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 that there has been a shift in gender ratios over the last 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 that 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 Dr. Jensen, a marine biologist with the National Oceanic and Atmospheric Administration in San Diego.