From the Amazon to the Nile to the Mekong, rivers are a lifeblood for many nations, filling taps and irrigation canals and generating hydroelectricity that is powering economic development. But a new study warns that changes to river flows caused by climate change threaten that. Thousands of hydrodams risk being left high and dry by mid-century as global warming takes hold.

On the face of it hydroelectricity seems an obvious antidote to climate change. Hydrodams are among the world’s largest power sources and free of carbon emissions. They could replace the burning of fossil fuels in dozens of countries, allowing economic development without booming emissions of greenhouse gases. Brazil, Egypt, China have led the way.

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But there is a problem. Future droughts risk leaving reservoirs empty. Michelle van Vliet of the Austria-based International Institute for Applied Systems Analysis, an influential east-west thinktank, has crunched the numbers to see how a range of standard climate models predict that river flows will change – and what this means for more than 24,000 hydroelectric power plants round the world. Her findings were published in Nature Climate Change this month.

The findings are stark. She concludes that “climate change will reduce the existing power capacities of hydropower in most regions worldwide”. Up to 74% of current hydrodams will likely see reductions in useable electricity-generating capacity by mid-century.

Where dams are built, the risks of financial disaster, human suffering and even water wars are significant

That is potentially a big threat to fast-growing economies like those in South America, which relies on hydropower for more than 60% of its electricity. Much of Africa, Australia, south-east Asia, the US and southern and central Europe can also expect significant declines in hydropower capacity, she warns.

The future is far from certain. Climate models disagree about the hydrological future for many river systems. Depending on which climate model you choose they may get more water – or less. Such uncertainty should make governments and investors nervous. Hydroelectric dams have a design life of 80 years and more. Betting on river flows remaining secure over that timescale is high risk. And where dams are built, the risks of financial disaster, human suffering and even water wars are significant.

Take the Nile. Egypt is heavily dependent on the river for both electricity from the High Aswan dam, and for irrigating its desert farms. Right now, upstream Ethiopia is building a giant new hydrodam, the Grand Renaissance dam, that Egyptians fear could jeopardise flows and threaten their power supplies.

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The two regional superpowers, along with the eight other nations on the world’s longest river, are trying to reach a deal for sharing the water. But any deal is likely to be based on current flows being maintained. Fat chance. Nobody has a clue what future flows of the Nile will be. The Intergovernmental Panel on Climate Change has reported that different model projections for the Nile for later this century range from 30% more water to 78% less.

Meanwhile the Niger, which flows through nine nations across west Africa, skirting the arid Sahel region, also faces risks of lost rains, just as Guinea and others are planning major dams that threaten water supplies in downstream countries such as Mali. Likewise, China, Laos and others are building dams on the Mekong, the lifeblood of south-east Asia, even though van Vliet’s study suggests the river will suffer reduced flows in future.

Some nations are heeding the warning signs of climate change. In South America, recent droughts in Venezuela and Brazil have dried up reservoirs and caused power outages. Partly as a result, Brazil is planning to reduce its reliance on hydropower in future.

But for many countries, the temptation to exploit hydropower as an alternative to fossil fuels is strong. China has grown its hydroelectric capacity hugely in recent years and, with plans to scale back coal burning, is likely to invest in further hydro expansion.

Hydroelectric dams are not the only electricity sources that require lots of water. Most power plants burning coal, natural gas or oil, as well as nuclear power plants, use water for cooling. Away from the coasts, that usually means taking water from rivers. But such plants face a double whammy, says van Vliet, because what water remains in the rivers will be hotter than today. So it won’t be so good at cooling. She finds that up to 86% of such power plants will see declining generating capacity as a result.

There have been early signs of this already, notably in 2003, during a European drought and heat wave whose intensity climate scientists say was due to global warming. At its height, flows on the Loire, France’s longest river and home to many nuclear power plants, were reduced to a trickle. French electricity generators had to shut or power down 17 reactors for want of cooling water.

River water is the little-noticed essential for much of our energy supply. As the climate changes, that looks increasingly like a problem.

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