Climate change, impacts and vulnerability in Europe ... - MemoFin.fr

Climate impacts on environmental systemsduring winter (McNeil and Matear, 2008). Ten percent of the Arctic Ocean may become corrosiveto calcium carbonate already by 2020 (Steinacheret al., 2009), and surface waters of the Baltic Seawill still become corrosive well before the end ofthe century. In the Black Sea and MediterraneanSea there is no danger of surface waters becomingcorrosive to calcium carbonate before 2100, butthey will suffer sharp reductions in carbonate ionconcentrations (Med Sea – 37 %; Black Sea – 45 %).These rapid chemical changes are an added pressureon marine calcifiers and ecosystems of the Europeanseas that are already heavily suffering from otheranthropogenic influences.Without dramatic actions to curb CO 2emissions,recovery from human-induced acidification willrequire thousands of years for the Earth systemto re-establish roughly similar ocean chemicalconditions (Archer, 2005; Tyrrell et al., 2007; Archerand Brovkin, 2008) and millions of years for coralreefs to return, based on palaeo-records of naturalcoral reef extinction events (Veron, 2008).3.1.3 Ocean heat contentRelevanceThe World Ocean is the dominant component of theEarth's heat balance. Oceans cover roughly 72 % ofthe planet's surface, and water has a heat uptakecapacity that is around 20 times greater than that ofthe atmosphere (Levitus et al., 2009, 2012). About90 % of the total warming caused by climate changeis manifested in increased global heat content.Hence, a precise estimate of Ocean Heat Content(OHC) is essential for understanding the role ofoceans in past climate change, and for assessingfuture climate change (Hansen, 2005; Church et al.,2011; Hansen et al., 2011). OHC is defined as theintegrated temperature change times the densityof sea water, times specific heat capacity from thesurface down to the deep ocean. Estimates of it aremade based on temperature measurements or onreanalyses made using a combination of models andobservations (see Section 2.1).Changes in heat content also cause the ocean toexpand or contract, thereby changing sea levelregionally and globally (Cazenave and Llovel,2010). This thermosteric effect has contributedabout one quarter to global sea-level rise since 1993(see Section 3.2.2).Past trendsThe warming of the World Ocean accounts forapproximately 90 % of the warming of the Earthduring the last 6 decades (Church et al., 2011;Hansen et al., 2011; Levitus et al., 2012).Figure 3.2 shows that the heat content of the WorldOcean has increased since around 1970. The lineartrend over the whole time series 1955–2010 of theuppermost 700 m and 2 000 m layer was 0.27 Wm -2and 0.39 Wm -2 (per unit area of the World Ocean),respectively. Two thirds of the observed increase ofglobal heat content has occurred in the upper 700 mof the ocean, with increases in the layers below700 m depth accounting for the remaining one third(Dore et al., 2009; Levitus et al., 2009, 2012; Purkeyand Johnson, 2010). Heat content has increased in allmajor sea basins of the World Ocean, in particular inthe Atlantic Ocean (Levitus et al., 2012).Several global ocean data assimilation products areavailable to compare observation-based estimateswith independent reanalysis data. Global andbasin‐scale heat content warming trends in theupper 700 m of the ocean computed from a set ofKey messages: 3.1.3 Ocean heat content• The warming of the World Ocean accounts for approximately 93 % of the warming of the Earth systemduring the last 6 decades.• An increasing trend in the heat content in the uppermost 700 m depth of the World Ocean is evidentover the last 6 decades. Recent observations show substantial warming also of the deeper ocean(between 700 m and 2 000 m depth).• Further warming of the oceans is expected with projected climate change, but quantitative projectionsof ocean heat content are not available.92 Climate change, impacts and vulnerability in Europe 2012