A sea turtle’s sex is determined by its nesting environment. The gender shift suggests climate change is having a significant effect on one of the biggest green-turtle populations in the world.

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Male sea turtles are disappearing from Australia’s Great Barrier Reef.

A new study of gender ratios found that 99 percent of immature green turtles born in the northern part of the reef are female. Among adult turtles, 87 percent are female, suggesting there has been a shift in gender ratios during the past few decades.

A sea turtle’s sex is determined by its nesting environment. As sands warm, more females will hatch relative to males; if the sand temperature tops 84.7 degrees during incubation, only females will emerge.

The gender shift suggests climate change is having a significant effect on one of the biggest green-turtle populations in the world, said Michael Jensen, lead author of the new study, published in Current Biology.

“We’re all trying to wrap our heads around how these populations are going to respond to those changes,” said Jensen, a marine biologist with the National Oceanic and Atmospheric Administration (NOAA) in San Diego.

At what biologists call the pivot temperature, turtles hatch as a mixture of males and females. For green sea turtles, this temperature is 29.3 degrees Celsius (85 Fahrenheit). A few degrees below 29.3 C, all the sea turtles are born male. Heat up the eggs, and only females are born.

“That transitional range, from 100 percent males to 100 percent females, spans a very narrow band of only a couple of degrees,” Jensen said.

The gender shift has been noticed before by people who study hatchlings, said Jeanette Wyneken, a sea-turtle expert and professor at Florida Atlantic University, who was not involved in the new research.

But it was not clear until this study that the shift was so dramatic and happening in such a large population across time, she said.

“This is the first paper that’s shown this multigenerational effect,” influencing the gender of juveniles, older adolescents and adults, Wyneken said.

It takes 35 to 40 years for a green sea turtle to reach sexual maturity, she said.

“These animals are teenagers for an awfully long time. We won’t see the effects of what’s happening today for several decades,” she said.

David Owens, a professor emeritus from the College of Charleston in South Carolina, was not involved in the new study but said he has dreamed of doing such research for years. He praised the way the study team — which included a wide range of expertise — was able to link temperature with turtle gender.

“We’ve had little pieces of this story come out before, but this is the first time anyone’s been able to orchestrate the whole story to bring in global climate change along with feminization,” Owens said.

If the green sea turtles of Australia are seeing this shift, the same thing is probably happening to many other sea turtles and other animals whose gender is determined by temperature, he said.

Sea turtles tend to lay their eggs on the same beaches where they were hatchlings decades earlier, so they cannot quickly adapt to warming temperatures, Owens said.

It is possible that they could lay their eggs earlier in the season, instead of waiting for the hottest weeks of the year, but Jensen said there is no evidence to suggest that such a shift has or could occur.

Sea turtles are the “lawn mowers of the ocean,” according to Camryn Allen, an author of the new study and a marine biological researcher with the Joint Institute for Marine and Atmospheric Research — University of Hawaii and located at NOAA in Hawaii. The turtles maintain coral reefs and are nutrient transporters. If they are lost, other species that depend on the same habitat will also be harmed.

“Sea turtles are sentinels,” Allen said. “They’re telling us something is going on in the oceans and we need to pay attention.”

Both she and Jensen said they do not yet see the problem as urgent, because there are enough adult males to sustain the population.

Highlighting the problem now will give researchers time to learn more about factors affecting the animals.

Allen said the government of Australia has started to take steps toward helping sea turtles, such as funding the Raine Island Recovery Project to study and support the local turtle population.

The group conducted its research over 16 days in July 2014, plying small boats around the Howick Group of islands in the north Great Barrier Reef — “an absolutely magical place,” according to Jensen.

They captured 411 foraging turtles, one at a time, drawing blood to measure gender hormones and taking skin samples for DNA. The genetic analysis allowed researchers to determine whether the turtle had been born in the northern or southern parts of the reef, which are separated by about 1,200 miles.

Allen said the reasons for the gender bias were still a mystery. “The exact mechanism is not well understood,” she said. “But we do know they don’t have sex chromosomes like humans do.”

Turtles born in the cooler south were only biased 65-69 percent female, the study showed. Researchers still do not know the ideal ratio, or how many males to females it takes to effectively sustain the population, Jensen said.

Without the new study, he said, scientists might not have recognized the gender skewing in the north for decades — perhaps missing the window to make a difference.

“The result is definitely alarming,” Jensen said. “But now we know and can focus our research on the right questions and start thinking about what can be done. So I’m hopeful as well.”