There are already many countries where the scarcity of water affects people’s lives. While water for drinking may be first to come to mind, as agriculture is the largest use of water worldwide water scarcity first and foremost is a threat to food supply. And as many industrial processes rely on water availability, it also hampers economic development.

In the simplest sense, water scarcity is supply falling short of demand. Demand for freshwater will increase in most regions of the world due to population growth. Between eight and ten billion people are expected to live on Earth in 2050, as opposed to six billion today. But as for supply, water resources will be affected by projected climate changes due to unabated greenhouse gas emissions, for instance through changes in the amount, pattern and timing of rainfall and evaporation.

While rising demand is hard to end, climate change could still be contained. This is why our study, published in the Proceedings of the National Academy of Sciences, tried to identify the relative impact of climate change on water scarcity. The effect turned out to be substantial. Assembled by an international team of researchers, simulations using a large range of hydrological models suggest climate change is likely to put about 40% more people at risk of absolute water scarcity than would be the case without climate change.

From scarcity to absolute scarcity

This is not a far-off future. The steepest increase of global water scarcity might happen before global warming reaches 3°C above pre-industrial levels. Which means the internationally adopted target of limiting warming to 2°C is very relevant to global water resources. But as the world seems largely set on “business as usual”, we are set to burst through that 2°C limit within a few decades. Today, between one and two people out of a hundred live in countries with absolute water scarcity. Allowing for a 3°C rise in temperature and rising population, this could increase to ten in a hundred.

UN/World Water Development Report 4

Absolute water scarcity is defined as less than 500m3 per person, per year. Some industrialised countries can cope with such a small amount through advanced water management and processing techniques. But in many others, the necessary infrastructure simply isn’t there, and absolute water scarcity puts lives and livelihoods at risk.

The planet’s changing water cycle

The impact of climate change is not uniform across the world, and there will be large differences from region to region. For example, the Mediterranean, Middle East, the southern states of North America, and southern China will very probably experience a pronounced drop in available water. In contrast, Southern India, western China and parts of Eastern Africa may see substantial increases in average water resources from increasingly tropical, wet weather. This could be beneficial if the extra water comes at the right time and if the means to use and manage it is in place. If not, it may just add to problems from water-logging, flooding, and causing sewage and drain systems to fail – something that happens regularly in Europe even now. The overall risks are thus growing along with the rapid changes in the global water cycle.

And these changes are still far from completely understood. Our study is part of an unprecedented worldwide scientific effort to more systematically assess the impact of climate change. We used a suite of hydrological models, fed with different future climate scenarios. But all these models show a similar trend; while the numbers differ, reflecting the underlying assumptions, water is increasingly scarce. Evaluating the differences between the model simulations will help us better understand what lies ahead.