Page 80 - FAO-IPCC Expert meeting on climate change

Appendix 01: Speakers’ summary notes
Three continents stand out as hotspots for hydrological impacts on agriculture as climate change progresses; South Asia, East Asia and North America. These are areas where water resources for irrigation are already highly stressed; where human impacts on the water cycle are equal to, or exceed, the impacts that can be expected from moderate climate change (Haddeland et al. 2014); where aquifer withdrawals for agriculture equals or exceeds recharge values (Richey et al. 2015), and where the impacts of climate change will have far reaching implications in terms of affected populations and global food production.
The Indus Basin in South Asia is considered the basin most affected by human impacts on a global comparative scale (Haddeland et al. 2014). Streamflow is projected to increase in some areas (Mahmood & Jia 2016; Mathison et al. 2015; Narsimlu et al. 2013; Pechlivanidis et al. 2015; Roy et al. 2015) but the timing of increases is critical, with
some studies projecting increases to occur entirely in monsoon season and reductions to occur at other times (e.g., Narsimlu et al. 2013). This may serve to reduce the utility to agriculture of the extra streamflow and increase the risk
of flooding (Mathison et al. 2015). In South Asia implications for irrigated agriculture depend critically on abilities to harvest increased water resources where they occur, yet high uncertainty in projections means that investing in large- scale infrastructure projects carries with it heavy risk. One global study identified uncertainty in projections as being particularly high for South Asia and East Asia (Gosling & Arnell 2016), and another projected that freshwater scarcity
in the regions identified here as hotspots (Western United States, China and West, South and Central Asia) could force between 20 and 60 Mha of cropland to change from rain-fed to irrigated agriculture by 2100 (Elliott et al. 2014), hence decisions around how to undertake management of water resources in preparation for future climate impacts must be taken very carefully in order to avoid maladaptation. Ultimately the reduction in available water for irrigation is likely to translate increasing food prices on the global market (Haddeland et al. 2014).
Multi-model ensembles are increasingly used within climate and hydrological studies to produce a range of climate projections that provide an estimation of the degree of certainty according to model selection. However, instances wherein studies seeking to project the hydrological impacts of climate change have incorporated both climate and hydrological ensembles were scarce within the literature identified by this review, with recent studies on the whole tending to employ multiple climate models, but only one hydrological model. Where studies used both, large uncertainties
were identified as coming from both GCMs and GHMs (Schewe et al. 2014), with greater uncertainty being attributed
by one study to the GHM ensemble outputs (Elliott et al. 2014). Further uncertainty in terms of model outputs can
arise from the use of different measures to define water scarcity between projects (Gosling & Arnell 2016). Most of the hydrological models used did not include any estimation of the effect of CO2 concentrations on evapotranspiration rates (Döll et al. 2016). There is thus some suggestion arising from the findings of vegetation models that hydrological studies may be overestimating probable irrigation requirements and scarcity (Elliott et al. 2014), and one ensemble study using
an eco-hydrological model that did include this effect projected a decrease in global irrigation demand of around 17% (albeit with large increases in certain areas) (Konzmann et al. 2013). Future studies should seek to interrogate these discrepancies. Additional advances in hydrological impacts modelling could be achieved by developing a more nuanced understanding of the seasonality of impacts and by increasing the coherence between different studies in terms of both the measures of water availability and terminology for describing water resources that are used. All these issues speak to the need for advances in hydrological modelling to increase the robustness of outputs concerning climate change impacts on water resources, as well as for greater interdisciplinary efforts to identify low regret adaptation options that are suitable under conditions of uncertainty.
References
Alexandratos, N. & Bruinsma, J., 2012. World agriculture towards 2015/2030: The 2012 Revision. ESA Working Paper, No. 12- 03(12), p.147.
Döll, P. et al., 2014. Integrating risks of climate change into water management. Hydrological Sciences Journal, 60(1), pp.4– 13. Available at: http://www.tandfonline.com/doi/abs/10.1080/02626667.2014.967250.
Döll, P. et al., 2016. Modelling Freshwater Resources at the Global Scale: Challenges and Prospects. Surveys in Geophysics, 37(2), pp.195–221.
Elliott, J. et al., 2014. Constraints and potentials of future irrigation water availability on agricultural production under climate change. Proceedings of the National Academy of Sciences of the United States of America, 111(9), pp.3239–3244.
Gosling, S.N. & Arnell, N.W., 2016. A global assessment of the impact of climate change on water scarcity. Climatic Change, 134(3), pp.371–385.
FAO-IPCC Expert meeting on climate change, land use and food security