Climate change projections for the Mediterranean region

Abstract

We present a review of climate change projections over the Mediterranean region based on the most recent and comprehensive ensembles of global and regional climate change simulations completed as part of international collaborative projects. A robust and consistent picture of climate change over the Mediterranean emerges, consisting of a pronounced decrease in precipitation, especially in the warm season, except for the northern Mediterranean areas (e.g. the Alps) in winter. This drying is due to increased anticyclonic circulation that yields increasingly stable conditions and is associated with a northward shift of the Atlantic storm track. A pronounced warming is also projected, maximum in the summer season. Inter-annual variability is projected to mostly increase especially in summer, which, along with the mean warming, would lead to a greater occurrence of extremely high temperature events. The projections by the global and regional model simulations are generally consistent with each other at the broad scale. However, the precipitation change signal produced by the regional models shows substantial orographically-induced fine scale structure absent in the global models. Overall, these change signals are robust across forcing scenarios and future time periods, with the magnitude of the signal increasing with the intensity of the forcing. The intensity and robustness of the climate change signals produced by a range of global and regional climate models suggest that the Mediterranean might be an especially vulnerable region to global change.

Introduction

The Mediterranean region lies in a transition zone between the arid climate of North Africa and the temperate and rainy climate of central Europe and it is affected by interactions between mid-latitude and tropical processes. Because of these features, even relatively minor modifications of the general circulation, e.g. shifts in the location of mid-latitude storm tracks or sub-tropical high pressure cells, can lead to substantial changes in the Mediterranean climate. This makes the Mediterranean a potentially vulnerable region to climatic changes as induced, for example, by increasing concentrations of greenhouse gases (e.g. Lionello et al., 2006a, Ulbrich et al., 2006). Indeed, the Mediterranean region has shown large climate shifts in the past (Luterbacher et al., 2006) and it has been identified as one of the most prominent “Hot-Spots” in future climate change projections (Giorgi 2006).

The climate of the Mediterranean is mild and wet during the winter and hot and dry during the summer. Winter climate is mostly dominated by the westward movement of storms originating over the Atlantic and impinging upon the western European coasts. The winter Mediterranean climate, and most importantly precipitation, is thus affected by the North Atlantic Oscillation (NAO) over its western areas (e.g. Hurrell 1995), the East Atlantic (EA) and other patterns over its northern and eastern areas (Trigo et al., 2006). The El Nino Southern Oscillation (ENSO) has also been suggested to significantly affect winter rainfall variability over the Eastern Mediterranean (along with spring and fall precipitation over Iberia and North-western Africa, Alpert et al., 2006). In addition to Atlantic storms, Mediterranean storms can be produced internally to the region in correspondence of cyclogenetic areas such as the lee of the Alps, the Gulf of Lyon and the Gulf of Genoa (Lionello et al., 2006b).

In the summer, high pressure and descending motions dominate over the region, leading to dry conditions particularly over the southern Mediterranean. Summer Mediterranean climate variability has been found to be connected with both the Asian and African monsoons (Alpert et al., 2006) and with strong geopotential blocking anomalies over central Europe (Xoplaki et al., 2004, Trigo et al., 2006).

In addition to planetary scale processes and teleconnections, the climate of the Mediterranean is affected by local processes induced by the complex physiography of the region and the presence of a large body of water (the Mediterranean Sea). For example, the Alpine chain is a strong factor in modifying traveling synoptic and mesoscale systems and the Mediterranean Sea is an important source of moisture and energy for storms (Lionello et al., 2006a, Lionello et al., 2006b). The complex topography, coastline and vegetation cover of the region are well known to modulate the regional climate signal at small spatial scales (e.g. Lionello et al., 2006a). In addition, anthropogenic and natural aerosols of central European, African and Asian origin can reach the Mediterranean, possibly influencing its climate characteristics (Alpert et al., 2006). Because of the interactions of processes at a wide range of spatial and temporal scales, the climate of the Mediterranean is characterized by a great diversity of features, resulting in a variety of climate types and great spatial variability (Lionello et al., 2006a).

Despite the importance of this region within the global change context, assessments of climate change projections over the Mediterranean are relatively sparse. Perhaps the most comprehensive review of climate change projections over the Mediterranean region is reported by Ulbrich et al. (2006) based on a limited number of global and regional model simulations performed throughout the early 2000s. A number of papers have reported regional climate change simulations over Europe, including totally or partially the Mediterranean region (e.g. Giorgi et al., 1992, Rotach et al., 1997, Giorgi et al., 1997, Jones et al., 1997, Deque et al., 1998, Machenhauer et al., 1998, Raisanen et al., 1999, Christensen and Christensen, 2003, Semmler and Jacob, 2004, Schar et al., 2004, Giorgi et al., 2004b, Raisanen et al., 2004, Deque et al., 2005). Finally, several studies have presented regional evaluations of different generations of global model projections, including the Mediterranean region (Kittel et al., 1998, Giorgi and Francisco, 2000, Giorgi et al., 2001, Giorgi and Bi, 2005a, Giorgi and Bi, 2005b). Reference will be made to these studies in the discussion of the results presented here.

The main motivation of this paper is that recent research efforts provide an opportunity to approach a Mediterranean climate change assessment on much stronger grounds than in the past. First, a worldwide effort has been recently carried out by which about 20 research groups around the world completed a large set of global climate simulations for the 20th and 21st century under different greenhouse gas forcing scenarios as a contribution to the fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC). Output from this ensemble of models, which is hereafter referred to as Multi Global Model Ensemble (or MGME), is stored at the Program for Climate Model Diagnosis and Intercomparison (PCMDI, http://www-pcmdi.llnl.gov) and is of public access. This dataset of unprecedented quality and comprehensiveness allows a much better assessment of climate change projections than in the past and has in fact spurred a large number of research projects on different climate change issues (see the PCMDI web site above).

The second effort we refer to is the completion of the European project Prediction of Regional scenarios and Uncertainties for Defining, EuropeaN Climate change and associated risks and Effects (PRUDENCE, Christensen et al., 2002), in which a wide range of global and regional climate models were used to produce climate change projections over the European region. To complement this project, fine scale (grid interval of 20 km) multi-decadal regional simulations of climate change over the Mediterranean area have been completed by Gao et al. (2006).

Based on the availability of these new sources of information, which represent the state of the art in both global and regional climate change simulations, in this paper we present an updated assessment of future climate change projections over the Mediterranean basin. We examine different climate variables and statistics, such as mean changes and changes in variability and extremes for surface climate variables as well as circulation patterns. We do not address issues of impacts, adaptation and mitigation, although we hope that our assessment might provide useful information for such issues.

Our assessment is based on both a new analysis of the above mentioned data and a review of published material deriving from the MGME and PRUDENCE data. Therefore, we begin this paper with a brief description of these datasets.