Will groundwater ease freshwater stress under climate change?

Today, groundwater is the source of about one third of global water withdrawals and provides drinking water for a large portion of the global population. In many regions it is subject to stress with respect to both quantity and quality. Hence, it is of utmost importance to improve our knowledge about the impacts of climate change on groundwater. Climate change will affect groundwater recharge, i.e. long-term average renewable groundwater resources, via increases in mean temperature, precipitation variability and sea level, as well as via changes in mean precipitation (increasing in some areas and decreasing in others). Over many areas groundwater recharge is projected to increase in the warming world (though less than river runoff), but many semi-arid areas that suffer from water stress already may face decreased groundwater recharge. The sea level rise that is likely to occur during the 21st century might leave many flat coral islands without a reliable groundwater source. However, in coastal areas with a land surface elevation of a few metres or more, groundwater availability is more strongly impacted by changes in groundwater recharge than sea-level rise. Under climate change, reliable surface water supply is likely to decrease due to increased temporal variations of river flow that are caused by increased precipitation variability and decreased snow/ice storage. Under these circumstances, it might be beneficial to take advantage of the storage capacity of groundwater and increase groundwater withdrawals. However, this option is only sustainable where groundwater withdrawals remain well below groundwater recharge. Groundwater is not likely to ease freshwater stress in those areas where climate change is projected to decrease groundwater recharge (e.g. Northeast Brazil and the Mediterranean basin).