Drivers of change. In: Climate and Environmental Change in the Mediterranean Basin - Current Situation and Risks for the Future. First Mediterranean Assessment Report
Cherif, S. ; Doblas-Miranda, E. ; Lionello, P. ; Borrego, C. ; Giorgi, F. ; Iglesias, A. ; Jebari, S. ; Mahmoudi, E. ; Moriondo, M. ; Pringault, O. ; Rilov, G. ; Somot, Samuel ; Tsikliras, A. ; Vila, M. ; Zittis, G.
Climate drivers 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. Pollution Across the Mediterranean Basin, ocean and inland pollution are ubiquitous, diverse and increasing in both quantity and in the number of pollutants, due to demographic pressure, enhanced industrial and agricultural activities, and climate change. Mediterranean seawater is generally oligotrophic (low nutrient), with decreasing levels from Gibraltar eastwards to the Levantine Sea. Several coastal regions are hotspots of human-induced nutrient inputs. This nutrient enrichment causes eutrophication and may provoke harmful and toxic algal blooms, whose frequency will likely increase. Harmful algal blooms may cause negative impacts on ecosystems and may represent serious economic threats for fisheries, aquaculture, tourism and human health. Emerging contaminants are well present across the Mediterranean Basin, and enhanced by increasing inflow of untreated wastewater. These substances may cause disorders of the nervous, hormonal and reproductive system. And the increasing frequency of extreme precipitation events in the north of the Mediterranean increases the supply of fecal bacteria and viruses to the coastal zone. The Mediterranean Sea is one of the most polluted large water bodies globally in terms of plastic and the level of this pollution is expected to increase in the future. The Mediterranean Basin is among the regions in the world with the highest concentrations of gaseous air pollutants (NO2, SO2 and O3). Its dry and sunny climate, and specific atmospheric circulation patterns enhance air pollution levels. Ships are among the major causes of increasing SO2 and NOx emissions in this region. Emissions of aerosols and particulate matter (PM) into the atmosphere arise from a variety of anthropogenic activities. Particular meteorological conditions and natural sources, including the proximity of the Sahara Desert, create particular patterns of aerosol concentrations that may influence particulate matter PM concentrations. The occurrence of critically high PM concentrations associated with dust outbreaks is higher in the southern Mediterranean (>30% of the annual days) than in the northern area (<20% of the annual days). Land and sea use change Landscapes and their use have changed over millennia in the Mediterranean Basin, however the rate of change has increased substantially since the second half of the 20th century, with rapid growth of urban and peri-urban areas leading to loss of biodiversity and habitats. Abandonment of agropastoralism (which will likely continue in the future) is causing unmanaged shrubs forest development in marginal lands, arid and mountain areas in European countries, while land overexploitation is causing widespread forest degradation in areas of North Africa and the Middle East. Future land use trends depend strongly on regional policies for urbanization, agriculture, forestry and nature conservation. Marine resource overexploitation and unsustainable fishing practices have increased in time and are the main drivers of the population decline of several species. Presently, more than 60% of marine stocks have collapsed or are overexploited. Sustainable management of marine resources requires the reduction of fishing pressure. Non-indigenous species The Mediterranean Sea (and particularly the Levantine Basin) is a hotspot for the establishment of many non-indigenous species (invertebrates, primary producers, and vertebrates), whose arrival and increase are linked to the decrease or collapse in populations of native species. Most marine nonindigenous species enter the Mediterranean Sea from the Red Sea and Atlantic Ocean, but those introduced by ships and aquaculture produce the largest impact on the environment. The number and spread of non-indigenous species will likely further increase with increasing shipping activity and impacts of climate change on the Mediterranean water masses. Mediterranean land areas currently host a high number of non-indigenous species (mostly plants and invertebrates) in human-modified ecosystems and in regions with high infrastructure development. Most invertebrate species are phytophagous pests that cause damages to crops and forests. Future warming is predicted to induce a northward shift at a speed of 37-55 km decade-1 of current major non-indigenous species and determine a window of opportunity for new nonindigenous species adapted to dry environments. The presently increasing trend of the numbers of introduced invertebrates and vertebrates (the latter generally caused by accidental escapes) will very likely continue, as they can be easily transported also as stowaways in air and maritime cargo. Interaction among drivers When ecosystems and societal sectors are threatened by multiple, co-occurring drivers, climate change, pollution, land and sea use change, and non-indigenous species can interact. Interactions cause effects that can be additive/cumulative, synergistic or antagonistic and result in alteration, intensification, and even in generation of new impacts. Examples of new threats are increase of flood events, due to a combination of climatic and land use changes, desertification, which is the result of increasing aridity and exploitation of resources, and wildfires, affected by forest encroachment and heat waves, among many other interactions.</p>
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