The various sections of text above bring us closer to a consideration of future climate change in relation to human welfare. This is the main focus of the next chapter. This section, by briefly considering water scarcity, begins to build a bridge towards a more rounded view of the future impacts of all aspects of global change. It is important to bear in mind distinctions between meteorological (deficit of precipitation), agricultural (largely soil-moisture deficit) and hydro-logical (low lake-levels and river flow) definitions of drought (Trenberth et al., 2004), as well as to recall that a warming atmosphere (by increasing both evaporation and absolute humidity) can increase the chances of both drought and floods in the same, or neighbouring regions.
The record of past human migrations (Dillehay, 2002), welfare and societal collapse is rich in examples of economic and cultural decline coinciding with prolonged drought (Haberle and Lusty, 2000). While always acknowledging that the effects of environmental stresses on human populations are mediated by a whole complex of interacting social factors, and that it is therefore unacceptable to invoke climate alone as the cause of civlisation collapse (Rosen and Rosen, 2001; Dahlin, B. H., 2002; Shennan, 2003; Dillehay et al, 2004), the coincides are still impressive, confirming that in many situations drought has been one of the factors contributing to major declines in civilisations as diverse as the Maya (Hodell, 1995; Haug et al., 2003), Anasazi (Larson et al., 1996), Hohokam (Nials et al., 1989), Tiwanaku (Chepstow-Lusty et al., 1997) and prehistoric cultures in the Atacama and Andean Altiplano (Nunez et al., 2002) in the New World; likewise the Akkadian (Weiss et al., 1997; Weiss and Bradley, 2001) and Harrapan empires (Singh et al., 1990; Staubwasser et al., 2003), and groups in the east Mediterranean (Rosen, 1995), the Sahara (Hoelzmann et al., 2001; Nicoll, 2004), South Africa (Tyson et al., 2002) and China (Huang et al., 2003) in the Old. Continuous records of recurrent drought linked to periodic cultural decline (and vice versa) come from east Africa (Verschuren, 2000). At the present day, much of the world's population suffers from serious water stress in some form or other and over the next two decades, rising-water demands linked to population growth and economic development will play a much greater role than climate change in defining the status of water resources (Vorosmarty et al., 2000). All these considerations prompt a closer look at the projected future availability of water in relation to human needs. Few authorities doubt the size and urgency of the problem (see, e.g., Arnell, 1999; Parry et al., 2001; Anon, 2003a), though signs that it is beginning to generate effective global action, as distinct from further study, are all too sparse.
In view of the difficulties involved in modelling the spatial distribution of precipitation for the present day, itself only one part of the challenge, it is hardly surprising that the task of developing hydrological models that can be used for projecting the potential availability of water resources in the future is fraught with a multitude of uncertainties. Arnell's (1999) analysis illustrates well the problems involved when models generate widely different climate-change and water-demand scenarios. Some progress has been made by applying improved climate models to well characterised past drought events. Giannini et al.
(2003) show that the major twentieth-century droughts in the North American 'dust bowl' and in the Sahel region can be simulated in atmospheric models by specifying SSTs. In their analysis, ocean forcing is more important than changes in land cover, though the latter may generate feedbacks that prolong the drought.
Doll (2002) attempts to evaluate the impact of climate change and variability on irrigation. She uses a global model to compute irrigation requirements under a range of conditions. The research strategy is designed to make possible comparisons between the effects of 'climate change' (in the future), and those of 'climate variability' (in the recent past). The former is defined in terms of differences between the 'baseline' climate (1961-1990) and decadal simulations for the 2020s and 2070s. The latter is described in terms both of inter-annual variability over the timespan 1901 to 1995, and long-term multi-decadal variability, defined by comparing values for 1901-30 and 1931-60. The two GCMs used, HadCM3 and ECHAM4 generate rather different future projections, with the former indicating more extensive areas of reduced annual precipitation in north Africa, around the Mediterranean and in the Middle East. Despite the careful approach taken, uncertainties abound in addition to those already cited for simulations of present-day precipitation. They span the whole range from neglected processes in the climate models to uncertainties about future crops and their growing seasons, and about the effects of increased atmospheric CO2 on crop physiology. Moreover, only for the largest river basins is the horizontal resolution of the models used appropriate. Tentative overall conclusions include an indication that around two thirds of the area currently equipped for irrigation may suffer from increased water requirements and that for up to half the total area the negative impacts of 'climate change' are more significant than those of climate' variability', as both are defined above. The latter observation is somewhat conditional on how variability is defined. This is one of several papers comparing 'change' with 'variability' (see, e.g., Hulme et al., 1999). The very exercise highlights two limitations in much of the current research on future climate change. First, variability, whether termed 'natural' or not, is defined in terms of part or parts of the all too short instrumental record, much of which lies within the period of growing anthropogenic impacts. As we have seen in Section 7.8, proxy records for the late Holocene show that for many parts of the world, especially when we consider rainfall, hydrology and drought incidence, this short period fails to capture the full range of variability that has occurred, both on inter-annual and, more especially, multi-decadal timescales. Furthermore, change and variability should not be seen as competing concepts but rather as complex interacting processes that may sometimes act in mutually compensating ways, but may also be mutually reinforcing to the point of driving future changes over critical thresholds and generating strongly non-linear responses.
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