HOGA ISLAND, Indonesia — He sat shirtless on his thin bamboo floor in a home built on posts rising out of the Banda Sea.
Tadi had just returned in his dugout canoe from scanning crevices in a nearby reef for octopus. He and his neighbors spend every day this way, scouring the ocean for something to eat or sell. Fishing, here, is about survival.
Their stilt village has no industry, no land, no running water. They dive without oxygen, wearing hand-carved wooden goggles, and carry spear guns hacked from logs with their machetes. They eat what they catch and sell the rest, using the money to buy everything else they need: boat fuel, root vegetables, rice, wood.
Without fishing, "how would I feed my family?" asked Tadi, who like many Indonesians has only one name.
Now Tadi's community, like countless others across the globe, is on a collision course with the industrialized world's fossil-fuel emissions.
Hundreds of millions of people around the world rely on marine life susceptible to warming temperatures and ocean acidification, the souring of seas from carbon dioxide emitted by burning coal, oil and natural gas. That includes Northwest oyster growers and crabbers in the frigid Bering Sea, who now face great uncertainty from shifts in marine chemistry.
But from Africa to Alaska, many coastal communities face a substantially greater risk. These cultures are so thoroughly dependent on marine life threatened by CO2 that a growing body of research suggests their children or grandchildren could struggle to find enough food.
The science of deciphering precisely who might see seafood shortages remains embryonic.
But with many of the most at-risk coastal communities already facing poverty, marine pollution, overfishing and rising seas, the potential for calamity is high.
"I can't tell you how many people will be affected," said Sarah Cooley, at Woods Hole Oceanographic Institution in Massachusetts, who studies links between acidification and food security. "But it's going to be a very big number."
Said Andreas Andersson, an acidification and coral reef expert with the Scripps Institution of Oceanography in San Diego: "These people are literally going to be fighting for their lives."
Among the most vulnerable to changing ocean conditions are people like Tadi and his 1,600 fellow villagers, even if they don't quite view it that way yet.
From his elevated perch sheltered from the midday sun, Tadi could see huts with wispy thatch roofs connected by the rickety planks of his village boardwalk. Everything stood a dozen feet above emerald waters.
Like many in his village, he's uncertain of his age. But for as long as he can remember, Tadi has netted, trapped, hooked or speared grouper, snapper, wrasses and parrotfish. Sometimes the men in his village disappear to the open sea for days to chase small tuna.
Among his peers, Tadi is considered one of the best spear fishermen. And no wonder: He said he stabbed his first fish when he was barely older than a toddler.
That childhood in the ocean left an impression. Every animal seemed huge. Sea life teemed wherever he looked.
"I could choose with my spear whatever fish I want," Tadi said through a translator. "I never caught any small fish."
The Sama people, or Bajau, are known as sea gypsies or sea nomads because they once lived mostly on boats. They roamed Southeast Asia between Malaysia, the Philippines and Indonesia, living off the sea, until governments began encouraging them to settle. Tadi's offshore village was built in the late 1950s.
Today, up to a million sea nomads are sprinkled throughout the Coral Triangle. Their lives are a blend of old ways and new.
Some in their village adopted the destructive practices of Asia's worst fishing fleets. A few here fish with coral-destroying bombs or cyanide. Some from the village and nearby islands gather colorful fish for sale to the aquarium trade. Commercial trawlers from elsewhere cause more damage.
A coming storm from CO2 will only make things worse.
Scientists navigate complex ways carbon dioxide can alter the marine world. Some impacts are clear.
Rising temperatures already wreak havoc on corals. Warming waters can cause corals to eject the algae that give them their vibrant color, weakening or killing reefs and turning them white. This process is known as bleaching. Without reductions in global emissions, 90 percent of reefs by midcentury are projected to see severe bleaching episodes every year.
Ocean acidification will compound the problem.A quarter of the CO2 spewed by cars and power plants winds up in the ocean. That lowers the pH, makes waters more corrosive and reduces carbonate ions, which then makes it harder for marine creatures to build their shells and skeletons.