Page 79 - FAO-IPCC Expert meeting on climate change

57
Appendix 01: Speakers’ summary notes
Freshwater availability and water scarcity: Projected effects on agricultural water scarcity
TIM HESS, CHLOE SUTCLIFFE & DAVID HARO, CRANFIELD UNIVERSITY, UK
PLENARY SESSION 4:
HUMAN-DIRECTED IMPACTS ON WATER SCARCITY, BIODIVERSITY AND IMPLICATIONS FOR FOOD SECURITY
Irrigation is key for global food supplies, and its importance is set to grow in future as demand for food increases, whilst the reliability of yields from rain-fed agriculture is projected to decline due to climate change. The global population is predicted to increase to 11.2 billion by 2100 (United Nations Department of Economic and Social Affairs Population Division, 2015), and to meet requirements food production will need to also increase (Alexandratos & Bruinsma 2012). Yet, even contemporary water resources in many global regions are already overstretched and two thirds of the global population are living under conditions of severe water scarcity for at least one month during the year (Mekonnen & Hoekstra 2016). For example, presently, aquifers where extraction is estimated to exceed recharge include the Ganges and the Indus Basin in South Asia, the Californian Central Valley Aquifer System in the United States, and the North China Aquifer System and the Tarim Basin in China (Richey et al. 2015). These are also areas where the timing of water supplies for agricultural production is an equally important consideration for sustainable agricultural production, with seasonal water shortages that coincide with peaks in summer demand causing concern in many important agricultural areas.
In the absence of climate change, water scarcity is projected to grow considerably due to beinggrowing human pressure on water resources (Gosling & Arnell 2016). The hydrological impacts of climate change may either mitigate or exacerbate the water scarcity (Haddeland et al. 2014). It is therefore essential that the potential impacts of climate change on water availability for agriculture are well understood so that adaptive water management measures can be put in place (Döll et al. 2014) .
Water availability for irrigation is determined by precipitation, and there is strong agreement between climate models that global mean precipitation will increase linearly as global temperatures rise (IPCC, 2014). However, there will
be major changes to the geographical distribution and timing of rainfall and snowmelt, and scaling down to the regional level, there is much less certainty about how future precipitation will be distributed (Schewe et al. 2014). Climate change will also impact water resources via other mechanisms, including the effects of temperature and atmospheric CO2 concentration on evapotranspiration from plants, and changes to the partitioning of excess rainfall between surface runoff and groundwater recharge stemming from changes in rainfall timing, intensity and soil surface conditions.
Climate change impacts on the hydrological system and water availability will undoubtedly be far-reaching, but it is difficult to predict with confidence what the outcomes will be at the regional scale because of high uncertainty within downscaled precipitation projections and the additional uncertainty stemming from the incorporation of hydrological models (Haddeland et al. 2014).
A review was carried out of 40 articles and conference proceedings published since the IPCC Fifth Assessment Report (published in 2012 and 2013). There was greatest coverage in Europe (9 articles), followed by South Asia and East Asia (both 6), the Middle East (5), North Africa (4), sub-Saharan Africa (4), South America (3), Central America (2), Southeast Asia (1) and Australasia (1). Five articles with global coverage were also identified. There was notably greater coverage of projections with regard to surface water impacts (33) than those which expressly concerned estimations of groundwater (7).
Global precipitation and therefore runoff is expected to increase overall as a result of climate change, and there is strong agreement that discharge will increase at high northern latitudes, eastern Africa and on the Indian peninsula, and that reductions in water resources are to be expected in mid-latitude regions including the Mediterranean and large parts of North and South America (Schewe et al. 2014). However, regional projections remain highly uncertain in many areas in terms of both the magnitude and even the sign of the change. Despite a mean intensification in global hydrology, the studies mainly concurred that climate change would, in most locations, result in reduced water availability due to amplified temporal-spatial variability of water supply (Leng et al. 2015) , including in areas where production from irrigated agriculture plays a major role in global food security.
FAO-IPCC Expert meeting on climate change, land use and food security