by Shelby Long
The production of food and economic prosperity are highly dependent on a sufficient water supply. Both the demand and supply for water are affected by climate change-induced adjustments of precipitation patterns, temperature, other climate variables, and shifts in population. Much uncertainty remains among climate change models regarding how precipitation levels and patterns, as well as temperature, will change (Meehl et al. 2007). Also, precipitation changes affect other hydrological variables, such as surface or subsurface runoff and river discharge, and, therefore, effective hydrological and climate change modeling takes these variables into account. Schewe et al. (2013) use multiple global hydrological models (GHMs) and greenhouse-gas concentration scenarios to examine how climate change impacts global water resources. They determined that climate change is likely to intensify regional and global water scarcity. They project that a 2°C increase in global temperature will result in approximately 15% more of the global population experiencing a severe decrease in water resources. They also project this 2°C rise in temperature to increase the number of people living under absolute water scarcity by at least 40%, while some models predict a 100% increase. Along with climate change, they expect future population growth to further exacerbate the water scarcity problem. They combined results from 11 global hydrological models forced from 5 global climate models (GCMs). Of the 11 global hydrological models they included one dynamic global vegetation model and one global land-surface model. They calculate water scarcity by dividing the sum of “blue” water (BW) by the country’s population, which equals the water crowding index. “Blue” water is defined as runoff redistributed across the river basin based on the distribution of discharge. They then summed the number of people living in countries below a given threshold of the index and divided it by the global population to calculate the percentage of the world population. In order to examine the effect of water discharge changes on the spatial distribution of population, they used population projections from the newly-developed Shared Socioeconomic Pathways (SSPs). For their analysis, they assumed a global population of 10 billion by the year 2090. Water scarcity was calculated annually and then averaged over 31-year periods. Schewe et al. calculated global mean temperature (GMT) from the GCM data. The GMT is given as the difference from the 1980-2010 baseline period average. They selected 31-year periods for each GCM whose average temperature correlates to the different levels of global warming.
Based on the multimodel assessment of water scarcity, Schewe et al. determined that a 2 °C increase in temperature would result in an increase in discharge at high northern latitudes. More specifically, these increases are projected to take place in eastern Africa and on the Indian peninsula. The models project a decrease in discharge in the Mediterranean region and large parts of North and South America. Under a 3 °C increase scenario, the models indicate similar but more enhanced increases and decreases in discharge for the particular regions. In other global regions, there is variation, or a large spread, between the discharge projections made by the GCMs and GHMs. Bias correction was applied to the GCM data, which greatly reduced their spread for present-day climatologies, but not so much for their future temperature and precipitation trends.
Schewe et al. examined how the projected discharge changes will affect the spatial distribution of population in the future. They applied two criteria for a severe decrease in average annual discharge. The first is a reduction in average annual discharge by more than 20%, and the second is a reduction by more than one standard deviation of the 1980-2010 annual discharge. The multimodel median (MMM) indicates that a 1, 2, and 3 °C increase in temperature will result in a severe reduction in water resources by at least one of the criteria for 8%, 14%, and 17% of the global population, respectively. When only one scenario is applied, a 2 °C increase results in 13% of the global population being projected to experience a reduction in discharge. There is a large spread across the model projections. For some of the regions that are expected to receive the strongest relative reduction in discharge, the GHM variance is larger than GCM variance. Also, the spread among GHMs is similar to or larger than the spread across GCMs. Schewe et al. determined that the two models in the study that examined vegetation distribution and dynamics, typically yield smaller reductions in water resources than most hydrological models. This difference in results is indicative of the systematic dissimilarities between the two types of models. Researchers determined that more people will be impacted by increases in discharge than by decreases. Although increases in discharge may improve the availability of water, it can also increase flood risk, deteriorate water quality, and lead to the malfunctioning of water-related infrastructure (Kundzewicz et al. 2008).
Schewe et al. consider the percentage of global population in two water scarcity classes. The two classes are “blue” water availability below 500 m3 per capita (absolute water scarcity)and below 1,000 m3 per capita (chronic water scarcity). It is estimated by the MMM that currently approximately 1.5% of the global population falls under absolute water scarcity and approximately 3% falls under chronic water scarcity. Population growth affects water scarcity, but climate change further exacerbates the problem. Based on the MMM, the level of future water scarcity is amplified by climate change by 40% for the <500 m3 class under a 1 °C and 2 °C temperature. Under the 3 °C temperature increase scenario, water scarcity is amplified by climate change by only 25%, which indicates that the additional climate change has a smaller effect in conjunction with population changes than under the 1 and 2 °C increase. Therefore, both population growth and climate change can lead to increased water scarcity. For the <1,000 m3 class, water scarcity is amplified by climate change by 30%, 20%, and approximately 0% for the 1, 2, and 3 °C scenarios, respectively. Under these scenarios, they determined that climate change is predicted to alleviate the population change-induced increase in water-scarce populations. Schewe et al. stress that future water scarcity is strongly affected by economic growth, lifestyle changes, technological developments, water management practices, water infrastructure, and seasonal availability of water resources.
Schewe, J., Heinke, J., Gerten, D. et al., 2013. Multimodel assessment of water scarcity under climate change. PNAS doi: 10.1073/pnas.1222460110. http://bit.ly/WGdaVW