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Recent warming of Mediterranean waters: How much is due to climate change?


An article by IES scientists published today in PLoS ONE finds that anthropogenic causes of the warming of the Mediterranean Sea could be masked by the effect of naturally occurring multidecadal oscillations in sea surface temperature.

The surface water temperature of the Mediterranean Sea has been rising at quite a high rate during the past two decades, mirroring the general global ocean-warming trend. Between 1985 and 2009, the surface temperature of the Mediterranean waters increased by an average of 0.75°C, peaking around 2005. This warming trend could modify sea levels and alter the general currents in the region, with a knock-on effect on marine ecosystems.

The authors used the 25 years of available satellite data (1985-2010) to calculate the Sea Surface Temperature (SST) trend for the Mediterranean Basin (the red ‘linear fitting’ line in the diagram to the right). The results, which showed increasing temperatures from 1985 to 2010, compared well with those of a hydrodynamic model. A low-frequency oscillation trend, which accounts for 53% of the total observed warming, emerged from the overall trend. This low-frequency trend closely followed the temporal dynamic of the Atlantic Multidecadal Oscillation (AMO) index.

The AMO index shows the natural long-term oscillation of SST in the North Atlantic for the past 150 years. More than half of the total warming observed in the Mediterranean Sea reflects warming attributed to the AMO, with human activities contributing the remaining 47% of the total temperature increase in the basin during the past 25 years. The authors argue that the influence of multidecadal oscillations such as the AMO must therefore be considered when evaluating projections of Mediterranean SST in the coming decades. For instance, the predicted shift of the AMO to a cold phase in the coming years could mask the effects of anthropogenic warming, potentially leading decision makers to underestimate the effects of anthropogenic causes of climate change.

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