Are We Running Out of Water?

How climate change is threatening the global water cycle

Only 3% of the water on the Earth’s surface is freshwater. Less than 0.5% of that is accessible for consumption as drinking water. If no urgent action is taken, an increasing number of cities worldwide are expected to experience severe water shortages. Recent analyses by the BBC ranked global cities such as Beijing, Tokyo and London among the those most likely to run out of drinking water in the near future.

Climate change is expected to severely alter the quantity, quality and spatial distribution of global water resources. Warmer temperatures increase evaporation, change the holding capacity of moisture in the air and alter rainfall patterns. The most recent IPCC report concluded that, in general, wet regions will get wetter and dry regions will get drier. Increases to the frequency and intensity of extreme events like droughts and heatwaves will also contribute to water stress.

A Changing Water Cycle

A ground-breaking study on the impacts of climate change to groundwater resources was recently published in Nature Climate Change. The study showed that groundwater stored in aquifers, which provides 36% of the world’s domestic water supply for over 2 billion people, is highly sensitive to future climate change.

Groundwater is stored in underground aquifers that are replenished by rainfall and soil moisture. Researchers found that 44% of all aquifers globally will be fully impacted and depleted as a result of climate change in the next 100 years due to changes in the intensity and pattern of rainfalls. Underground water reserves in drier regions are naturally slow at adjusting to above-ground atmospheric and climactic changes, but over-abstraction and other impacts of extreme drought may still exacerbate regional water stress.

Figure 1 Groundwater response time: measure of the time in number of years it takes a groundwater system to re-equilibrate (replenish and discharge into rivers, lakes or streams) to changing conditions

Another recent study concluded that total water storage in landlocked river basins has declined significantly over the past few decades. Using gravity satellite observations from the NASA GRACE satellite, researchers calculated that water storage has declined by 100 billion tonnes per year. An event that attributable to climate change and unsustainable water management. Given that most of the landlocked basins are in arid regions, there are significant implications to regional water stress.

A consequent impact of water storage decline is its contribution to sea level rise. Because of conservation of mass in the earth system, water lost in landlocked basins impacts global sea level through changes to the water vapour flux. Water loss in landlocked river basins accounted for about 10% of global sea level rise observed in the past 10 years.

Figure 2 Total water storage change in mm per year in global landlocked river basins

Why This Matters

There is a growing consensus around the idea that anthropogenic climate change is already significantly changing the global water cycle and that the sustainability of freshwater sources is being compromised.

Urbanisation and an exponential increase in freshwater demand for new households are driving factors behind water scarcity, especially in regions with a precarious water supply. Cape Town, the first modern city to effectively run out of drinking water in 2018, has suffered because of the confluence of extreme drought, poor water resource management and over-consumption. Pipes were dry and thousands were left queuing for drinking water.

Disruptive technologies like artificial intelligence and machine learning may hold the key for new and bold solutions. Smart hand-pumps that leverage AI to analyse groundwater use and predict pump failures has been experimented in rural Kenya resulting in water use optimisation and thus reducing wasteful dispersion of this increasingly precious, liquid gold.


Cuthbert, M. O., Gleeson, T., Moosdorf, N., Befus, K. M., Schneider, A., Hartmann, J., & Lehner, B. (2019). Global patterns and dynamics of climate–groundwater interactions. Nature Climate Change

Wang, J., Song, C., Reager, J.T., Yao, F., Famiglietti, J.S., Sheng, Y., MacDonald, G.M., Brun, F., Schmied, H.M., Marston, R.A. and Wada, Y., 2018. Recent global decline in endorheic basin water storages. Nature Geoscience

BBC News 2018. ‘The 11 cities most likely to run out of drinking water – like Cape Town’, <>, 11 February 2018

OxWater Ltd ‘What is a smart handpump?’ <> University of Oxford, 2018

This article was written by Wilson Chan, MSc Climate Change graduate at University College London and research assistant at the University of Hong Kong