The atmospheric concentrations of carbon dioxide (CO2), methane, and nitrous oxide have increased to levels unprecedented in at least the last 800.000 years. Carbon dioxide concentrations have increased by 40% since pre-industrial times, primarily from fossil fuel emissions and secondarily from emissions from land use change¹Intergovernmental Panel on Climate Change (IPCC)(2013)Climate Change 2013: Physical Science Basis. Working Group 1 Contribution to the fifth assessment report on the Intergovernmental Panel on Climate Change.

Warming of the climate system is unequivocal, and since the 1950s, many of the observed climate changes are unprecedented over decades to millennia. The atmosphere and oceans have warmed, the amounts of snow and ice have diminished, sea level has risen and the concentrations of greenhouse gases have increased. The oceans have absorbed about 30% of the emitted anthropogenic carbon dioxide, causing ocean acidification²IPCC as above.

Sea turtles already face significant threats that challenge their survival (incidental captures during fishing, degradation of nesting areas, maritime traffic, marine pollution and especially plastic litter, illegal trade and consumption etc.). Climate change places sea turtles under threat that we cannot fully estimate (see EuroTurtle for more information).

Global temperatures have already increased by 0.8°C and are predicted to increase by over 2°C by 2100, skewing gender ratios – feminization of the population. Sea turtles exhibit temperature-dependent sex determination, which means that the gender of sea turtles is determined by the temperature of the sand in which they are laid. Cooler temperatures produce males and warmer temperatures produce females. With rising temperatures, the sand will also increase in temperature, which eventually will lead to an all-female population. If temperatures exceed the upper limit for egg incubation (34º C), the eggs will not hatch. Research to date across most the Mediterranean loggerhead sea turtle nesting beaches has shown that nest clutches are already producing a female bias in the gender ratio³Godley, B., Broderick, A., Downie, J., Glen, F. and others (2001). Thermal conditions in nests of loggerhead turtles: further evidence suggesting female skewed sex ratios of hatchling production in the Mediterranean. Journal Exp Mar Biol Ecol 263: 45-63.

However, sea turtles naturally exhibited some resilience to a female-biased hatchling sex ratios. Research tracking male sea turtles in Zakynthos, Greece has shown that males breed twice as frequently as females⁴Hays, G., Fossette, S., Katselidis, K., Schofield, G., Gravenor, M. (2010). Breeding Periodicity for Male Sea Turtles, Operational Sex Ratios, and Implications in the Face of Climate Change. Conservation Biology, Volume 24: 6:1636-1643. and will breed with more than one female per breeding season.

The females will also breed with more than one male and are able to store the sperm from multi males, resulting in multiple paternities within each clutch of eggs per nest. These life history breeding strategies exhibited by both the males and females lessens the need for a 1:1 gender ratio to ensure a healthy population and provides some alleviation to the effects of rising temperatures.  However, the current maintained rapid increases in temperatures will continue to skew the female- biased gender ratios placing the status of current and future populations at risk. 

Ocean heat has been increasing steadily since 1955⁵CSIRO and BoM (2012). State of the climate 2012 increasing sea turtle distribution.

Sea turtles preferred habitat is warmer water temperatures in tropical or sub-tropical seas (e.g. loggerhead sea turtles prefer over 15°C). Thermal niche modelling has shown that as sea temperatures rise, the preferred temperatures for sea turtles across the Mediterranean will spatially expand⁶Mazaris, A., Kallimanis, A., Sgardelis, S. and Pantis, J. (2008). Do long-term changes in sea surface temperature at the breeding areas affect the breeding dates and reproduction performance of Mediterranean loggerhead turtles? Implications for climate change. Journal of Experimental Marine Biology and Ecology 367 (2208) 219-226.. This means that regions across the Western basin will exhibit warmer temperatures and could increasingly become  more favorable during winter months where once it was too cold⁷Witt, M., Hawkes, L., Godfrey, M., Godley, B. and Broderick, A. (2010). Predicting the impacts of climate change on a globally distributed species: the case of the loggerhead turtle. The Journal of Experimental Biology 213: 901-911..

Although temperatures predominantly define sea turtle distributions, it is not the only determining factor. Changes in the sea temperature will also cause changes to ecosystems and could impact food availability. For species exhibiting specialised diets such as the green turtle, feeding primarily on seagrasses and algae, their ability to expand their current distribution will be limited and dependent on the availability of food sources. 

Global mean sea level has risen by 0.19m between 1901 and 2010. Using current global greenhouse gas emissions, estimates of sea level rise is projected between 0.26m to 0.97m by 2100, jeopardising nesting beaches. As sea levels rise alongside increasingly more frequent and severe storm surges, nesting beaches will increasingly face devastating impacts from erosion and tidal inundation. Already low-lying sand beaches such as Bonaire, the Maldives and the Great Barrier Reef have been seriously eroded threatening sea turtle nesting. The low-lying narrow nesting beaches in the Marine Protected Area of the National Marine Park of Zakynthos are at risk, especially those where coastal development prevents the natural landward migration of the beach⁸Dimitriadis, C. (2012). Climate Change Impacts on the Nesting Beach of the loggerhead sea turtle in the National Marine Park of Zakynthos.

However there is the possibility that as increasing temperatures reduce the suitability of current nesting sites, new areas previously inaccessible to nesting – thermally or geographically may become available⁹Witt et al., (2010) as above. Hawkes, L., Broderick, A., Godfrey, B. and Godley, B. (2009). Climate change and marine turtles. Endangered Species Research. Volume 7: 137-154..

Across the western Mediterranean and within the northern sea regions (e.g. Adriatic), sporadic sea turtle nesting events are being monitored. Typically, these regions are considered too cold for sea turtle nesting, but their occurrence and frequency can provide insight into sea turtle adaptive behaviour towards climatic changes.

The ocean has absorbed about 30% of the emitted anthropogenic carbon dioxide, causing ocean acidification¹⁰IPCC, 2013 as above., thus threatening food sources.

Dissolved carbon dioxide forms a weak acid. As the concentration of carbon dioxide increases the pH of the ocean decreases, causing the ocean to become more acidic. Acidification will decrease the availability of key nutrients, including nitrogen, phosphates, silica and iron and in turn reduce the growth of plankton. Green turtles are significantly at risk, having a very restricted diet, primarily consisting of sea grasses and algae. Loggerheads have a more generalised diet, which provides some resilience to this threat¹¹Witt et. al., 2010, as above.

Resilience is the capacity to withstand threat or catastrophe. Research suggests that sea turtles will have some resilience to the impacts of climate change¹²Hays, G., Broderick, A., Glen, F., Godley, B. (2003). Global Change Biology. Volume 9, Issue 4: 642-646. Hays et al.,  (2010) as above and indeed, have an evolutional ancestry spanning millions of years, surviving multiple climate change events. The Mediterranean Sea has been found to be one of the most resilient seas turtle management areas¹³Fuentes, M., Pike, D., Dimatteo, A. and Wallace, B. (2013). Resilience of marine turtle regional management units to climate change. Global Change Biology 19: 1399-1406..

Natural resilience of sea turtles may include shifting their nesting season to cooler months of the year, selecting cooler beaches to nest or selecting cooler sites to lay their eggs¹⁴Witt et. al., 2010 as above. Today the additional pressures exerted by humans is placing sea turtle’s ability to adapt at risk.  Artificial active management could also be used such as shading nests and relocating to cooler sites. However, the risks of active management techniques are not known, neither  is it known how successful they will be¹⁵Fuentes et. al., 2013 as above.