The Mystery of How They Will Adapt: How will climate change affect sea turtles?
By Jeff Miller
Flotilla of mating green turtles at Raine Island, Great Barrier Reef, Australia. © 2006 DAVID DOUBILET
Global warming is the environmental problem of the century. The Earth’s climate is warming quickly, by geological standards. Climate change will affect weather patterns, cause polar ice caps to melt, and ultimately result in sea levels rising. There are other lesser known and perhaps related phenomena—the consequences of which could be enormous—such as changes in ocean water quality. Given what we know of sea turtles’ ecological roles, at best we can only speculate about the long-term impacts of the changing climate on sea turtle survival, but we can identify some venerable parts of their lifecycle where climate change will likely impact. Not surprisingly, in 2005, the IUCN Marine Turtle Specialist Group identified climate change as one of five key hazards to sea turtles worldwide, making the issue a high priority for further study.
During the next 100 years, the predicted rise in sea level of up to 120 centimeters, combined with the increased frequency and intensity of storms and higher air and water temperatures (estimated at an average 4°C rise), will impact sea turtles in both their foraging areas and at their nesting beaches.
In the foraging areas—seagrass beds, coral reefs, and the open ocean—increasing storm frequency and intensity will add to the turbidity or murkiness of the water. Combined with greater water depth, this turbidity will disrupt the growth of seagrasses, sponges, corals, mollusks, and crustaceans on which sea turtles feed, in turn affecting the frequency of sea turtle reproduction, which is linked closely to food availability.
Sea turtles are perhaps most vulnerable to climate change at their nesting beaches. The females require nesting sites that are accessible from the sea and stable for digging; the sites must also be suitable for incubation of the eggs and adjacent to ocean currents to facilitate hatchling dispersal. These beach characteristics must coincide and be stable through time, because nesting sites are used by successive generations. An increase in storm intensity and frequency will likely alter the topography of beaches through erosion. As primary nesting beaches erode, turtles will be forced to use sub-optimal nesting sites where incubation may not be as successful. Further, as the sea level rises, some nesting areas will be lost altogether. The combined impacts will cause a reduction in hatchling production, affecting the viability of turtle populations decades later.
Before this occurs, however, several less obvious impacts of climate change will add to the pressure. When deposited on the beach, sea turtle eggs are subject to changes in beach conditions—temperature, moisture, and oxygen availability—during development. As the atmospheric temperature increases, so will that of the sand surrounding the eggs, and too much exposure to temperatures over 34°C can be lethal to embryos. At sublethal levels, incubation temperature determines the sex of sea turtle hatchlings; hence, as beach temperatures rise, more females will be produced.
Beach sand moisture content of 2 to 12 percent provides adequate hydric conditions for egg development. As beaches are eroded by storms, waves inundate beaches, and as sea spray splashes ashore, salinity will build up in the nesting beaches. Higher salt content sand reduces available moisture and can cause eggs to dehydrate and die.
The increase in tidal height may also flood eggs from underneath. If the sand is saturated by storm-driven waves or subsurface flooding and does not drain adequately, the embryos will drown.
It is not clear precisely how great the effects of climate change on sea turtle reproduction will be. Comprehensive long-term data sets are needed to fully research the matter. Until then, how the turtles will respond to climatic change remains a matter of speculation. Will sea turtles respond in concert with changing environmental conditions, or will environmental change outpace their ability to adapt to change? The answers to these questions will determine the long-term survival of these remarkable species.
This article originally appeared in SWOT Report, vol. 2 (2007). Click here to download the entire article as a PDF.