Every three days, researchers from the Caribbean Conservation Corporation walk the beaches here counting sea turtle nests. Four species nest in its black sands – leatherback, green, loggerhead, and hawksbill – each at a different time of year, at a different area on the beach, and at a different depth in the sand. The researchers record whether the female laid eggs or just poked around and, unsatisfied, returned to the sea. And they note if humans, the greatest threat to these ancient mariners, have dug up any eggs.
Sea turtles have navigated the world’s oceans for some 200 million years. They saw the dinosaurs come, and they saw them go. But of the seven species of sea turtles, six are endangered. While conservation efforts have led to some rebound in the Atlantic and Caribbean, the picture is much grimmer in the Pacific. Last year, 46,784 green turtles visited an 11-mile stretch of Tortuguero that researchers have monitored since 1971 – a 450 percent increase. But between 1980 and 2002, female leatherbacks nesting along the west coast of the Americas dropped from an estimated 91,000 to less than 2,000. [Editor's note: The original version of this story misstated the number of green sea turtles at Tortuguero National Park. The figure quoted refers only to an 11-mile stretch of beach researchers have been monitoring since 1971, not the entire beach at the park.]
Scientists worry that climate change, on top of ongoing stresses, could deal the final blow to these creatures. Ongoing threats include habitat loss, poaching, and being caught in nets and on hooks intended for other prey. Now, in a warming world, scientists also foresee stronger storms increasing erosion of the sandy beaches the turtles use to nest. Rising seas will inundate existing beaches, even as human development halts natural beach migration inland and upland. Shifting currents may alter the ocean's upwelling patterns, which turtles depend on for food.
Finally, there's global warming's most direct effect – more heat.
Turtles lack sex chromosomes. Their genes do not directly determine whether a hatchling comes out male or female. Instead, buried eggs take their cue from the ambient temperature. For leatherbacks, temperatures below 29.4 degrees C (85 degrees F.) produce a clutch that is mostly male; above that, it's mostly female. With a mere 2 degree C (3.6 degrees F.) increase, a nest will produce all females. A few degrees higher yet, and the "boiled" eggs don't hatch at all.
In order to maintain a viable breeding population, a cool, male-producing year has to come at least once every five to 10 years, says James Spotila, a professor of environmental science at Drexel University in Philadelphia. If male years begin to come only every 20 years because of climate change, the turtle could become extinct.
Beaches at Playa Grande on Costa Rica's Pacific coast are already producing clutches that are 70 to 90 percent female, depending on the year, Dr. Spotila says. At Junquillal (Hoon-ki-YAL) Beach on the Pacific coast, where it is often too hot for eggs to hatch at all, scientists have begun moving eggs to nurseries – essentially holes dug to a certain depth on cooler areas of the beach. When the hatchlings emerge, rangers chaperone them from the nest to the water, protecting them against human and animal predators alike.
But many scientists don't see direct human meddling in nature as a viable long-term solution.
"I think it will be inescapable that people will to try to work around these problems," says ecologist Carl Safina, author of "Voyage of the Turtle." "But constant intervention as a way of keeping a wildlife population in existence strikes me as probably a losing battle.
"If the expectation becomes that we will simply adjust to [climate change], ... I think we're just asking for an overwhelming slew of problems," he adds.
In pristine conditions, sea turtles might be able to adapt. Females lay several clutches of eggs each nesting season, on different areas of the same beach or different beaches entirely. Some scientists speculate that by dispersing her eggs, a mother turtle increases the chances of some eggs hatching.
Indeed, for years, puzzled scientists have observed leatherbacks mysteriously nesting in southern Baja California where temperatures weren't quite warm enough for incubation. The effort seemed a waste. But in the context of climate change – and sea turtles have survived past climate shifts – these northerly nests take on a new function.
"It will be those eggs that will hatch as the climate warms," says Carlos Drews, the World Wildlife Foundation's marine and species coordinator for Latin America and the Caribbean. "That gives them an adaptive window."
But turtles must live in a human-dominated world. Shifting north is not so simple: People and turtles tend to like the same sandy beaches. And while people and turtles can coexist – look at Juno Beach, Fla., says Dr. Safina, with "thousands of people and thousands of turtles" – people can alter beaches in ways that are harmful to turtles. Humans remove the vegetation immediately behind the beach to build houses, inadvertently causing erosion. They try to stabilize the beach with metal, stone, and concrete reinforcement. With predicted sea level rises – and stronger storms – the beach cannot migrate, and inevitably shrinks.
People also replenish shrinking beaches with sand dredged from offshore. Some ecologists think that this sand, high in fine clay particles, could suffocate hatchlings as they try to dig out of the nest.
"As climate changes, these turtles are going to have to find other beaches to nest on," says Frank Paladino, a professor of biology at Purdue University's campus at Fort Wayne, Ind. But human development "will have destroyed their ability to adjust and adapt to changing climatic conditions."
But the gravest threat may be on beaches where turtles already nest. Of the 13 nests counted on a recent May patrol here in Tortuguero National Park, eight nests – or 75 percent – had been poached. Even without human interference, only 1 in every 1,000 leatherback hatchlings makes it to adulthood.
"If you had a healthy population, the populations would have a better chance of adapting," says Emma Harrison, CCC's scientific director in Costa Rica. Already stressed, sea turtles have a greater chance of disappearing from a changing climate, she says. Helping turtles adapt ultimately means changing people's ways – showing those who live near nesting sites that turtles are worth much more alive.
In Costa Rica, as in many countries where turtles nest, custom has it that eggs eaten raw enhance male virility. But after Dr. Drews's team began an antipoaching campaign in communities near Junquillal in 2005, poaching dropped from almost 100 percent to near zero in one year. Residents saw that tourists would come to see the turtles, bringing money. Those who might have been poachers are now rangers, he says. They have a stake in the turtle's wellbeing.
When the project ends in a few years, "we want full appropriation by these young people," says Drews. "So the community will be the owners forever of the turtles."
Scientists aren't sure what would happen if sea turtles disappeared. But they suspect that whatever the turtles feed on will proliferate. Leatherbacks, which can weight up to a ton, eat jellyfish, lots of them. Jellyfish eat the young of fish species consumed by people. If you remove leatherbacks, one of the few jellyfish predators, the outlook for fish, an important source of food for humans, grows grimmer.
But while Safina sees these impact assessments as important, he says they don't capture the entirety of what is lost if such an ancient and mysterious creature disappears.
"This is a really wondrous world," he says, "and to eliminate [the] various opportunities to discover things that are wonderful [such as sea turtles] – even if you never see them – it, to me, greatly diminishes the prospects for being alive and for being a fully human being."