General observations regarding scenarios future climate and water resources

The IPCC prepared five reports, the latest of which, in a preliminary version, was released in January 2007. The conclusions of this report most relevant to water resources and groundwater are (IPCC 2007):

• Projected warming in the twenty-first century shows geographical patterns similar to those observed over the last few decades. Warming is expected to be greatest over land and at the highest northern latitudes, and least over the Southern Oceans and parts of the North Atlantic Ocean;

• Snow cover is projected to contract. Widespread increases in thaw depth are projected over most permafrost regions;

• The more optimistic globally averaged rises in sea level at the end of the twenty-first century are between 0.18-0.38 m, but an extreme scenario gives a rise up to 0.59 m;

• It is very likely that hot extremes, heat waves and heavy precipitation events will continue to become more frequent; and

• Increases in the amount of precipitation are very likely at high latitudes, whereas decreases are likely in most subtropical land regions.

The IPCC scenarios (global and regional) are based on the results from Global Circulation Models (GCMs), traditionally considered by the IPCC to be the most reliable tools for obtaining indications regarding the future climate (Troen 1993; Kattenberg et al. 1996; IPCC 2007). Uncertainties conspire to make the model output, a rough approximation at best, of what could happen under various assumptions of greenhouse gases emissions (Covey 2003; Friedlingstein et al. 2003; Bender et al. 2006; Hegerl et al. 2006; Schmidt et al. 2004; Masson-Delmotte et al. 2006; van Ulden & van Oldenborgh 2006; Zhang et al. 2006; IPCC 2007; Schneider 2007). Future scenario outputs may even be contradictory (Rosenberg et al. 1999; Gagnon & Gough 2005; Stephenson et al. 2006; IPCC 2007; Kripalani et al. 2007; Li et al. 2007) and the results are averages over vast areas (IPCC 2007; Jacob et al. 2007; Ruosteenoja et al. 2007). By using Regional Circulation Models (RCMs), nestled within a GCM, one can arrive at averaged results (in terms of rainfall and temperature) for areas as small as 600 to 2500 km2, but the results often depend more on the choice of the initial GCM than on the choice of the emission scenarios (Hay et al. 2006; Graham et al. 2007; Ruosteenoja et al. 2007; Olesen et al. 2007). This is unsatisfactory for defining the impact of climatic variations on water resources, and for planning intervention strategies for mitigating the likely impacts.

The inadequacy of the GCMs suggests that other approaches, although empirical, should be used together with the GCMs. The 'analogue approach' gives information that is more specific than that given by the GCMs by reconstructing past climates (i.e. temperature and precipitation) in a given area. These reconstructions can be used to construct future scenarios by analogy. The analogue approach assumes that, if a given average temperature variation corresponded to a given variation in rainfall or in water resources in the past, a similar temperature variation in the future will cause similar effects. Thus it is accepted that this occurs regardless of the causes of the variations in temperature, which may also be of different types, such as variations in the solar constant or the concentration of greenhouse gases in the atmosphere. This assumption can only be maintained if the temperature variations being compared are similar and if they occur during similar atmospheric boundary conditions, i.e. during time periods in which the planet has cryo-sphere, oceans and land masses in similar conditions (Wigley et al. 1986; Dragoni 1998). Similarity can be accepted only if the 'palaeoanalogue periods' go back no more than a few millennia or, on a more detailed scale, if the palaeoanalogues consist of multi-annual instrumental series, such as those of the warm years at the beginning and the end of the twentieth century. Of course the future scenarios based on such palaeoanalogues are not quantitatively sound, and cannot be extrapolated confidently into the future beyond a few decades. Thus, despite the progress made by GCMs and by information obtained by the analogue approach, it must be recognized that a definition of scenario given about twenty years ago is still valid: 'scenarios are not meant to be predictions of future climate; rather they are meant to be internally consistent pictures of a plausible future climate, a basis for other workers to evaluate the possible impacts of climatic change on Man and society' (Wigley et al. 1986).

Data and information on past climatic and hydrological conditions are important for verifying whether a GCM or RCM is potentially reliable: only a model that provides good results for the present and/or for past climate can be reliably used for constructing future scenarios (Bell et al. 2003; Karl & Trenberth 2003; Dearing 2006; Sloan 2006).

Confidence is increased if one or more models and the analogue approach independently indicate a similar future scenario in terms of temperature and rainfall. Nevertheless, the actual intensity and spatial and time variability of rainfall and temperature for a given scenario and a given region still remain uncertain. The same degree of uncertainty is retained when translating rainfall and temperature to evapotranspiration, runoff and aquifer recharge, whatever procedure is adopted (Strzepek & Yates 1997; Di Matteo & Dragoni 2006).

Another consequence of the uncertainties intrinsic to the climatic scenarios is that the impact of the conditions provided by the scenarios on hydrogeo-logical systems are tentatively simulated for different, and more or less arbitrary values for the climatic factors (Rosenberg et al. 1999; Loaiciga et al. 2000; Taeuea et al. 2000; Nijssen et al. 2001; Yusoff et al. 2002; Allen et al. 2004; Gagnon & Gough 2005; Jha et al. 2006; Vicuna & Dracup 2007; Olesen et al. 2007). The results provided by this approach show the most probable direction of change, the sensitivity to different factors which regulate hydrological systems, and point to the processes which will be modified most by future climate variation.

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