Uncertain future for global sea turtle populations in face of sea level rise

Abstract

Sea level rise has accelerated during recent decades, exceeding rates recorded during the previous two millennia, and as a result many coastal habitats and species around the globe are being impacted. This situation is expected to worsen due to anthropogenically induced climate change. However, the magnitude and relevance of expected increase in sea level rise (SLR) is uncertain for marine and terrestrial species that are reliant on coastal habitat for foraging, resting or breeding. To address this, we showcase the use of a low-cost approach to assess the impacts of SLR on sea turtles under various Intergovernmental Panel on Climate Change (IPCC) SLR scenarios on different sea turtle nesting rookeries worldwide. The study considers seven sea turtle rookeries with five nesting species, categorized from vulnerable to critically endangered including leatherback turtles (Dermochelys coriacea), loggerhead turtles (Caretta caretta), hawksbill turtles (Eretmochelys imbricata), olive ridley turtles (Lepidochelys olivacea) and green turtles (Chelonia mydas). Our approach combines freely available digital elevation models for continental and remote island beaches across different ocean basins with projections of field data and SLR. Our case study focuses on five of the seven living sea turtle species. Under moderate climate change scenarios, by 2050 it is predicted that at some sea turtle nesting habitats 100% will be flooded, and under an extreme scenario many sea turtle rookeries could vanish. Overall, nesting beaches with low slope and those species nesting at open beaches such as leatherback and loggerheads sea turtles might be the most vulnerable by future SLR scenarios.

Figure 1

Vulnerability of sea turtle nests under sea level rise at IPCC’s RCP 4.5. Seven study sites at sea turtle rookeries spanning countries in the Caribbean Sea, Gulf of Mexico and Pacific Ocean, with five species represented. In the Caribbean and Gulf of Mexico: Mondonguillo beach, Costa Rica; Guanahacabibes peninsula, Cuba; Saona Island, Dominican Republic; Zeelandia, Turtle, Kay bay, Tumbledowndick, Crooks and Oranjebaai beaches, St Eustatius; and St George Island, Florida, USA. In the Pacific Ocean: Coast of Ecuador; and Raine Island, Australia. Map Data: Google Earth and free images.

Figure 2

Summary of model predictions. Predictions of the models for different DEMs and empirical data (in situ) indicate the best fit of probability of nest flooding for (a) Costa Rica and (b) Ecuador turtle nests. IPCC sea level rise predictions are included for RCP 4.5 (blue) and RCP 8.5 (red). For Ecuador, the RCP climate change scenario did not influence the probability of nest flooding. Cohen’s kappa (κ) values for prediction (or not) of flooding for each nest compared in situ data with respective DEM datasets. Bold values correspond to a small but significant relationship between the in situ and DEM data.

Figure 3

Summary of actual vs. estimates of slope and elevation by DEM. Comparison of the relationship between slope (radians) and elevation (m) using empirical data (in situ measurements) and DEMs for turtle nesting beaches in Costa Rica.

Figure 4

Predictions of sea turtle nest flooding in a subset of rookeries. Sites include (a) Florida (USA) (2050 and 2100), (b) Cuba (2050), and (c) St Eustatius (three species, 2050). Column 1: Proportion of nests likely to be flooded at each location. Column 2: Heatmap of nests predicted to be free from flooding by 2050. Column 3: Heatmap of nests predicted to be flooded by 2050, identifying the areas at highest risk. Probability values of 1 (yellow), 0.5 (blue) and 0 (purple) represent the density of unflooded or flooded nests within respective heatmaps. A Climate Central moderate scenario was adopted for these scenarios (Kopp et al. 2017).