Worldwide impacts: Water resources
Although overall global rainfall is expected to increase, climate change is likely to have negative impacts on water resources in many regions, through a combination of shifting rainfall patterns, melting glaciers and more evaporation. A warmer world will also lead to an intensified water cycle, with heavier bursts of rain punctuated by longer dry spells. These effects could increase water stress for many people around the world.
Rainfall changes are more difficult to predict than temperature changes, so scientists can’t be sure of the likely impacts in specific regions. But climate models can help scientists understand how rainfall could respond to higher global temperatures. Average yearly rainfall is expected to increase in both equatorial and high-latitude regions, although this will vary from season to season. In contrast, arid subtropical areas are expected to receive less rainfall. This could be amplified by a feedback effect, with reduced soil moisture making less water available for evaporation and cloud formation. Wet regions in general are likely to get wetter, while dry regions are likely to get drier.
As well as changing geographical patterns of rainfall, rising temperatures will lead to increased evaporation rates. This in turn leads to faster cloud formation and a larger amount of moisture being moved around the climate system – known as the ‘intensification’ of the water cycle. Scientists calculate that every degree of global temperature rise will lead to about a 7% increase in global rainfall, though not all regions will see an increase, and much of the extra rain will fall over the oceans. An intensified water cycle also means that more rain falls in heavier bursts, with a decrease in the amount of lighter rainfall and longer dry spells in-between heavy downpours.
Scientists predict that most wet regions are likely to receive higher annual rainfall as global temperatures rise. High-latitude areas such as northern parts of Europe, Asia and America are expected to see an increase in both winter and summer rainfall. Monsoon rainfall in parts of Asia, Africa and Australia could increase, and tropical areas such as South America and equatorial Africa could receive less rainfall in summer but more in winter. However, increased flooding could threaten freshwater supplies with contamination. Combined with more seasonal variability and faster evaporation, this means climate change is likely to have impacts on water resources in all regions – even where annual rainfall is predicted to rise.
Scientists predict that continued global warming could cause a general decrease in rainfall in areas that already have arid climates, particularly the Mediterranean and southern Africa. A 2 °C rise in global temperatures is expected to lead to about a 50% increase in the frequency of droughts in southern Africa and the Mediterranean. A rise of 4 °C or more could see droughts happening twice as often in these regions and in southeast Asia too, with a reduction of up to 70% in river runoff in some areas. Changing rainfall patterns and reduced runoff could have widespread impacts on water resources around the world, potentially increasing water stress for up to a billion people by the end of the century.
Many communities in India, China and South America rely on glaciers to provide fresh water. Water is stored in glaciers over the winter through snow accumulation and then released during the spring and summer melt season, boosting the flows of many rivers. More than one-sixth of the world’s population live in river basins fed by seasonal glacier and snow melt. As temperatures rise, faster spring melt rates should increase water availability in these regions. But scientists predict that further warming and shrinking glaciers will cause more water to be released earlier in the year – or possibly not stored as snow in the first place – resulting in reduced spring and summer water flows with consequences for drinking water supplies and irrigation.