1 Introduction

In the last decades we have witnessed changes in climate and significant variations in all the key components of the hydrological cycle, which have profoundly altered water availability, river flow regimes, frequency and intensity of floods and droughts in many regions of the world (Ehsani et al. 2017, Kahaduwa and Rajapakse 2021). These changes may directly affect the spatial and temporal distribution of water resources, with important fallout in many socio-economic fields, such as agriculture, human health, and energy production, sometimes increasing the risk of a conflict between different sectors (e.g., agriculture, industry, domestic) and needs (e.g., urban, rural) in several regions (Flörke et al. 2018), or also causing flood- and drought-related disasters. In this regard, following the sixth assessment report (AR6) of the working group I (WGI) of the Intergovernmental Panel on Climate Change (IPCC) (Douville et al. 2021), since the 1970, 44% and 7% of all disaster events worldwide have been flood- and drought-related, respectively, many of which in the Mediterranean area.

Acclaimed increases in air temperature in Mediterranean regions, where water budgets are strictly dependent on the evapotranspiration (ET) fluctuations (e.g., Ambas and Baltas 2012, Gharbia et al. 2018), may imply higher levels of surface drying, potentially declining runoff, groundwater recharge, and water availability, and resulting in an increase in drought duration and severity (Ramos et al. 2018, Samaniego et al. 2018), especially when increases in atmospheric evaporative demand is combined with a decrease in precipitation (Caloiero et al. 2018b, Gudmundsson and Seneviratne 2016, Spinoni et al. 2017, Stagge et al. 2017).

With the aim to explore future changes in the intensity and frequency of short-duration extreme rainfall under a warming climate (Pall et al. 2007, Westra et al. 2014), the past decade has witnessed an increasing interest also on studies aimed at inferring a possible scaling relation between rainfall extremes and variables representative of the near-surface humidity, typically the surface air temperature or the dew point temperature (e.g., Blenkinsop et al. 2015, Mishra et al. 2012, Pumo et al. 2019, Pumo and Noto 2021, Roca 2019). The outcomes of such works confirm that an increase in air and sea temperature will likely lead to an increase in heavy precipitation, especially across the Mediterranean areas, due to the warmer air masses that, moving over the hotter and hotter water of the Mediterranean Sea, increase the atmospheric moisture-holding capacity to generate convection processes and, in turn, consequent pluvial and flash floods (Hilmi et al. 2022).

With the widespread warming, especially during the summer season, the Mediterranean area has run into an increase in extreme events, such as droughts and heavy precipitation (Arnone et al. 2013, Luu et al. 2018, Treppiedi et al. 2021, Zittis et al. 2021), especially at the sub-hourly durations, with some exceptions only for the northern areas. Such climate change effects are projected to become even more intense during the 21st century throughout the entire region. Nevertheless, extreme rainfall increase is not expected to compensate the acclaimed decrease in total precipitation of some areas, as the losses from decreasing frequency of low-medium-intensity precipitation are projected to dominate gains from more extreme precipitation events (Polade et al. 2017, Tramblay and Somot 2018).

In the light of the above, it is fundamental to understand the trajectory of climate for the Mediterranean area in the near future to efficiently assess the potential impacts of climate change on water resources and, consequently, on the different economic sectors strictly connected to water availability, as well as their possible societal repercussions in one of the most complex, history- and biodiversity-rich regions of the world (Tramblay et al. 2020).

The present work aims to provide a comprehensive and updated overview of the observed, i.e., historical, and predicted effects of climate change on the main processes involved in the hydrological cycle and directly related to water availability in the regions bordering the Mediterranean Sea. This area, hereinafter referred to as Mediterranean Basin (MB), is depicted in Fig. 1, where black polygons filled with oblique lines show the main basins of the Mediterranean area as reported in the WMO (World Meteorological Organization) Basins and Sub-Basins (WMOBB) dataset, which is an ongoing GIS project of the Global Runoff Data Centre (GRDC), available at: https://www.bafg.de/GRDC/EN/01_GRDC/grdc_node.html.

The manuscript is organized as follows. Sections 2 and 3 introduce the main detected and projected changes in rainfall and other key variables such as evaporation, surface runoff, and groundwater, while the last section is devoted to conclusions. A critical analysis concerning the various sources of uncertainty for climate predictions and their impacts on hydrological variables is provided in a companion paper (Noto et al., 2022).

Fig. 1
figure 1

Mediterranean Basin. Black polygons filled with oblique lines show the main basins of the MB as reported in the WMOBB dataset. Spatial reference system: WGS84 – UTM33N (EPSG: 32,633)

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2 Change Signals on Rainfall and Components of the Hydrological Cycle

2.1 Precipitation

Any change in the radiative balance of the Earth may affect temperature and precipitation patterns (Houghton et al. 1990), with rising temperatures leading to greater water holding capacity of air and, hence, to an increased amount of water potentially available in the atmosphere for precipitation. However, since changes in climate imply also changes in the wind patterns and ocean currents (Houghton et al. 1990), which drive the world’s climate system (Hogg et al. 2009, Niiler 2001), this does not necessary mean an increase in overall precipitation (Houghton et al. 1990). Indeed, some areas may experience variations on the frequency and intensity of storms, which not always imply significant modifications in the total seasonal or annual precipitation. Analogously, an increase in the length of dry periods might offset the increased precipitation falling during heavy events.

With reference to the MB, in the last decade, many studies have demonstrated a general decrease in the mean annual precipitation (MAP) and the winter precipitation, while extreme rainfall events have generally become more frequent, both in frequency and severity, especially at the smallest temporal scales (e.g., hourly and sub-hourly). Such a behavior is confirmed also in Table 1, which lists different studies related to observed changes in MAP, seasonal, and extreme precipitation for the MB, also reporting some details about the direction and the magnitude of the observed trend.

Table 1 Overview of the recent literature findings on observed precipitation trends over the MB.
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2.2 Climate Change Induced Alterations on the Hydrological Process

Beyond determining fundamental alterations in climate forcings (e.g., temperature and rainfall), climate changes strongly impact the hydrological response of the systems at different spatial scales (e.g., urban, watershed, continental), with relevant implications on all the water fluxes involving evapotranspiration, runoff generation, and groundwater recharge processes. Also, climate change directly impacts water supply, because of changes in rainfall and water lost by evapotranspiration, and has the potential to modify soil water infiltration dynamics and water partitioning into aquifer recharge and surface runoff, affecting both the quantity and the quality of water in rivers, lakes, and aquifers (Panwar and Chakrapani 2013, Pumo et al. 2016).

2.2.1 Atmospheric Evaporative Demand and Actual Evapotranspiration

Evapotranspiration is one of the major components of the water cycle and represents a critical link among terrestrial water, carbon, and surface energy changes.

The actual water amount evaporated and transpired from a specific area, i.e., the actual evapotranspiration (AET), is very challenging to measure and predict, especially at large spatial scales; for this reason, most of the works aimed to detect past and future trends in ET refers to other types of evapotranspiration (Allan et al. 1998), such as pan evaporation (PE), potential evapotranspiration (PET), and reference evapotranspiration (RET), which are all representative of the atmospheric evaporative demand (AED). While AED mainly reflects the impact of climate change on several climate variables, such as solar radiation, air temperature, humidity, and wind speed (Dimitriadou and Nikolakopoulos 2021a, Jiang et al. 2019), AET also critically depends on water availability, which is often the real limiting factor for AET in arid and semi-arid environments.

Several studies derived an increase in AED worldwide over approximately the last half century, as well as in the MB (Noguera et al. 2022, Scheff and Frierson 2014, Vicente-Serrano et al. 2020), with more highlighted patterns in summer. For example, Azorin-Molina et al. (2015) derived an increasing trend of PE (5.3 mm decade− 1) and PET (from 29 to 39 mm decade− 1) for Spain over the period 1961–2011. Similar results have been found for RET in Turkey (Dadaser-Celik et al. 2016), with an average increase of 1.2 mm year− 1, and also in Spain and the Balearic Islands (Tomas-Burguera et al. 2021), with a growth ranging from 7.0 to 12.6 mm decade− 1.

Despite different studies evaluated an overall increase in AET as a response to increased air temperature and vapor pressure deficit (Naumann et al. 2018, Scheff and Frierson 2014), trends in AET do not necessarily follow the same trends detected for AED using PE, PET, and RET as representative values, also because many arid and semi-arid regions of the MB are prevalently water-limited rather than energy-limited. Under such conditions, precipitation is often the prominent driver for AET. The diffuse use in climate change impact studies of PET and/or RET as surrogate of AET, without taking explicitly into consideration water availability and the possible changes in vegetation and land cover can, in fact, be questionable (Tramblay et al. 2018).

2.2.2 Surface Water and Groundwater

The surface water, i.e., runoff and streamflow, is probably the component of the hydrological cycle most representative to describe freshwater availability. Although no clear trends of changing runoff and streamflow emerge on the global level, significant trends have been observed at a regional level, different in magnitude and direction. The variability of such trends across different areas is related to both climatic factors, especially changes in precipitation and evaporation (Ficklin et al. 2018, Hannaford 2015), and anthropogenic forcings (Gudmundsson et al. 2021, Gudmundsson and Seneviratne 2016), such as water diversions and reservoirs/dams regulation.

With reference to the MB, analyses of in situ measurements of streamflow detected an overall decrease of mean annual values in between 1971 and 2010, thus showing a drying tendency for that period (Gudmundsson et al. 2021, Tramblay et al. 2019). Masseroni et al. (2021), after an analysis on historical data in Europe, detected for some regions of the MB a negative trend of -103 m3km− 2year− 1 of the annual streamflow volume in the 90% of the gauge stations considered over the period 1950–2013.

According to the AR6 WGI (Seneviratne et al. 2021), the increase in extreme precipitation has globally produced an associated increase in the frequency and magnitude of river floods, with several impacts across human and natural systems. An analysis of a large dataset of observations of annual maximum flood (i.e., highest daily mean of instantaneous discharge in each calendar year for each station) over the period 1960–2010 in Europe (Blöschl et al. 2019) detected a decreased trend in the mean annual flood discharge per decade (mainly within the range − 5% to -12%), with a maximum percentage decrement around − 24% for the European areas over the MB.

In the MB, groundwater often provides the main water resource supply for agricultural, as well as for industrial and domestic use, during drought periods. Since infiltration from rivers is the main groundwater recharge process in arid and semiarid environments (Leduc et al. 2017), a decline in groundwater levels may be often linked to a reduction in surface runoff (Han et al. 2020, Hellwig and Stahl 2018, Schreiner-McGraw and Ajami 2021, Wossenyeleh et al. 2021), especially in the driest areas of the MB, such as the north Africa (Benabdallah et al. 2018, Meddi and Boucefiane 2013).

Global hydrological models (Herbert and Döll 2019) and a global-scale analysis of the Gravity Recovery and Climate Experiment (GRACE) measurements over the period 2002–2016 (Shamsudduha and Taylor 2020) showed nonlinear negative trends in groundwater storage occurring in the major aquifers systems of many parts of the world, including the MB. Especially since the beginning of the 21st century, mean annual values of groundwater storage has declined, also due to the intensification of groundwater-fed irrigation induced by global warming. Moreover, groundwater in aquifers across arid and semi-arid regions of the MB tends to have long response times and appears to be rather resilient to climate change as enhanced recharge is observed to occur mostly episodically from intense precipitation and flooding events (Cuthbert et al. 2019, Opie et al. 2020).

3 Potential Future Impacts of Climate Change in the MB

3.1 Projected Alterations in Precipitation

Many studies in the last years have been focused on precipitation projections under different climate change scenarios in the MB, and on the analysis of how different future precipitation patterns might affect the water cycle and, in turn, the availability of water resources. In this regard, Table 2 reports many studies at different time scales with reference to the MB. Many of these studies are based on the four Representative Concentration Pathways (RCPs), namely RCP 2.6, RCP 4.5, RCP 6.0, and RCP 8.5, developed in the AR5 of IPCC, for the possible future scenarios. Indeed, there are still few studies involving the very recent five Shared Socio-economic Pathways, SSPs (i.e., from SSP1 to SSP5), and the seven levels of radiative forcing (i.e., 1.9, 2.6, 3.4, 4.5, 6.0, 7.0, 8.5 W/m2), introduced in the AR6 (Douville et al. 2021).

Looking at a future perspective of water resources management, it is worth to analyze the MAP and seasonal precipitation changes under various possible future climate scenarios. With this regard, many studies predicted a general MAP decrease, by using different Regional Climate Models (RCMs), which can reach values of about − 60% under the most pessimistic RCP 8.5. The decreasing pattern of MAP is accentuated especially in the warmer and arid areas of southern Mediterranean regions. While many analyses on the seasonal precipitation agree on an overall decreasing trend in spring-summer months, some other studies show conflicting results for the winter months depending on the considered region of the MB.

Since changes in precipitation extremes may have important consequences on the society, it is also worth to look at the impacts of future climate change scenarios on the upper tail of precipitation distributions. In this case, some works reported in Table 2 show a general increase of precipitation extremes expected in the future, especially by the end of the 21st century under the RCP 8.5. Almost all the MB will likely be subject to more severe precipitation, with an increase up to 30% in winter and a decrease up to -20% in summer of daily precipitation extremes, evaluated as the difference between the 99th and the 90th percentiles in both present and future conditions (Scoccimarro et al. 2016).

Table 2 Overview of the recent literature findings on future projections in precipitation over the MB
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3.2 Projections on the Future Hydrological Response to Climate Change

Mediterranean regions are expected to be particularly vulnerable to the foreseen increase in AET, with important implications on vegetation water stress, water availability, and soil salinization (Dimitriadou and Nikolakopoulos 2021b, Pumo et al. 2010). Tramblay et al. (2018) evaluated climate change impacts on AET and, in turn, on surface water resources for the largest artificial basins in Algeria, Morocco, and Tunisia, finding out an increase in RET and a decrease in AET, with a consequent decrease of water availability.

The atmospheric evaporative demand in the MB, often analyzed in terms of PE and/or PET, will likely rise in future due to the consistently warmer conditions. For instance, by the end of the century and under the RCP8.5, increasing trends in the mean annual PET values are predicted to increase up to 21% in France (Lemaitre-Basset et al. 2022) and up to 17% in Northern Tunisia (Dakhlaoui et al. 2020). As a consequence of the increase in annual PET for 2070–2100, Lemaitre-Basset et al. (2022) predicted a reduction in annual runoff over France, while Dakhlaoui et al. (2020) projected a decreasing discharge in Northern Tunisia over mid-term (2040–2070) and long-term (2070–2100).

Many studies, whose details can be found in Table 3, confirmed that mean annual streamflow/runoff and, more in general, water availability in the Mediterranean region are overall projected to decrease in the future. Significant alterations are also projected for streamflow seasonality and extreme flows in the MB. In particular, the occurrence of extreme climate events, such as droughts, is expected to increase in the future, particularly in semi-arid regions of the MB (Tramblay and Hertig 2018).

Many other studies, instead, are concordant in predicting an intensification of low flows in summer and more irregular discharge in winter (García-Ruiz et al. 2011). River runoff, especially during the summer, and low flows are, in fact, expected to decrease in most Mediterranean locations due to reduced precipitation (Marx et al. 2018, Yeste et al. 2021).

With reference to floods in the MB, a change in both intensity and timing is expected. Specifically, the flooding frequency is projected to increase in 2050 and beyond (Arnell et al. 2016), even if, in many studies, systematic differences between projections of changes in flood hazard are present in Italy, Greece and Iberian Peninsula; these difference can be attributed to the use of large-scale hydrological models (Kundzewicz et al. 2017) with different climate model types, scenarios and downscaling approaches. Statistically significant increase in flood risk was found in most regions under specific warming levels of 1.5, 2, and 4° C (Alfieri et al. 2017). In some areas of the MB, the frequency of 10-yr return flood estimated at the end of the 20th century is expected to increase, with such return floods then occurring every two years in the middle of the 21st century (Quintana-Seguí et al. 2011). Conversely, decreases of the order of − 10% on average are projected in some Mediterranean regions, such as the Iberian Peninsula and the Balkans (Thober et al. 2018). More intense floods are expected to occur up to 14 days earlier per-decade in some areas, such as north of Italy, south of France, and eastern Greece, and 1 day per-decade later in other areas, such as the northeastern Adriatic coast, eastern Spain, the south of Italy, and Greece (Alrteimei et al. 2022, Blöschl et al. 2017).

As for the surface runoff generation, also groundwater recharge processes will be likely highly affected by climate change. According to the AR6 WGI, groundwater recharge in Mediterranean regions, especially in southern areas, is projected to decrease due to a predicted decrease in precipitation and an increasing evaporative demand (Cramer et al. 2018, Guyennon et al. 2017, Koutroulis et al. 2016). Overall, projected groundwater recharge could either increase or decrease, mainly depending on groundwater depletion and over-abstraction, due to the projected irrigation expansion and increased evapotranspiration in a warmer climate (Condon et al. 2020).

Table 3 Overview of the recent literature findings on future projections in evapotranspiration and surface water over the MB
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4 Conclusion

Climate change poses an important challenge for our society, as it obliges us to adapt our actions to efficiently manage future and likely more limited water resources and the risks associated with a likely future intensification of the hydrological cycle. This review of the most recent works across the regions bordering the Mediterranean Sea, including portions of the southern Europe, southwestern Asia, and northern Africa, demonstrates how actually the Mediterranean basin can be considered a hot spot for changes in most of the key climatic and hydrological components (Lionello et al. 2012).

Historical trends and future projections in the Mediterranean basin, despite characterized by unavoidable uncertainties (Noto et al., 2022), show clear climate change signals and depict an increasingly warmer and drier scenario for the future. This happens even under the most optimistic greenhouse gases emission scenarios, with relevant modifications also in the seasonality and in some characteristics, as magnitude and frequency, of extreme values for rainfall regimes. As it emerges from many studies, total rainfall reduction will affect all seasons, while heavy rainfall events are likely to intensify especially in the late summer and the autumn, thus increasing the risk for floods as well.

In the Mediterranean region, evapotranspiration processes and, therefore, surface water and groundwater levels variability are mostly governed by precipitation. Many studies agree in predicting, on annual basis, an increasing atmospheric evaporative demand in the Mediterranean basin in response to the temperature increment, while actual evapotranspiration will likely decrease due to a reduced soil water availability that constrains the evapotranspiration process.

The projected reduction in precipitation on annual basis will affect water resources in the Mediterranean basin, declining water availability from both natural and artificial surface water bodies and groundwater. Moreover, besides the trends in water availability, several studies project a strong increase in water demand, use, and consumption for the Mediterranean basin due to the population growth and economic development, indicating a higher water stress in the future, as it is discussed in the companion paper (Noto et al., 2022).

In order to face the new challenges that climate change is posing to the Mediterranean regions, it is crucial to develop appropriated and coordinated strategies to improve our understanding of the climate and its changes, starting from the enhancement of the monitoring systems, which must be capable to efficiently integrate satellite observations, ground-based data and forecast models to monitor and forecast changes in the climate.