These are tough times for sea turtles. Historically plundered for their meat, shells, skins, and eggs, turtles continue to be poached even where they're legally protected.

And they suffer injury and drowning when they come into contact with fishing operations.

As a result of the pressures on them, all seven species of marine turtles are considered at risk globally.

A warming climate will present new threats to these ancient reptiles. Warming may upset turtle population sex ratios. The sex of turtle hatchlings is determined by the temperature at which the eggs develop in the nest, with higher temperatures favoring the production of females.

Finely Tuned Creatures

Turtles have evolved to synchronize their nesting with times of year when the incubation temperature produces roughly equal numbers of male and female hatchlings. If that ratio goes out of whack, populations could plummet, owing to a scarcity of one or the other of the sexes.

The problem could become acute in the Scattered Islands, which dot the Mozambique Channel in the southwest Indian Ocean. (See "A Tale of Two Atolls.") Here, five species of sea turtles feed and breed. Biologists have been focusing their research on the green turtle, one of the most widespread species.

Greens form rookeries of up to 50,000 nests on some of the islands, and their nesting times differ latitudinally. In the cooler southern islands, such as Europa, the peak is in summer, and in the more northerly islands, such as Mayotte and the Glorieuses, egg laying is most prolific in winter.

In theory, a warming climate could make Europa’s nesting peak shift toward the cooler winter months, allowing those turtles to maintain a healthy sex ratio.

But that option isn’t available for turtles on Mayotte and the Glorieuses, where they’re already nesting at the coolest time of year. For these rookeries, warmer air temperatures could result in an excess of females and a paucity of males, disrupting the equilibrium of the population.

View Images The sex of a green turtle is determined by how hot or cold it is while the egg develops in the nest. PHOTOGRAPH BY THOMAS P. PESCHAK

Other Threats

Skewed sex ratios aren’t the only challenge posed by climate change. A predicted increase in extreme weather events triggered by a warmer atmosphere heightens the risk of storm surges that can inundate turtle nests. Storm surges can also destroy nesting beaches.

And as global sea levels rise, nesting habitat will shrink. It’s hard enough for turtles to find suitable egg-laying beaches now, as real-estate development spreads along coastlines, let alone in a future when seas may be several feet higher. (See "Rising Seas.")

Sea turtles face other, less predictable climate impacts. Increases in sea temperature and acidity might limit the growth of sea grasses and other fodder on which some turtles browse. Increasing air humidity could make eggs more susceptible to disease, resulting in higher mortality.

Rising temperatures could also lead to profound changes in ocean circulation—a serious concern in regions such as the Mozambique Channel, with its complex eddies and currents.

Instead of being carried to traditional foraging sites by long-established gyre systems, turtle hatchlings might end up in places that are far from ideal for their survival and growth. Familiar migratory pathways might also be lost.

Uncertainty surrounds all these potential effects, but of one thing biologists are certain: Almost every aspect of turtles’ lives—both on land and in the sea—is linked tightly to environmental conditions. Their ability to adapt to a rapidly changing environment will be crucial to their survival.

View Images Scientists are using tracking technologies to better understand the nesting areas of green turtles. PHOTOGRAPH BY THOMAS P. PESCHAK

Shore Them Up Now

To help sea turtle populations cope with unknown future threats, one of the best things we can do is protect them from existing known harms—fisheries mortality being one of the most grave.

Biologists at the French Research Institute for Exploration of the Sea, in La Réunion, have been working to identify the migratory corridors green turtles use to travel between their feeding and nesting areas.

Where those corridors bring turtles close to areas of fishing activity—especially coastal gillnetting, which entangles and kills thousands of sea turtles each year—mitigation strategies could include altering the design of nets to make them less likely to snag turtles, illuminating nets with light sticks, building in turtle escape devices, and, most importantly, say the biologists, educating fishers about the need to avoid turtle bycatch.

Tracking technologies have transformed turtle research, and a recent study led by conservation biologist Jérôme Bourjea has shown where turtles that nest in the Scattered Islands go to feed.

Europa’s turtles, for instance, swim an average of 1,500 kilometers (more than 900 miles) in 21 days to reach their feeding grounds, traversing the offshore waters of five countries.

There's still much to learn, especially about the movements of newly hatched turtles, which are too small (weighing less than an ounce) to have conventional transmitters attached to their shells. Researchers would like to know at what point in their lives hatchlings become active swimmers as opposed to passive drifters in ocean currents.

Shadowing all this work is the question of whether sea turtles will be able to adapt quickly enough to the environmental changes that are predicted to occur in the oceans over the coming decades.

In the case of green turtles, Bourjea draws at least a modicum of hope from history.