The effectiveness of population policy in reducing the combined impacts ofpopulation change and climate change on water resources is explored. Oneno-policy scenario and two scenarios with population policy assumptions areemployed in combination with water availability under the SRES scenariosA1b, B1 and A2 for the impact analysis. The population data used are from theWorld Bank. The river discharges per grid of horizontal resolution0.5° are obtained from the Total Runoff Integrating Pathways(TRIP) of the University of Tokyo, Japan. Unlike the population scenariosutilized in the SRES emission scenarios and the newest representativeconcentration pathways, the scenarios employed in this research are based,even after 2050, on country-level rather than regional-level growth assumptions.Our analysis implies that the heterogeneous pattern of population changesacross the world is the dominant driver of water stress, irrespective offuture greenhouse gas emissions, with highest impacts occurring in thealready water-stressed low latitudes. In 2100, Africa, Middle East and partsof Asia are under extreme water stress under all scenarios. The sensitivityanalysis reveals that a small reduction in populations over the region couldrelieve a large number of people from high water stress, while a furtherincrease in population from the assumed levels (SC1) might not increase thenumber of people under high water stress considerably. Most of thepopulation increase towards 2100 occurs in the already water-stressed lowerlatitudes. Therefore, population reduction policies are recommended for thisregion as a method of adaptation to the future water stress conditions.Population reduction policies will facilitate more control over their futuredevelopment pathways, even if these countries were not able tocontribute significantly to greenhouse gas (GHG) emission cuts due to economic constraints.However, for the European region, the population living in water-stressedregions is almost 20 times lower than that in the lower latitudes.For countries with high population momentum, the population policy scenariowith fertility-reduction assumptions gained a maximum of 6.1 times the wateravailability in Niger and 5.3 times that in Uganda compared with theno-policy scenario. Most of these countries are in sub-Saharan Africa. Thesecountries represent 24.5% of the global population in the no-policyscenario, and the scenario with fertility-reduction assumptions reduces itto 8.7% by 2100. This scenario is also effective in reducing the areaunder extreme water stress in these countries. However, the policy scenariowith assumptions of population stabilization at the replacement fertilityrate increases the water stress in high-latitude countries. Nevertheless,the impact is low due to the high per capita water availability in theregion. This research is expected to widen the understanding of the combinedimpacts of climate change in the future and of the strategies needed toenhance the space for adaptation.
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