PLAYA GRANDE, Costa Rica — This resort town was long known forLeatherback Sea Turtle National Park, nightly turtle beach tours and even a sea turtle museum. So Kaja Michelson, a Swedish tourist, arrived with high expectations. “Of course we’re hoping to see turtles — that is part of the appeal,” she said.
But haphazard development, in tandem with warmer temperatures and rising seas that many scientists link to global warming, have vastly diminished thePacific turtle population.
On a beach where dozens of turtles used to nest on a given night, scientists spied only 32 leatherbacks all of last year. With leatherbacks threatened with extinction, Playa Grande’s expansive turtle museum was abandoned three years ago and now sits amid a sea of weeds. And the beachside ticket booth for turtle tours was washed away by a high tide in September.
“We do not promote this as a turtle tourism destination anymore because we realize there are far too few turtles to please,” said Álvaro Fonseca, a park ranger.
Even before scientists found temperatures creeping upward over the past decade, sea turtles were threatened by beach development, drift net fishing and Costa Ricans’ penchant for eating turtle eggs, considered a delicacy here. But climate change may deal the fatal blow to an animal that has dwelled in the Pacific for 150 million years.
Sea turtles are sensitive to numerous effects of warming. They feed on reefs, which are dying in hotter, more acidic seas. They lay eggs on beaches that are being inundated by rising seas and more violent storm surges.
More uniquely, their gender is determined not by genes but by the egg’s temperature during development. Small rises in beach temperatures can result in all-female populations, obviously problematic for survival.
“The turtles are very good storytellers about the effect of climate change on coastal habitats,” said Carlos Drews, the regional marine species coordinator for the conservation group W.W.F. “The climate is changing so much faster than before, and these animals depend on so much for temperature.”
If the sand around the eggs hits 30 degrees Celsius (86 degrees Fahrenheit), the gender balance shifts to females, Mr. Drews said, and at about 32 degrees (89.6 Fahrenheit) they are all female. Above 34 (93), “you get boiled eggs,” he said.
On some nesting beaches, scientists are artificially cooling nests with shade or irrigation and trying to protect broader areas of coastal property from development to ensure that turtles have a place to nest as the seas rise.
In places like Playa Junquillal, an hour south of here, local youths are paid $2 a night to scoop up newly laid eggs and move them to a hatchery where they are shaded and irrigated to maintain a nest temperature of 29.7 degrees Celsius (85.4), which will yield both genders.
On a recent night, Dennis Gómez Jiménez, a 22-year-old in a red baseball cap and jeans, deftly excavated the nest of a three-foot-wide Olive Ridley, one of the smaller sea turtle species. The turtle had just finished the hourlong task of burying 100-plus eggs and then lumbered back into the water.
One by one, Mr. Jiménez placed what looked like table tennis balls into a plastic bag and transferred them to an ersatz nest he had dug in a shaded, fenced-off portion of sand that serves as a hatchery. Sandbags are positioned to protect against tides that could rip nests apart.
When the turtles hatch, in 40 to 60 days depending on the species, they are carried in wicker baskets to the ocean’s edge and make a beeline for the water. Gabriel Francia, a biologist who oversees the youths, known locally as the “baula” or leatherback boys, likens their work to delivering an endangered infant by Caesarean section.
“In some ways we’re playing God — this is a big experiment,” he said. The long-term hope, he said, is to build a robust turtle population that will slowly adapt by shifting to cooler, more northern beaches or laying eggs at cooler times of the year.
Worldwide, there are seven sea turtle species, and all are considered threatened. (Turtle populations in the Atlantic have increased over the last 20 years because of measures like bans on trapping turtles and selling their parts.)
The leatherback is considered critically endangered on a global level. Populations are especially depleted in the Pacific, where only 2,000 to 3,000 are estimated to survive today, down from around 90,000 two decades ago. Cooler sands alone will not save them, given the scope of the threats they face. At Playa Junquillal, markers placed a decade ago to mark a point 55 yards above the high tide line are now frequently underwater.
“It’s happened really fast — we have no rain, but water pouring in from the ocean,” said Adriana Miranda, 30, the manager of a local hangout that serves beer and rice and beans.
Beachside tables have been removed because rising tides have destroyed the restaurant’s concrete terrace and uprooted shading trees there. In different circumstances, the beaches could gradually extend backward as the sea level rose. But along much of Costa Rica’s Pacific coast, the back of the beach is now filled with hotels, restaurants and planted trees, giving the sand no place to go. “The squeezing of the beaches where turtles nest is going to be a big problem,” said Carl Safina, head of the Blue Ocean Institute, a conservation group.
In Playa Grande, the turtle issue has pitted environmentalists against developers and the national government. To ensure a future for the leatherbacks and the national park, biologists wanted a large section of land extending about 140 yards back from the current high-tide line protected from development. Beachfront property owners, many of them foreigners with vacation homes, demanded hefty compensation.
Arguing that the government cannot afford the payouts, President Óscar Arias has instead proposed protecting the first 55 yards, and allowing about 80 yards of somewhat regulated mixed-use development to the rear. But Costa Rica’s leading scientists have protestedthat the new boundaries will lead to “certain extinction.”
Turtles will not nest if there are lights behind the beach, Mr. Drews said, and those first 55 yards will be underwater by midcentury.
“Turtles will have to find their way between the tennis courts and swimming pools,” he said dryly.
In a country where turtle eggs are traditionally slurped in bars from a shot glass, uncooked and mixed with salsa and lemon, biologists are also promoting cultural change.
“Of course 25 years ago, you went out with your friends or family and dug up the eggs,” said Héctor García, 42, shopping at the Junquillal market. “It was a tradition. They are delicious, cooked or raw.”
Today egg collecting is illegal in Costa Rica, but poaching is still common in many towns. It is frowned on at Playa Junquillal, where the five baula boys, with their piercings and baseball caps, patrol for poachers and are idolized by many younger children. Dr. Francia, the biologist, has also invited local families to watch the babies being released. “There were a lot of people who had eaten eggs but never seen a turtle,” he said.
