Climate change and groundwater a short review

1Dipartimento di Scienze della Terra, Universita di Perugia, Piazza Universita 1, 06001 Perugia, Italy (e-mail: [email protected])

2National Geophysical Research Institute, Hyderabad-500 007, India (e-mail: [email protected])

Abstract: There is a general consensus that climate change is an ongoing phenomenon. This will inevitably bring about numerous environmental problems, including alterations to the hydrolo-gical cycle, which is already heavily influenced by anthropogenic activity. The available climate scenarios indicate areas where rainfall may increase or diminish, but the final outcome with respect to man and environment will, generally, be detrimental. Groundwater will be vital to alleviate some of the worst drought situations. The paper analyses the main methods for studying the relationships between climate change and groundwater, and presents the main areas in which hydrogeological research should focus in order to mitigate the likely impacts.

This article has two aims. The first is to present a summary of the current knowledge of the relationships between climatic variations and water resources, with emphasis on groundwater. The second aim is to review the main issues that ground-water specialists will have to face and study in order to minimize the impact of climatic variation and to protect groundwater resources.

The climate has changed in the past, is changing presently and will change in the future. The scale of the fluctuations varies from hundreds of millions of years to decades or less (for example Huggett 1991; Goudie 1994; Issar 2003; Lamy et al. 2006; Yang 2006). The present climatic trend (i.e. a warming trend), which is no longer a hypothesis but a planet-wide observation, may correspond to a natural warming phase, probably at the scale of a few hundreds years, which began in the nineteenth century; the warming is being accelerated and increased because of the anthropogenic release of greenhouse gases from fossil fuels burnt during the last two centuries. The main concern raised by global warming is that climatic variations alter the water cycle; indeed, in many cases, the data show that the hydro-logical cycle is already being impacted (Dragoni 1998; Buffoni et al. 2002; Labat et al. 2004; Huntington 2006; IPCC 2007).

Today there is a very large consensus, supported by an impressive set of observations and analyses, that anthropogenic activity is the main factor causing the present global warming (Trenberth et al. 2006; Kerr & Balter 2007). However, the Intergovernmental Panel on Climate Change (IPCC), does not provide total certainty to this view, but only indicates a probability greater than 90% (IPCC 2007) and a few recent papers raise some doubts about the driving role of greenhouse gases (de Jager & Usoskin 2006; Stanhill 2007; Svensmark 2007). Indeed, a heated dispute is going on, as there is a minority of scientists who claim that the main reason for the present climatic behaviour is natural (sun variability being the most probable) and that, very likely, the future warming will be moderate (Essex & McKitrick 2003; Landscheidt 2003; Santer et al. 2004; Michaels 2005; Singer & Avery 2006; IDAG 2005; Shaviv 2005; Scafetta & West 2006; Zastawny 2006; Lockwood & Fröhlich 2007). This issue is critical, because the worst possibilities considered by the IPCC indicate that the temperature will rise by several degrees and the warm phase will last for centuries, with dramatic consequences beyond those that can reasonably be defined at present. In any case, today, there is an unanimous consensus on the forecast that the warming will persist for decades, no matter what action is taken (Michaels 2005; Singer & Avery 2006; Trenberth et al. 2006; IPCC 2007). As the warming process continues, it will bring about numerous environmental problems, among which the most severe will relate to water resources (Loai-ciga 1996, 2000; Milly et al. 2005; Holman 2006; IPCC 2007).

The magnitude of future consequences can be inferred from the dramatic effects caused by the natural and 'moderate' climatic changes that occurred during the last millennium, during which millions of deaths all over the world were caused directly by the alternation of droughts and short cool-warm periods (Lambe 1977; Goudie 1994; Dragoni 1998; Brown 2001; Fagan 2001; Davis 2002). The development (and in some cases the

From: Dragoni, W. & Sukhija, B. S. (eds) Climate Change and Groundwater. Geological Society, London, Special Publications, 288, 1-12.

DOI: 10.1144/SP288.1 0305-8719/08/$15.00 © The Geological Society of London 2008.

disappearance) of many civilizations was determined by natural and 'moderate' climatic change (Stewart 2005; Brooks 2006; Cremaschi et al. 2006; Kumar et al. 2006; Issar & Zohar 2007). Clearly a comprehensive knowledge of climate variations in space and time is vital in order for human society to adapt and survive. The key issues in the study of climate change (Oldfield 2005) are:

(i) what will be the amplitude and rate of global climate change over the next century and beyond;

(ii) how will the global mean climate be expressed in terms of extreme droughts and floods, sea level changes, groundwater recharge, soil degradation, deforestation, loss of biodiversity, and changes in ecosystem functioning, especially in view of the human-induced greenhouse effect; and

(iii) how do the complex changes involved affect the key issues of vulnerability and sustaina-bility of water resources for the human population in general and groundwater in particular.

The importance of the relationship between groundwater and climatic change cannot be overstated. The global volume of groundwater is estimated at between 13% and 30% of the total volume of fresh water of the hydrosphere (Jones 1997; Babklin & Klige 2004) and groundwater provides 15% of the water used annually (Shiklomanov 2004b), the remainder being from surface water. Aquifers mitigate droughts as they have a high storage capacity and are less sensitive to climate change than surface water bodies. Surface water baseflow is, of course, groundwater discharging from store.

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