Why are sea turtles turning female?

The sex of sea turtles is not determined purely genetically, as it is in humans. The temperature during the development of the embryo will determine whether a male or a female turtle hatches. Higher temperatures result in females, lower in males.

Increasing sand temperatures on nesting beaches can therefore shift the sex ratio of hatchlings to almost entirely female. As a result, turtles may have a problem reproducing in the future.

Nesting female olive ridley turtles, Aribada. Image. — Nesting female olive ridley turtles.

Olive ridley turtle hatchling leaving the nest. Image — Olive ridley turtle hatchling leaving the nest.

References:

Abella Perez E, Marco A, Martins S and Hawkes L 2016. Is this what a climate change-resilient population of marine turtles looks like? Biological Conservation 193: 124-132.
Chaloupka M, Kamezaki N and Limpus C 2008. Is climate change affecting the population of the endangered Pacific loggerhead sea turtle? Journal of Experimental Marine Biology and Ecology 356: 136-143.
Fuentes MMPB, Limpus CJ and Hamann M 2011. Vulnerability of sea turtle nesting grounds to climate change. Global Change Biology 17: 140-153.
Pike DA, Antworth RL and Stiner JC 2006. Earlier Nesting Contributes to Shorter Nesting Seasons for the Loggerhead Sea Turtle, Caretta caretta. Journal of Herpetology 40: 91-94.