Rationale

Justification for indicator selection

Relevance
Sea surface temperature (SST) is closely linked to one of the strongest drivers of climate in western Europe: the ocean circulation known as the Atlantic Meridional Overturning Circulation (MOC). This circulation (also known as the great conveyor belt) carries warm upper waters north in the Gulf Stream and returns cold deep waters south. It is widely accepted that the MOC is an important driver of low frequency variations in sea surface temperature on the time scale of several decades (Griffies et al., 1997). It is also widely accepted that the NAO index (a proxy of atmospheric circulation variability) plays a key role in forcing variations in MOC as well as the northward extent of the Gulf Stream (Frankignoul and Kestenare, 2005; De Coetlogon et al., 2006). At present, changes in sea surface temperatures of the global ocean and the regional seas of Europe are consistent with the changes in atmospheric temperature (Levitus et al., 2000; Rayner et al., 2006).
The sensitivity of the MOC to greenhouse warming, however, remains a subject of much scientific debate. Observations indicate that there has indeed been a freshening of the North Atlantic since 1965 due to increased freshwater inputs from rivers, precipitation and melting glaciers (Curry and Mauritzen, 2005), and thus possibly a weakening of the Atlantic MOC. The freshening calculated by these authors occurred mainly before 1970 and does not yet appear to have substantially altered the MOC and its northward heat transport. Uncertainties regarding the rates of future climate warming and glacial melting limit the predictability of the impact on ocean circulation, but do not exclude the possibility of a weakening of the MOC. Recent observations, have, however, shown that the variability of the MOC is large. The yearlong average MOC is 18.7 ± 5.6 Sverdrup (Note: Sverdrup = 10⁶ m³s^-1) but with large variability ranging from 4.4 to 35.3 Sverdrup (Cunningham et al., 2007). A recent study has shown that the variability of the MOC may be predictable on decadal time scales, and the study predicts that North Atlantic and European sea surface temperatures will fall slightly in the next decade as natural climate variability off-sets the projected anthropogenic warming (Keenlyside et al., 2008). The plausibility of the Keenlyside et al., 2008 projections are, however, also subject to intense debate in the scientific community (see e.g. http://www.realclimate.org).
One of the most visible ramifications of increased temperature of the ocean is the reduced area of sea ice coverage in the Arctic polar region and there is an accumulating body of evidence suggesting that many marine ecosystems are responding both physically and biologically to changes in regional climate caused predominantly by the warming of air and SST, as shown in the following sections.

Scientific references

• References Coppini, G. and Pinardi, N., 2007. Compiled for EEA by the Istituto Nazionale di Geofisica e Vulcanologia (INGV) based on datasets made available by the Hadley Center HADISST1: http://hadobs.metoffice.com/hadisst/data/ download.html. Coppini, G.; Pinardi, N.; Marullo, S. and Loewe, P., 2007. Compiled for EEA by the Istituto Nazionale di Geofisica e Vulcanologia (INGV) based on datasets made available by the Hadley Center. HADISST1: http://hadobs.metoffice.com/hadisst/data/download.html . ENEA within Mediterranean Operational Oceanography Network (MOON), and Bundesamt für Seeschifffahrt und Hydrographie (BSH) within the Baltic Operational Oceanography System (BOOS). Coetlogon, G. de; Frankignoul, C.; Bentsen, M.; Delon, C.; Haak, H.; Masina, S. and Pardaens, A., 2006. The gulf stream variability in five oceanic circulation models. Journal of Physical Oceanography 36 (11): 2119-2135. Cunningham, S. A.; Kanzow, T.; Rayner, D.; Baringer, M. O.; Johns, W. E.; Marotzke, J.; Longworth, H. R.; Grant, E. M.; Hirschi, J. M. M.; Beal, L. M.; Meinen, C. S. and Bryden, H. L., 2007. Temporal variability of the Atlantic meridional overturning  circulation at 26.5 degrees N. Science 317: 935-939. Curry, R. and Mauritzen, C., 2005. Dilution of the Northern North Atlantic Ocean in Recent Decades. Science 308: 1772-1774. Frankignoul, C. and Kestenare, E., 2005. Observed Atlantic SST Anomaly Impact on the NAO: An update. Journal of Climate 18: 4089-4094. Griffies S. M. and Bryan, K., 1997. Predictability of the North Atlantic Multidecadal Variability. Science 275: 181-184. IPCC, 2007a. Cimate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Solomon, S.; Qin, D.; Manning, M.; Chen, Z.; Marquis, M.; Averyt, K. B.; Tignor M. and Miller H. L. (eds.), Cambridge University Press, Cambridge, UK. Keenlyside, N. S.; Latif, M.; Jungclaus, J.; Kornblueh, L. and Roeckner, E. 2008. Advancing decadal-scale climate prediction in the North Atlantic sector. Nature 453: 84-88. Levitus, S.; Antonov, J. I.; Boyer, T. P.; Stephens, C., 2000. Warming of the world ocean. Science 287: 2225-2229. Lindley, J. A., 1987. Continuous Plankton Records: the geographical distributions and seasonal cycles of decapod crustacean larvae and pelagic post-larvae in the North-eastern Atlantic Ocean and the North Sea, 1981-83. Journal of the Marine Biological Association of the United Kingdom 67: 145-150. Liu, Z.; Vavrus, S.; He, F.; Wen, N. and Zhong, Y., 2005. Rethinking Tropical Ocean Response to Global Warming: The Enhanced Equatorial Warming. Journal of Climate 18: 4684-4700. Rayner, N. A.; Brohan, P.; Parker, D. E.; Folland, C. K.; Kennedy, J. J.; Vanicek, M.; Ansell, T. J.; Tett, S. F. B., 2006. Improved analyses of changes and uncertainties in sea surface temperature measured in situ sice the mid-nineteenth century: The HadSST2 dataset. Journal of Climate 19(3): 446-469. Stouffer R. J., 2004. Time scales of climate response. Journal of Climate 17: 209-217. Watterson, I. G., 2003. Effects of a dynamic ocean on simulated climate sensitivity to greenhouse gases. Climate Dynamics 21: 197-209. Xu, Y.; Zhao, Z.-C.; Luo, Y. and Gao, X., 2005. Climate change projections for the 21st century by the NCC/IAP T63 with SRES scenarios. Acta Meteorologica Sinica 19: 407-417.