiribati's people to learn job skills, such as nursing and carpentry, that could give them an economic lifeline to Australia, New Zealand, or other countries if their homeland becomes uninhabitable—a concept he calls "migration with dignity."

"Do you understand what climate change means, what it means to lose a home?" Tong says when I ask how he decided on the phrase. "I had to find something to inspire our people, that at the end of the day, something highly positive could come out of it." 

The "migration with dignity" mantra has resonated with upwardly mobile Kiribati people who are eager to go abroad. But poor people in the outer islands, who aren't interested in leaving, have resisted it. While traveling the country for KiriCAN, Anterea talks with many impoverished Kiribati citizens, and they often complain to her about coastal erosion, salty wells, and rains that come either too often or not enough. But when she broaches the idea of migration, they look surprised. They ask, "How can we move to a new ocean that we don't know how to fish?"

Though Anterea was born and raised on an outer island, today she's not as economically vulnerable as many villagers are. Yet she, too, is anxious about the future. Her ramshackle home on South Tarawa is only a few hundred feet from the ocean, making it vulnerable to severe storms or floods. She also worries about the prospect of leaving her homeland if—or perhaps when—Kiribati can no longer sustain humans. After leaving the Good Sams convent, she married and had a child. She will do anything to keep her family safe. But she doesn't want to become a "second-class citizen" in Australia or another foreign land. While much of Kiribati is desperately poor, its people tend to look out for one another, she says. "I don't want to lose that . . . if we ever migrate."

On a languid Sunday morning, Anterea walks into Kiribati's main Catholic church and sits down to pray in a circular hall with large windows open to the Pacific. As harsh tropical sunlight pounds the church's roof, and beads of sweat pour off the congregation, the priest relates the biblical parable of the prodigal son. "Maybe we've experienced what this son has experienced: Life is bitter," he says, standing at a white altar in purple vestments, as Anterea and her neighbors nod in agreement. Then the priest urges the crowd to "be reconciled with God; it's the only thing that God is waiting for."

Reconciling Kiribati's religious leaders to climate change is another story. Some are actively hostile to Anterea's group because they view it as an attack on their faith, or a kind of Trojan horse for Tong's "migration with dignity" campaign. "If it's for climate change, don't bother to come to my island," some tell her when she asks if they would like to receive climate-adaptation assistance. Such resistance can prevent villages from accepting rainwater tanks or mangrove seedlings that could help fight coastal erosion and groundwater salinization.

Faith-based resistance to climate change is not limited to Kiribati's remote atolls. On the northern end of Tarawa, the nuns at Anterea's former convent have struggled to win over skeptics. Tibwau Matia, a nun there, tells me that recurrent flooding near the convent has prompted some villagers to conclude that the ocean is rising. But others disagree, quoting God's promise to Noah, in the book of Genesis, to never again create a "flood to destroy the earth."

"We'll stick with His promise," they tell Matia. "We believe in Him, and we trust Him."

But such attitudes may be fading. A growing number of Catholic, Protestant, and Mormon leaders across Kiribati have partnered with KiriCAN since it started in 2009, Anterea says—some of them inspired by Pope Francis's calls for action on climate change. They also see migration with dignity as a means of economic advancement rather than an insult to their faith.

Titau Davita, the president of Kiribati's Seventh-day Adventist Church and a KiriCAN supporter, sees no conflict between climate science, natural disasters, and the holy scripture. "The Bible is very clear," he tells me one evening in his home. "It mentions most of these things, like earthquakes and pestilence, and right now this is what we are seeing. It seems like climate change is going along with what the Bible said."

After a tea-and-toast breakfast in the church's courtyard, Anterea and her three-year-old daughter, Baneua Tangaroa, stroll down a grassy path toward a windy, ocean-side cemetery. Baneua is in a buoyant mood, and Anterea smiles as the girl scampers among the weathered gravestones. Then, after she tires out, Baneua sits with her mother at the base of a stubbly tree near the water's edge. Shallow waves slap harmlessly against the coral reefs that lie a couple of hundred feet from shore. 

On a crowded island with so little land, this is among the few places where Anterea can be alone. Her grandfather is buried here, alongside 19th-century missionaries, and she pays her respects to all of them every All Souls' Day. "We pray for them and also ask them to pray for us," she says.

Church members say that this cemetery, like most coastal properties on the atoll, is fighting against alarmingly swift erosion. A seawall that church volunteers threw up at the cemetery's edge is keeping the ocean at bay for now. "But how many more times will we keep on building this seawall?" Anterea asks. She lets the question flutter in the offshore breeze, and no one answers it.

Reporting for this article was financed by the Access to Energy Journalism Fellowship and Discourse Media.

This story was funded by the Sierra Club's International Climate and Energy Program (sierraclub.org/international).