Fred Conning pulled up Google Earth on his phone, and zoomed in on the coastline that had been the backdrop of his childhood in a small village in the Solomon Islands.

Here was the beach of white sands where he would splash about and while away the days. Here was the point along the shoreline where banana trees stood. Here was a sand dune.

Now, it’s all sea.

“For us on the coast, the indication that things are changing is obvious,” said Mr. Conning, a 55-year-old engineer who now lives in the capital, Honiara.

On the other side of town, a few miles down the dusty, traffic-choked main road, the threat of climate change was high on the agenda at the annual gathering of the leaders of 18 Pacific island nations and territories. They conferred on ideas about what could be done to address a crisis they are most at the mercy of, and yet least empowered to ward off.

In recent years, these meetings of the Pacific Islands Forum, the region’s top intergovernmental body, have witnessed splashy announcements and pledges from the United States on support for climate adaptation and disaster preparedness. Washington was re-engaging with the region after years of disregard, just as China was expanding its interests there.

But many of those U.S. promises have been scrapped or hang in limbo. The Trump administration abruptly shut down the U.S. Agency for International Development, pulled back on a host of climate policies, including exiting the Paris Agreement, and gutted other aid and development programs.

That about face stings more in a part of the world that years ago described climate change as its “single greatest threat.” In private, one senior Pacific official called the fickle U.S. engagement in the region the “yo-yo” policy.

Prime Minister Feleti Teo of Tuvalu, a nation of fewer than 10,000 people that is expected to be among the first to be fully engulfed by rising sea levels, expressed that dismay at the forum. He said the U.S. policy shifts would undoubtedly undermine the international agreements that matter most to the world’s smallest nations.

China, on the other hand, has consistently been investing in relationships in the region and has taken the opportunity to start painting itself as an alternative — and more dependable — partner for climate projects.

In May, China hosted the foreign ministers of 11 Pacific island nations that it has diplomatic relationships with and announced an initiative to deepen cooperation with the region on combating climate change, saying it would fund 100 “small and beautiful” climate projects in the region in the next three years.

Asked for a list of those projects, China’s embassy in Honiara pointed only to one agreement signed in August to provide mini solar power equipment to remote areas of the Solomon Islands. The U.S. embassy did not respond to a request for comment.

“There’s a gap that’s been left with the withdrawal of the U.S., and China’s trying to fill that gap,” said Sindra Sharma, a climate economist and the international policy lead for the Pacific Islands Climate Action Network.

Geopolitical jostling has often overshadowed existential conversations for the region at the Pacific Islands Forum.

In 2012, the secretary of state at the time, Hillary Clinton, landed in the Cook Islands in a rare high-level U.S. visit to the forum. In a move widely viewed as a counter to Chinese influence, she promised assistance on an array of issues including climate change, promising America was in it “for the long haul.”

Last year’s gathering, hosted by Tonga, ended with a kerfuffle over language initially included in a communiqué that reaffirmed the region’s relations with Taiwan. That prompted an angry response from China, which claims Taiwan as its territory, and the reference was removed from the final version. Only three of the forum’s members still maintain diplomatic ties with Taiwan.

This year, after some back and forth about whether Taiwan should be invited, Prime Minister Jeremiah Manele of the Solomon Islands asked all external partners — including China, Taiwan and the United States — not to come. The move prompted criticism from Australia and New Zealand, which are members.

Without the major powers interloping, the nations spent this week discussing ideas on how the region could take matters into its own hands: a homegrown fund to finance climate projects without the bureaucratic hurdles of existing funds; a declaration to stand against militarization in the region; and ways to build on the historic ruling at an international court holding nations to account for failing to act on climate change, in a case brought by a member, Vanuatu.

Still, the Pacific island nations, which are among the world’s poorest and most underdeveloped, need deep-pocketed support to face the calamity of climate change. They recognize that donors and partners’ attention and funds are being sucked into conflicts far away from their vulnerable shores.

“We need partners in this region, big and small,” said Finau Soqo, the general manager of the Pacific Resilience Facility, the regional climate fund that was officially established on Wednesday. “Our small island states are really right there on the front line.”

The United States has provided $5 million in seed money for the fund, and last year it pledged an additional $20 million — the fate of which hangs in the balance. But Pacific island nations have no choice but to forge ahead, looking for solutions and support wherever they can, Ms. Soqo said.

“If it’s not Ukraine, if it’s not the Trump administration, it’s going to be something,” she said. “We can’t be deterred by that.”

Ralph Regenvanu, Vanuatu’s minister for climate change, said the abrupt shutdown of U.S. programs had an immediate effect on projects that had been underway in the region. In one U.S.A.I.D.-funded project in one of Vanuatu’s most remote islands, water tanks and pipes had been taken on ships, on trucks and on workers’ backs — only to be stranded after the agency was shuttered, he said. Australia eventually stepped in with the funding to pick up the pieces, he said.

But in terms of global leadership on climate issues, the region already knows Washington cannot be counted on, he said.

“The U.S. has always been out of the picture anyway in terms of ambition,” Mr. Regenvanu said.

Mr. Conning, the engineer, has worked for years on the largest renewable energy project in the Solomon Islands, a hydroelectric power plant. Even so, he said he was not holding his breath for global super powers to swoop in and help gird the country against the effects of climate change.

He has been taking matters into his own hands — hiring and teaching young people from his native village, Hulavu, to build a jetty on the beach that was his childhood playground, on his own dime. As a result, the white sand has been slowly coming back, he said, beaming as he showed off photos. He is hoping to build up the area, devastated by logging, as a tourist destination, providing jobs for young people in a country with few options.

As for the international bigwigs, he said, they will always serve their immediate interests first and foremost, flitting in and out of the Solomons as the geopolitical winds blow.

“If they come, they come,” he said.