Urban growth, climate change, and freshwater ecosystem services

Background/Question/Methods

Nearly 3 billion additional urban dwellers are forecasted by 2050, an unprecedented wave of urban growth. While cities struggle to provide water to these new residents, they will also face equally unprecedented hydrologic changes due to global climate change. Here we use a detailed hydrologic model, demographic projections, and climate change scenarios to describe how freshwater ecosystem services crucial to urban dwellers may change. We present a conceptual framework of urban water provision as composed of three axes: water availability, water quality, and water delivery. For each axis we calculate quantitative measures for the world’s cities, and then briefly discuss the strategies cities are using in response if they are deficient on one of the axes. We end by discussing the need for a global City Water Map, to better enable quantification of freshwater ecosystem service values for cities.

Results/Conclusions

We show that globally 523 million people are in cities where water availability may be an issue, 890 million people are in cities where water quality may be an issue, and 1.3 billion people are in cities where water delivery may be an issue. Modeled results for water availability for the largest cities in the developing world indicate that currently 150 million people live in cities with perennial water shortage, defined as having less than 100 liters/person/day of sustainable surface and groundwater flow within their urban extent. By 2050, demographic growth will increase this figure to almost a billion people. Climate change will cause water shortage for an additional 100 million urbanites. Freshwater ecosystems in river basins with large populations of urbanites with insufficient water will likely experience flows insufficient to maintain ecological process. Freshwater fish populations will likely be impacted, an issue of special importance in regions such as India’s Western Ghats where there is both rapid urbanization and high levels of fish endemism.  Tapping into groundwater is a widespread response, regardless of the management challenge, with many cities unsustainably using this resource.