By Fanny Adloff
The Mediterranean is the largest semi-enclosed sea on our planet. Acting as a miniature ocean, this basin is appropriate to study climate change impact on the ocean. The residence time of the Mediterranean waters – of about a century – is smaller than in the world ocean, so that a quicker response to climate change is expected in this vulnerable basin. Hot-spot of marine biodiversity, the Mediterranean hosts 10,000 to 12,000 marine species, one fourth of them being endemic. The Mediterranean suffers from high anthropic pressure at the shore and at the open sea, which substantially increases its ecosystem vulnerability.
IPCC global climate models do not have enough resolution accurately to represent the complex circulation and the water masses of the Mediterranean basin. High-resolution regional ocean models have been thus developed to study the impact of climate change on this specific region.
A six-member ensemble regional climate simulation covers the period 1961-2099 to study the oceanic response to climate change and its uncertainty (Adloff et al., 2015). The ocean response can vary depending on (i) the socio-economic scenario, (ii) the Atlantic boundary conditions, (iii) the fresh water fluxes from river input and Black Sea inflow, or (iv) the atmospheric surface fluxes. To assess the impact of climate change, the 30 year reference period 1961-1990 is compared to the ‘end of 21st century’ period 2070-2099.
The spatially averaged sea surface temperature increase is predicted to be 1.7°C under low-emission scenario, and up to 3°C under the larger emission scenario (Figure 1). The warming could reach 4°C in some specific regions like the Balearic Islands. The surface warning spreads non-homogenously toward depth, and the deep-water formation dynamics are also affected.
Figure 1: Composite of sea surface temperature anomalies maxima (top) and minima (bottom) for the 2070–2099 period w.r.t. 1961–1990 (°C). The largest (maxima) or smaller (minima) anomaly out of the six scenario simulations is represented at each grid point
Sea surface salinity could increase by 1 g of salt per kilogram of water by the end of the century. These hydrographic changes will lead to substantial modification of the water masses properties of the Mediterranean and of its thermohaline circulation (THC). This three-dimensional circulation is mainly driven by deep water formation processes taking place in winter when surface water becomes denser and sinks towards the bottom of the ocean, bringing oxygen to the deepest layers.
In the present climate, this phenomenon mainly occurs in the western Mediterranean (in the Gulf of Lion and in the Adriatic Sea). In future climates, the simulations show considerable changes of the Mediterranean THC, with a large increase of deep water formation in the Eastern Mediterranean (in the Levantine basin).
Significant modifications of surface currents and of mean sea level are also simulated, the latter being very sensitive to the chosen Atlantic boundary conditions (Figure 2, Slangen et al. 2017).
Figure 2: Cumulative thermosteric sea-level change w.r.t. 1961–1990 (cm), averaged over the Mediterranean Sea from the six-member ensemble scenario simulations from Adloff et al. (2015). In blue, the uncertainties linked to the choice of the prescribed hydrographic conditions of Atlantic waters west of Gibraltar, and in red, the uncertainties linked to the choice of the socio-economic scenario
These changes to the physical characteristics of the Mediterranean Sea could severally affect its marine ecosystems. Because of the warmer waters, many species migrate northward and get trapped by a ‘cul-de-sac’ effect at the north coasts of the Gulf of Lion, the Adriatic Sea and the Aegean Sea. Also, massive extinction of non-migratory species such as Gorgons or Posidonias could take place under climate change.