Sub-chapter 1.2.2. The climate of the Mediterranean regions in the future climate projections
p. 83-91
Texte intégral
Introduction
1Giorgi (2006) defined hot-spots as the most sensitive regions to climate change. On the basis of a regional climate change index (RCCI) calculated from temperature and precipitation projections, the Mediterranean region was revealed to be one of the most prominent hot-spots over the globe. Considering the last global climate change projections in the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC, 2013), the singularity of the region, particularly for a future large reduction of precipitation, was confirmed, as illustrated in figure 1. This figure reproduces the projected average percentage change in precipitation in winter (December-January-February) and in summer (June-July-August), under the conditions of the high emission RCP8.5 scenario, for two 20-year periods in the middle and at the end of this century compared to the period 1985-2005. It clearly shows that the Mediterranean region could be affected by a decrease in precipitation in both seasons, becoming more significant with time. This could occur while other regions, particularly those at the same latitude, do not undergo a change that is distinguishable from climate variability due to internal processes of the climate system.
2This great sensitivity to climate change can be understood as a consequence of the location of the region in a transition zone between the arid climate of North Africa and the temperate and rainy climate of central Europe, making it vulnerable to climate shifts caused by climate change (Lionello et al., 2012). Moreover, there is great consistency between models concerning the main characteristics of these projections over the region.
Mean temperature and precipitation
3Giorgi and Lionello (2008) reviewed climate change projections over the Mediterranean region based on a large ensemble of 17 global climate model (GCM) simulations, part of the ensemble of simulations analyzed in the fourth assessment report AR4 (IPCC, 2007). With respect to a median emission scenario (A1B), robust and large warming is projected at the end of this century (2071-2100) compared to the end of 20th century (1961-1990), with a maximum in the summer season. In summer (JJA) values would reach 3 °C to 4 °C over the sea, and 4 °C to 5 °C in inland areas, with maxima higher than 5 °C in the Sahara and Middle East. Substantial temperature increases will also occur in other seasons: + 2 °C to 3 °C in winter and spring, and + 3 to 4°C in fall. Changes would be about half a degree higher under the scenario with the highest emissions (A2 similar to the RCP8.5 scenario used in the AR5), and 1 °C less under the scenario with the lower emissions (B1). Climate change affects land areas more than the sea with differences between the Mediterranean Sea and the surrounding land regions of between 0.4 °C and 0.6 °C, reaching 0.9 °C in summer. The reduction in precipitation would be smallest in winter, where it would vary from no change in the northern Mediterranean to a 40% reduction in the south. In spring and fall, reductions in precipitation would vary between 10% and 40%. The average value of the large summer reduction would be in the range 25% to 30%, but some areas in the northeast and the south would undergo a reduction of more than 50%. In most model simulations, the reduction in mean annual precipitation, when averaged over the whole basin, is larger over sea than over land, except in winter. Like changes in temperature, changes in precipitation in the lower (higher) emission scenarios are smaller (larger) than in the median emission scenario. Both precipitation and temperature signals are projected to become progressively more severe during the course of the 21st century.
Figure 1
Multi-model CMIP5 average percentage change in seasonal mean precipitation relative to the reference period 1985–2005 averaged over the periods 2045–2065 and 2081–2100 under the RCP8.5 forcing scenario. Hatching indicates regions where the multi-model mean is less than one standard deviation of internal variability. Stippling indicates regions where the multi-model mean is greater than two standard deviations of internal variability and where 90% of models agree on the sign of change.
From fig. 12.22 WGI AR5, IPCC 2013
4The most recent results of emission scenarios of similar amplitude in IPCC AR5 are very similar. For instance, the maps presented in IPCC, 2013: Annex I, show that in the RCP8.5 comparable to the A2 emission scenario, warming over inland regions in about one century would be of the order of 3 °C to 5 °C in winter, and of 5 °C to 7 °C in summer. According to the same scenario, the decrease in precipitation over land during the October-March period would range from no change in the north to a 40% decrease in the south, and from 10% to 40% from north to south during the April-September period. However, according to the lower emission scenario (RCP2.6) that assumes the application of drastic mitigation climate policies, and has no corresponding scenario in AR4, the simulated changes would be much smaller. In this case, mean warming would not exceed 1.5 °C in winter and 2 °C in summer, even over the land areas. In addition, in this scenario, mean precipitation change would never exceed one standard deviation of internal climate variability.
5These orders of magnitude are also consistent with the results of climate change scenarios run using regional climate models (RCMs). These models enable a better representation of the orography thanks to the improved resolution of the calculation, but they only cover a limited geographical area. Some results from an ensemble of simulations performed in the context of the ENSEMBLES European research project are reported in Planton et al. (2012). They consist in climate change projections over the Mediterranean region based on an ensemble of six regional climate change simulations with a resolution of about 25 km. Here we reproduce the simulated temperature and precipitation changes between 1961-1990 and 2071-2100.
6In agreement with the GCM simulations, over land, the warming calculated by RCMs is roughly within the range of 2.5 °C to 3.5 °C in winter and 4 °C to 5 °C in summer (fig. 2). Warming over the sea is roughly in the range of 2 °C to 3.5 °C with less seasonal dependence. Precipitation changes simulated by RCMs (fig. 3) also generally agree with those simulated by GCMs with substantial future drying in all seasons and over all areas of the Mediterranean region, except in the northern part in winter.
Figure 2
Multi-model ENSEMBLES average percentage change in seasonal mean temperature relative to the reference period 1985–2005 averaged over the period 2071–2100 under the A1B forcing scenario.
From fig. 8.1 Planton et al. 2012.
7The main conclusion to be drawn from this synthesis of global and regional climate simulations is that in the Mediterranean at the end of the 21st century intense warming is almost certain and drying is very likely. The amplitude of these changes after 2050 depend to a great extent on the emission scenario concerned when the full range of the AR5 emission hypotheses is considered. However, actual values and the detailed spatial distribution of changes in precipitation, remain uncertain as they are strongly model dependent (Paeth et al. 2016).
Figure 3
Same as fig. 2 for precipitation.
From fig. 8.1 Planton et al. 2012.
Mean hydrological cycle
8Mariotti et al. (2008) studied changes in the Mediterranean water cycle associated with the A1B emission scenario from GCM simulations synthesized in AR4. These authors showed that a simulated decrease in precipitation in the 20th century precipitation decrease would be followed by rapid drying from 2020 onwards (precipitation decrease is –0.02 mm/d per decade or –7.2 mm/y per decade). This amounts to roughly 15% less precipitation in 2070-2099 compared to 1950–2000, and an 8% decrease as early as 2020-2049. Since the multi model ensemble average has internal variability with reduced amplitude, actual variability would be larger than that depicted by the model ensemble mean. As precipitation is the main driver of the land surface hydrological cycle, other major hydrological indicators would also change similarly (see chapter 2.3). Concerning land surfaces, evapotranspiration would also decrease because of the drier soils, but, as increased surface temperature favors higher evaporation, the rate would be half that of precipitation. By 2070–2099, the projected difference between precipitation and evaporation decreases over land would be –0.09 mm/d (–0.01 mm/d per decade or -3.6 mm/y per decade). Some specific results concerning the future hydrological budget of the Mediterranean Sea are presented in sub-chapter 1.2.3.
9The amplitude of the mean precipitation anomaly foreseen by 2020–2049 (about 0.1 mm/d or 36 mm/y) is comparable to that of the driest decadal spells experienced by the Mediterranean region in the 20th century. The Mediterranean region is indeed subject to climatic variability at a decadal time scale resulting partly from teleconnection with other regions (Ulbrich et al., 2012). Hence, at least in the short term (10–30 years), regional decadal anomalies and any potential for decadal predictability is likely to be critically dependent on the regional impacts of decadal modes of variability “internal” to the climate system.
Warm spells
10In addition to changes in mean values, climate projections also include significant changes in variability. Temperature and precipitation distributions would be subject to both a considerable shift and deformation, becoming broader in future climate scenarios. Increased interannual variability, especially in summer, along with the increase in mean warming, would lead to more frequent occurrence of extremely high temperature events. Like for precipitation, the decrease in mean precipitation and the increased frequency of large negative anomalies would increase the intensity and frequency of drought events (Giorgi and Coppola, 2009).
11The detailed impacts of climate change on extremes linked to the water cycle (heavy precipitation, strong winds, drought events, flash floods) are described in chapter 1.3. We consequently focus here on warm spells or heat waves whose length, frequency, and/or intensity are judged to be very likely to increase throughout the whole region (IPCC, 2013).
12The analysis of the RCM simulations in the context of the PRUDENCE and ENSEMBLES European projects revealed a general increase in temperature extremes projected for the end of the 21st century in different emission scenarios (Planton et al. 2012). Using a statistical downscaling approach Hertig et al. (2010) also confirmed the trends in temperature extremes in the median A1B emission scenario. In general, the results indicate that changes in temperature extremes do not follow a simple shift of the whole temperature distribution to higher values but also due to a broadening of the frequency distributions of Mediterranean temperatures.
Box 1
Future climate change in Morocco
Several socio-economic sectors (e.g. water resources and agriculture) are climate dependent in Morocco like in any many other Mediterranean countries. For example, dry years (e.g. 1980-1985 and 1990-1994) reduce crop yields and can negatively influence the GDP of the country. In the context of climate change, it is important to assess possible future changes in precipitation and temperature, two aspects of climate that directly influence agriculture, water resources and many other sectors.
Using the regional climate model ALADIN-Climat with a high resolution (12 km) over Morocco, we assessed future changes under two emission scenarios (RCP4.5 and RCP8.5). The main result obtained with the RCM is a quasi-general decrease in annual and winter rainfall amounts and an increase in temperature in all seasons, in both scenarios and at different time horizons.
Between 1971-2000 and 2036-2065, annual precipitation should decrease by 5% to 30% under RCP8.5 and the longest winter dry period should be extended by between 2 and 6 days from north to south. The number of high precipitation events is projected to decrease at both annual and winter scales whereas the amount of precipitation during this type of event should increase slightly, indicating fewer but more severe episodes. Warming is projected to concern all regions and mean temperature should increase by 1 °C to 1.6 °C from west to east under RCP4.5 and from 1.4°C to 2.2°C under RCP8.5. Warming should also manifest itself as more frequent summer heat wave days throughout the country (+1 to +4 days).
When future precipitation and temperature changes are combined, soil moisture is expected to be negatively impacted (see chapter 2.3). Taking into account the dependence of many vital sectors on climate and projected future changes, it is easy to deduce that adaptation is unavoidable to limit the negative impacts of climate change on the country to the greatest possible extent.
Figure 4
Future changes in the maximum dry period in winter (in days) and in summer heat waves (in days) projected by ALADIN-Climat for Morocco under the RCP8.5 scenario between 2036-2065 and 1971-2000.
13In more quantitative terms, Barriopedro et al. (2011) showed in particular that, according to the RCMs of the ENSEMBLES project, weekly heat spells of magnitude of the second week of August 2003 (7-day anomaly of 3.7 standard deviation of the 1979-1999 climatology), will probably occur in 2020–2049, with a best-guess return period of about 15 years in Western Europe, including the northern part of the western Mediterranean area. By the end of the 21st century, such extreme weekly heat spells are expected to occur about every 4 years, whereas some models show 2003-like anomalies about every second summer. In a study of projected heat extremes over the eastern Mediterranean and the Middle East (EMME) with a specific RCM, Lelieveld et al. (2014) projected that by the end of this century, in most cities, the coolest summers may be warmer than the hottest ones of the 1961-1990 period, according to all the scenarios considered (B2, A1B and A2).
14Projections of extreme temperature changes always show regional variability that can be partly attributed to mechanisms involving coupling between the surface and the atmosphere. For instance, a study by Zittis et al. (2014) also covering the EMME region, showed that heat waves might be intensified by a decrease in soil moisture due to reduced cooling caused by evaporation. This mechanism has been shown to play a role in particular in Italy, the Balkans and Turkey, but this result remains to some extent model dependent.
Bibliographie
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