The world has plenty of water but not where people actually live. Arid regions lack both water and the energy to make water potable. Ironically, the energy needed to make water potable or ship it to arid regions will result in greenhouse gas emissions that result in less water, according to a new paper.

The authors write that current targets for greenhouse gas emissions - which would set the mean temperature increase at around 3.5°C above pre-industrial levels - will expose 668 million people worldwide to new or aggravated water scarcity. 

The authors calculated that a further 11 percent of the world's population, taken from the year 2000, will live in water-scarce river basins or, for those already living in water-scarce regions, find that the effects will be aggravated. People in the Southwest of the USA, southern Europe, the Middle East and North Africa will experience the most significant changes, they contend.

If the global mean temperature increases by 2°C – the internationally agreed target – their model suggests that 8 percent of the world population (486 million people) will be exposed to new or aggravated water scarcity.

Threshold level of ΔTg leading to significant local changes in water resources (a) and terrestrial ecosystems (b). (a) Coloured areas: river basins with new water scarcity or aggravation of existing scarcity (cases (1) and (2), see section 2.3.1); greyish areas: basins experiencing lower water availability but remaining above scarcity levels (case (3)); black areas: basins remaining water-scarce but without significant aggravation of scarcity even at ΔTg = 5 °C (case (4)). No population change is assumed here (see figure S5 available at maps including population scenarios). Basins with an average runoff <10 mm yr−1 per grid cell are masked out. (b) Regions with severe (coloured) or moderate (greyish) ecosystem transformation; delineation refers to the 90 biogeographic regions. All values denote changes found in >50% of the simulations. Credit and link: 

Lead author of the research Dr Dieter Gerten, from the Potsdam Institute for Climate Impact Research, said, "Our global assessments suggest that many regions will have less water available per person. Even if the increase is restricted to 2°C above pre-industrial levels, many regions will have to adapt their water management and demand to a lower supply, especially since the population is expected to grow significantly in many of these regions.

"The unequal spatial pattern of exposure to climate change impacts sheds interesting light on the responsibility of high-emission countries and could have a bearing on both mitigation and adaption burden sharing."

According to Gerten, the main driver of new or aggravated water scarcity is declining precipitation; however, increased temperatures will also lead to an increase in evapotranspiration of water and, thus, decrease the resources.

The anticipated increase in population will have even stronger effects on the ratio of water demand and water availability in some regions.

To assess the impacts of different mean global warming levels, the international group of researchers combined existing simulations from 19 climate change models with eight different global warming trajectories. The latter ranged from 1.5°C to 5°C increases above pre-industrial levels, resulting in a total of 152 climate change scenarios that were examined.

In addition to water shortages, the researchers assessed the impact that future climatic changes may have on global terrestrial ecosystems. They sought to discover what areas will be affected by strong ecosystem changes, and whether these areas are rich in biodiversity and/or contain unique species.

"At a global warming of 2°C, notable ecosystem restructuring is likely for regions such as the tundra and some semi-arid regions. At global warming levels beyond 3°C, the area affected by significant ecosystem transformation would significantly increase and encroach into biodiversity-rich regions," continued Gerten. "Beyond a mean global warming of 4°C, we show with high confidence that biodiversity hotspots such as parts of the Amazon will be affected."

Citation: Dieter Gerten, Wolfgang Lucht, Sebastian Ostberg, Jens Heinke, Martin Kowarsch, Holger Kreft, Zbigniew W Kundzewicz, Johann Rastgooy, Rachel Warren and Hans Joachim Schellnhube, 'Asynchronous exposure to global warming: freshwater resources and terrestrial ecosystems', Environmental Research Letters Volume 8 Number 3  doi:10.1088/1748-9326/8/3/034032