Chapter 2 Drivers of change

During recent decades, observations of several variables provide evidence of the ongoing anthropogenic climate change in the Mediterranean region, particularly increase of mean and extreme temperatures, and dry environmental conditions. Climate projections show that the region will among the most affected regions by climate change, specifically regarding precipitation and the hydrological cycle, but also mean warming and heat extremes (in both the terrestrial and marine environment), sea level rise and sea water acidification. Basin-wide, annual mean temperatures are now 1.5°C above the preindustrial level. In the last decades dry conditions have become more frequent and a large reduction of glaciers across high mountains of the Mediterranean has occurred at a progressively increasing pace. Mediterranean Sea waters have become warmer and saltier, Mediterranean sea level has risen at a rate (1.4 mm yr-1) similar to the global trend at centennial scale. In the future, the regional average warming will exceed the global mean value by 20% and it might reach 5.6°C at the end of the 21st century in the RCP8.5 high emission scenario. Heat waves and warm temperature extremes will intensify. Total annual precipitation is expected to decrease over most of the region (the average reduction rate is approximately 4% per each degree of global warming). However, magnitude and spatial distribution of changes are uncertain, because of differences among models. Dry conditions will be further enhanced by increasing evapotranspiration over land. At the same time, the inter-annual variability of the hydrological cycle will increase, with longer dry spells especially in the southern areas. Extreme precipitation events will become more intense over large parts of the northern Mediterranean areas. Mediterranean mean sea level is projected to be at the end of the 21st century in the range from 20 to 110 cm higher than at the end of the 20th century, depending on the level of anthropogenic emissions. Sub-regional and local relative sea level rise will be further modulated by vertical land motions and regional circulation features (with deviations in the order of 10 cm from the basin average). Therefore, though in the future milder marine storms are expected, coastal hazards, floods and erosion will increase, because of mean sea level rise. Widespread seawater warming will continue. Annual mean surface temperature will increase 2.7-3.8°C and 1.1-2.1°C in one century under the RCP8.5 and the RCP4.5 scenarios, respectively. Marine heat waves will become longer, more intense than today and their spatial extent will increase. Seawater acidification will continue, with a pH reduction that might larger than 0.4 units at the end of the 21st century