Humans figure out whether to develop as males or females based on the presence of a single gene on the Y chromosome. But that’s just one of a dizzying number of ways that plants and animals determine their sex. A large group of reptiles, including crocodilians and many species of turtles, use the ambient temperature. If the eggs are above a certain temperature during a critical period in their incubation, the animal will be likely to develop as a female; below that temperature, you’re more likely to get a male.

And that, in a world where temperatures are rising, is a problem.

A new study of sea turtles that live near the Great Barrier Reef has found that populations closest to the equator, where the temperatures are warmest, have been producing over 99 percent females for two decades. While turtles have obviously weathered changing climates in the past, the current rate of change, coupled with sea turtles’ long life span, raise concerns about how well they’ll cope with our current warming.