National Geographic News
A photo of a Green Turtle.

Green turtles (Chelonia mydas) are currently listed as endangered on the IUCN Red List. The species faces a number of threats, including illegal poaching and unbalanced sex ratios caused by warmer temperatures in nesting areas.

PHOTOGRAPH BY THOMAS P. PESCHAK

Kennedy Warne

for National Geographic

Published March 17, 2014

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.

A photo of two Green Turtles mating.
PHOTOGRAPH BY THOMAS P. PESCHAK
The sex of a green turtle is determined by how hot or cold it is while the egg develops in the nest.

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.

A photo of a Green Turtle swimming.
PHOTOGRAPH BY THOMAS P. PESCHAK
Scientists are using tracking technologies to better understand the nesting areas of green turtles.

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.

“This species is one of the oldest in the world,” he says. “They have weathered many climate crises in the past, so maybe their capacity to adapt will keep working for them.”

17 comments
XU JIMIN
XU JIMIN

Try our best to protect these pretty animals.

Ralph Calvert
Ralph Calvert

I have just finished reading a " Geographical  Magazine,  [ Climate change Here and Now]    of which I found very interesting ,  on the changing weather,   However  while answering   some problems  I thought it   opened up other areas of problems ,   But over all very interesting.      The magazine used   Penguin's   Seal's,  and Walrus's   for some of the examples.      Ralph.. 

Mukund More
Mukund More

result of global change has arrived after long time

Austin Zoppi
Austin Zoppi

I think that last statement is brilliant. It gives the reader a little bit of hope for the turtles; it's powerful to evoke a response like that. On a side note, I think that the turtles are facing something serious here, and it is really rather intimidating to them because they can only do what they are biologically permitted to do.

katherine Esposito
katherine Esposito

Thanks Will!!!! I <3 these turtles i would risk my life for them!

William R.
William R.

I find the last statement in the article quite interesting as it almost makes the article's topic irrelevant.


to wit: “This species is one of the oldest in the world,” he says. “They have weathered many climate crises in the past, so maybe their capacity to adapt will keep working for them.”

m s
m s

It's already killing off large quantities of Coral in tropical waters where the temperature has shifted a couple of degrees. It will soon test all creatures of the sea.

Noorah Ahmed
Noorah Ahmed

Wow.. Every day we here about a species who is endangered. Is there any safe species at all!!

At least we are glad that there are people who care and tell about the animals who suffers from different problems and threats. These efforts will be effective for sure. Maybe later.. but any good work gets a good result after it.

craig hill
craig hill

Despite the blindness, deafness and dumbness of those who cannot bring themselves to grasp that Man is a part of Nature, and so impacts it with, in this case, untold trillions of tons of burned carbon aka greenhouse gases, the Earth has a fever that is only rising for the release of much more powerful methane from the melting permafrost. The winter in the Northern hemisphere fools the easily fooled into "thinking" there is no global heating going on; when the jet stream from the Arctic is bringing down Arctic weather across North America, Europe and Northern Asia, within the Arctic Circle temperatures are hitting 90 degrees F! Which is why the ice is melting across the Arctic and Greenland, dumkopfs!

Paul M.
Paul M.

32 years of a 95% consensus of certainty from the United Nations and Geo news editors for the worst crisis imaginable isn't good enough and former climate blame believers are relieved a crisis for our children was thankfully just a tragic exaggeration. The rest of you remaining “believers” just hate Humanity itself.

Ken Ricklin
Ken Ricklin

@William R. I question any climate statements from Heartland Institute.  It is a well known climate denier think tank funded by the Koch brothers, Exxon-Mobil and that ilk.

Melissa E.
Melissa E.

@m s

We live in the Marshall Islands and the biggest challenge that our coral suffers from is the human species not climate issues. Trash and destruction due to lack of knowledge and care. Our turtles here are protected and live here all the time.

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