The 1320 MW Glen Canyon Dam in northern Arizona in the United States generates power for residents of seven states
[1]. But in February 2023 the dam’s ability to deliver that electricity was in danger. After more than two decades of drought, the water in Lake Powell, the reservoir behind the dam, fell to a record low of 1074 m above sea level (
Fig. 1)
[2]. If the water level went down another 10 m, the dam might stop producing power; if the level decreased to 1027 m, water would stop flowing through the dam altogether
[2].
Catastrophe was avoided this time. By May 2024 precipitation had increased the reservoir’s water level to about 1085 m above sea level
[3]. Still, the lake remains less than half full
[4], and the dam’s power output has declined by more than 50% since the 1990s
[5].
Lake Powell is a prime example of the troubles affecting hydropower facilities as the climate changes. Even as countries are looking to hydropower to provide more low-carbon electricity, the effects of climate change, such as more severe droughts and reduced runoff, are undermining the ability of hydropower facilities to generate electricity
[6]. Researchers cannot predict what all the impacts will be, but studies so far suggest that climate change could upend arrangements for electricity generation around the world
[7],
[8]. In addition, rising temperatures threaten hydropower and other power infrastructure because they can unleash disastrous floods, landslides, and debris flows
[9]. “Climate change is going to affect all parts of the world’s electricity supply system, but it will affect hydropower the most,” said Ranjit Deshmukh, assistant professor of environmental studies at the University of California, Santa Barbara, CA, USA.
Still, experts say a variety of measures might reduce climate change’s impact on electricity generation, including hydropower. “Hydropower output will decrease in most areas, or it will become less reliable,” said Giacomo Falchetta, a research scholar in the energy, climate, and environment program at the International Institute for Applied Systems Analysis in Laxenburg, Austria. “But we can mitigate those effects, at least in part.”
Almost 6% of the United States’s electricity, about 249 TW·h per year, comes from hydropower
[10], but other countries rely on it more. The roughly 1300 TW·h China generates annually makes it the world’s largest hydropower producer and accounts for about 15% of the country’s electricity supply
[11]. Paraguay tops the list of hydropower-dependent countries, with 99.7% of its electricity coming from dams
[11]. The global amount of hydropower capacity has been growing rapidly, climbing by 538 GW between 2001 and 2020
[12]. Two giant dams, the Baihetan Dam in China and the Grand Ethiopian Renaissance Dam in Ethiopia, have opened since 2020 and between them generate 76 TW·h of electricity annually
[13]. Although the rate of increase in hydropower capacity has slowed
[12], many more dams are on the drawing board. Africa alone may add an additional 300 dams
[14].
Hydropower provides several climate benefits. The International Energy Agency estimates that it cuts global CO
2 production by 9%
[15]. In addition, said Brian Tarroja, an associate professional researcher in civil and environmental engineering at the University of California, Irvine, CA, USA, “hydropower plays some important roles for the energy system, and they will become more important” as the global economy shifts toward carbon neutrality. For instance, he said, hydropower increases grid reliability because it can furnish electricity when solar and wind are unavailable. It is also valuable because it can rapidly ramp up its electricity output during temporary shortfalls, such as when power producers’ forecasts of demand are too low or when other generating facilities fail. By helping to smooth out electricity generation, Tarroja said, hydropower can make the switch to other renewable sources of power easier.
Despite its benefits, experts emphasize that hydropower is no panacea; its electricity is associated with steep environmental, social, and cultural costs not usually factored into its price. Dams and the reservoirs they create devastate river ecosystems, drown terrestrial ecosystems, destroy fisheries, force residents of the flood zone to leave their homes, and inundate cultural sites
[13],
[16],
[17]. Dam construction produces greenhouse gases. Globally, for all purposes, manufacturing of cement and concrete accounts for as much as 9% of all CO
2 emissions
[18],
[19], and most large dams contain a lot of concrete. By boosting decomposition of vegetation, which yields methane, reservoirs also increase greenhouse gas emissions
[20]. How much methane reservoirs release has proven difficult to measure, said Deshmukh, because the gas bubbles up slowly over large areas. However, one study estimated that reservoirs could be responsible for 1.3% of global greenhouse gas emissions
[20].
The consequences of climate change and its impact on local hydrology—and hydropower—will not be the same everywhere, said Asphota Wasti, a water resources specialist in the Sacramento, CA, USA, office of the global engineering consulting company HDR (Omaha, NE, USA). In California, for instance, the Sierra Nevada mountains “act like a second reservoir,” said Wasti. The snow that falls on them during the winter slowly melts during the spring, helping to recharge the groundwater that feeds lakes during the hot summer months. When the snow melts faster, though, the water rushes to reservoirs without replenishing aquifers, and dams release much more water earlier in the year. This can disrupt water supply management and, in some cases, reduce electricity generation, Wasti said.
With the extended drought in California and the rest of the western United States, hydropower production there has hit record lows
[21]. But some parts of the globe will likely get wetter as the climate changes. The extra water may not be a boon for hydropower, however, as Tarroja and colleagues found when they projected climate impacts on China’s Three Gorges Dam on the Yangtze River
[22]. With a capacity of 22 500 MW, the dam produces more electricity than any other dam in the world
[22]. The researchers’ modeling suggested that the reservoir’s catchment basin will receive up to 11.7% more precipitation by the end of the 21th century. However, much of the runoff from this precipitation will enter the reservoir either in the spring or at the end of the flood season in early fall. While the spring runoff will boost power generation, the fall runoff will provide only a small benefit. This is because the reservoir’s water level at that time of the year is minimized as a flood-control measure and dam operators may not be able to take advantage of the extra runoff
[22]. The researchers’ results suggest that greater runoff will not increase overall electricity generation until 2080, and then by only 5% to 8%
[22]. The study also suggested that climate change will increase the year-to-year variability in power generation. “It is not just where you get more or less water. It is the timing and the intensity,” said Tarroja.
Hydropower could also take a hit—literally—as climate change triggers landslides, avalanches, and other catastrophic changes to the landscape. In 2021, for example, a 2.7 × 10
7 m
3 piece of rock and ice fell from Ronti Peak in the Himalaya mountains of northern India and hurtled down the valley below
[23]. The debris slide killed 204 people and severely damaged two hydroelectric dams; most of the victims were workers at the dams. By studying video, satellite images, and other evidence, Simon Cook, senior lecturer in environmental sciences at the University of Dundee in the United Kingdom, and colleagues determined that the collapse likely occurred because the permafrost that held the rock and ice to the side of the mountain had melted
[24]. They estimated the disaster’s financial cost to be about 223 million USD
[24].
With permafrost and glaciers melting, hydropower dams elsewhere in the world are increasingly vulnerable to similar disasters. Countries such as Peru and Switzerland may be particularly at risk because they are dependent on hydropower facilities in mountainous areas, Cook said. “It will be a huge problem in parts of the world.”
Many power producers have compensated for reduced hydropower output by increasing their use of fossil fuels for power generation. This was reflected in 2023 when the world generated 200 TW·h less from hydropower than in 2022, largely due to weather-related causes
[25],
[26]. As a result, greenhouse gas emissions spiked by about 170 Mt to a new record high of 37.4 Gt
[26].
But other approaches may be able to compensate for reduced hydropower output without further contributing to climate change. Wind and solar can replace some of the lost generation capacity, and their plunging prices may obviate the need to build some dams. Deshmukh and colleagues estimated that about half of the planned dams in southern Africa no longer make sense from an economic perspective because the costs of solar and wind facilities have dropped so low
[27]. As alternatives to hydropower, solar and wind projects make sense in other ways as well, Deshmukh said. They are faster to build than dams, and their environmental and social impacts are smaller.
Some of the grid reliability provided by hydropower backup could be replaced by the increasing use of battery storage. The US Energy Information Administration predicts that utility-scale battery storage in the United States will almost double in 2024, from 16 to 31 GW
[28]. Another technology, pumped hydropower, already stores far more power than batteries; in 2023, its global capacity was almost 140 GW
[29],
[30]. Pumped hydropower facilities move water from one water source, such as a river or reservoir, into a reservoir at a higher elevation (
Fig. 2)
[29]. Pumped hydropower is advantageous, Falchetta said, because solar and wind power—or off-peak excess—can provide the electricity for pumping the water. When the pumped water flows back downhill through pipes, it turns turbines and generates electricity. Using the pumped water to generate electricity at night or when the air is calm can even out the variability in generation by wind and solar. China has more pumped hydropower storage capacity than any other country, about 51 GW, and is reported to be constructing 66 new such facilities
[29],
[31].
Other solutions to counter declining hydropower output could include greater sharing of electricity between and within countries. For example, Falchetta noted, models suggest that central and southern Africa will become drier as the climate alters, but eastern Africa will become wetter
[32]. As a result, hydropower facilities in eastern Africa might be able to send power to the southern and central parts of the continent. For this to happen, however, the countries would need to build transmission lines, Falchetta said.
Structural upgrades could potentially protect existing dams and other energy infrastructure at high risk from extreme weather
[33]. The central Asian country of Tajikistan, for instance, has reinforced its Kayrakkum dam to be able to withstand a once-in-10 000-year flood
[33]. Smarter siting could also help, Cook said. Two similar collapses had previously occurred near the site of the 2021 Himalayan dam disaster, he said. “Why on Earth did they build another hydropower station in this place?”
Despite its declining output, hydropower remains a global pillar of energy generation. “Hydropower is still important because a lot of it exists and it is relatively cheap,” said Tarroja. With renewable strategies available to address the deficits, the problem is less than it could be, said Deshmukh. “We have the technology to fill the gap.”
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