The Ocean
likely that temperatures, along with salinity, have also increased at depth (400 m or more) in the western Mediterranean Sea over the past 30 to 40 years which, when analyzed in the context of heat budget and water flux of the Mediterranean, is consistent with anthropogenic greenhouse warming (Bethoux et al., 1990; Rixen et al., 2005; VargasYáñez et al., 2010). Large-scale variability such as the AMO and NAO can obscure or accentuate the overall warming trend (Marullo et al., 2011; WGI AR5 Sections 14.5.1, 14.7.6). Relatively warm episodes in the 1870s, 1930–1970s, and since the mid-1990s, for example, exhibit an influence of the AMO (Kerr, 2000; Moron, 2003). Reported temperature anomalies in the Mediterranean, often locally manifesting themselves as periods of low wind, increased water column stratification, and a deepening thermocline, are associated with positive phases of the NAO index (Molinero et al., 2005; Lejeusne et al., 2010). Sea levels have increased rapidly in some areas over recent decades and are also strongly influenced by NAO phases. The rate has been approximately 3.4 mm yr–1 (1990–2009) in the northwest Mediterranean (high confidence; Calvo et al., 2011). These influences are reduced when measurements are pooled over longer time scales, resulting in a lower rate of SLR (Massuti et al., 2008). If the positive phase of the NAO is more frequent in the future (Terray et al., 2004; Kuzmina et al., 2005; WGI AR5 Section 14.4.2), then future SLR may be slightly suppressed as a result of atmospheric influences (medium confidence; Jordà et al., 2012). As temperatures have increased, the Mediterranean has become more saline (+0.035 to 0.040 psu from 1950 to 2000; Rixen et al., 2005) with the length of the thermal stratification period persisting twice as long in 2006 as it did in 1974 (Coma et al., 2009). Conditions within the Mediterranean Sea changed abruptly and synchronously with similar changes across the North, Baltic, and Black Seas in the late 1980s (Conversi et al., 2010), which possibly explains the lack of trend in SES SST when examined from 1950 to 2009 (Table 30-1). These changes in physical conditions (increased temperature, higher sea level pressure, positive NAO index) also coincided with step changes in the diversity and abundance of zooplankton, decreases in stock abundance of anchovies and the frequency of “red tides,” and increases in mucilage outbreaks (Conversi et al., 2010). Mucilage outbreaks are strongly associated with warmer and more stratified water columns (high confidence), and lead to a greater abundance and diversity of marine microbes and potentially disease-causing organisms (likely; Danovaro et al., 2009). Increasing temperatures are also driving the northward spread of warm-water species (medium confidence) such as the sardine Sardinella aurita (Sabatés et al., 2006; Tsikliras, 2008), and have contributed to the spread of the invading Atlantic coral Oculina patagonia (Serrano et al., 2013). The recent spread of warm-water species that have invaded through the Straits of Gibraltar and the Suez Canal into cooler northern areas is leading to the “tropicalization” of Mediterranean fauna (high confidence; Bianchi, 2007; Ben Rais Lasram and Mouillot, 2008; CIESM, 2008; Galil, 2008, 2011). Warming since the end of the 1990s has accelerated the spread of tropical invasive species from the eastern Mediterranean basin (Raitsos et al., 2010; Section 23.6.5). In addition to general warming patterns, periods of extreme temperatures have had large-scale and negative consequences for Mediterranean marine ecosystems. Unprecedented mass mortality events, which affected at least 25 prominent invertebrate species, occurred during the summers
Chapter 30
of 1999, 2003, and 2006 across hundreds of kilometers of coastline in the northwest Mediterranean Sea (very high confidence; Cerrano et al., 2000; Garrabou et al., 2009; Calvo et al., 2011; Crisci et al., 2011). Events coincided with either short periods (2 to 5 days: 2003, 2006) of high sea temperatures (27°C) or longer periods (30 to 40 days) of modestly high temperatures (24°C: 1999; Bensoussan et al., 2010; Crisci et al., 2011). Impacts on marine organisms have been reported in response to the extreme conditions during these events (e.g., gorgonian coral mortality; Coma et al., 2009), shoot mortality, and anomalous flowering of seagrasses (high confidence; Diaz-Almela et al., 2007; Marbà and Duarte, 2010). The frequency and intensity of these types of heat stress events are expected to increase as sea temperatures increase (high confidence). Longer-term data series (over several decades) of changes in relative acidity of the Mediterranean Sea are scarce (Calvo et al., 2011; The MerMex Group, 2011). Recent re-analysis, however, has concluded that the pH of Mediterranean waters has decreased by 0.05 to 0.14 pH units since the preindustrial period (medium confidence; Luchetta et al., 2010; Touratier and Goyet, 2011). Anthropogenic CO2 has penetrated the entire Mediterranean water column, with the western basin being more contaminated than the eastern basin (Touratier and Goyet, 2011). Studies that have explored the consequences of ocean acidification for the biology and ecology of the Mediterranean Sea are rare (Martin and Gattuso, 2009; Rodolfo-Metalpa et al., 2010; Movilla et al., 2012), although insights have been gained by studying natural CO2 seeps at Mediterranean sites such as Ischia in Italy, where biodiversity decreases with decreasing pH toward the vents, with a notable decline in calcifiers (Hall-Spencer et al., 2008). Transplants of corals, molluscs, and bryozoans along the acidification gradients around seeps reveal a low level of vulnerability to CO2 levels expected over the next 100 years (low confidence; Rodolfo-Metalpa et al., 2010, 2011). However, periods of high temperature can increase vulnerability to ocean acidification, thereby increasing the long-term risk posed to Mediterranean organisms and ecosystems as temperatures warm. Significantly, some organisms such as seagrasses and some macroalgae appeared to benefit from local ocean acidification (Hall-Spencer et al., 2008).
30.5.3.2. Key Risks and Vulnerabilities SES are highly vulnerable to changes in global temperature on account of their small volume and landlocked nature. Consequently, SES will respond faster than most other parts of the Ocean (high confidence). Risks to ecosystems within SES are likely to increase as water columns become further stratified under increased warming, promoting hypoxia at depth and reducing nutrient supply to the upper water column (medium evidence, high agreement). The impact of rising temperatures on SES is exacerbated by their vulnerability to other human influences such as over-exploitation, pollution, and enhanced runoff from modified coastlines. Due to a mixture of global and local human stressors, key fisheries have undergone fundamental changes in their abundance and distribution over the past 50 years (medium confidence). A major risk exists for SES from projected increases in the frequency of temperature extremes that drive mass mortality events, increasing water column stabilization leading to reduced mixing, and changes to the distribution and abundance of marine organisms. The vulnerability of marine 1685
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