Plotting A Nature Positive Path To A Sustainable Energy Future

The approaching UN Climate Change Conference (COP27), to be held in Egypt in November, focuses attention on the pathways needed to achieve global climate targets. A rapid decarbonization of economies is central to stabilizing the climate, including achieving net zero power systems by 2050. But with the world also facing a nature/biodiversity crisis and striving to achieve a set of development goals, these pathways must factor in their impact on communities and ecosystems; stabilizing the climate should strive to be consistent with maintaining the Earth’s life support systems.

Several of the projections for what is needed to achieve power systems consistent with the 1.5° C climate target feature a doubling of global hydropower capacity, such as those from the International Energy Agency (IEA) and the International Renewable Energy Agency (IRENA). While that is a smaller proportional increase than other renewables such as wind and solar PV, which are projected to increase more than twentyfold, a doubling of global hydropower capacity nonetheless represents a dramatic expansion of major infrastructure that will affect the world’s rivers – and the diverse benefits they provide to societies and economies from freshwater fisheries that feed hundreds of millions to flood mitigation and stable deltas.

Only one-third of the world’s largest rivers remain free-flowing – and a doubling of global hydropower capacity would result in the damming of about half of those, while generating less than 2% of the needed renewable generation in 2050.

Nearly all new energy projects, including wind and solar, will cause some negative impacts, but losses of a major ecosystem type—large, free-flowing rivers—at that scale will have major tradeoffs for people and nature at a global level. As such, hydropower expansion merits particularly careful planning and decision making. Here, I examine some major issues relevant for evaluating hydropower, including issues that are frequently misunderstood.

Small hydropower is often assumed to be sustainable or low impact, but that is often not the case. Small hydropower is not consistently defined (e.g., some countries classify ‘small hydropower’ as anything up to 50 MW) but is often categorized as projects below 10 MW. Because projects of that size are often assumed to have minor impacts on the environment, small hydropower projects often receive incentives or subsidies and/or benefit from limited environmental review. However, the proliferation of small hydropower dams can cause considerable cumulative impacts. Further, even a small project in a particularly poor location can cause surprisingly large negative impacts.

Run-of-river hydropower is also often presented as having limited negative impacts, but some of the dams with the highest impacts on rivers are run-of-river dams. Run-of-river dams do not store water for long periods of time; the amount of water flowing into the project is the same as the amount flowing out of the project – at least on a daily basis. However, run-of-river projects can store within a day when they operate for “hydropeaking,” storing water throughout the day and releasing it during a few hours of peak demand. This mode of operation can cause major negative impacts on downstream river ecosystems. Because run-of-river dams don’t have large storage reservoirs, they do not cause some of the major impacts to people and rivers associated with large storage reservoirs, including large-scale displacement of communities and disruptions to seasonal patterns of river flow. But these differences too often lead to more sweeping generalizations that run-of-river projects do not have impacts on rivers – or even that run-of-river hydropower does not require a dam. While some run-of-river projects do not include a dam across the entire channel, many large run-of-river projects do require a dam that fragments a river channel (see photo below). This inappropriate generalization becomes particularly problematic when proponents of a project point to its run-of-river status as short-hand for arguing that it will have minimal impacts. That “hasty generalization” was employed by proponents of the Xayaboury Dam on the Mekong River, which is having major impacts on both fish migration and the trapping of sediment needed by the downstream delta.

While environmental reviews of hydropower dams often focus on local conditions, negative impacts can actually manifest even hundreds of kilometers away from a dam. When hydropower dams block the movement of migratory fish, they can cause negative impacts on ecosystems across a whole river basin, both upstream and downstream of the dam. And because migratory fish are often among the most important contributors to freshwater fisheries, this translates to negative impacts to people, even some who may live hundreds of kilometers from a dam site. Hydropower dams have been a primary contributor to dramatic global losses of migratory fish, which have declined by 76% since 1970, with high profile examples such as the Columbia and Mekong rivers. A second long-distance impact is sediment. A river is more than a flow of water, it is also a flow of sediment, such as silt and sand. Rivers deposit this sediment when they enter the ocean, creating a delta. Deltas can be extremely productive—for both agriculture and fisheries—and more than 500 million people now live on deltas around the world, including those of the Nile, Ganges, Mekong and Yangtze. However, when a river enters a reservoir, the current slows considerably, and much of the sediment drops out and is “trapped” behind the dam. Reservoirs now capture approximately one quarter of the global annual flux of sediment—silt and sand that would otherwise help maintain deltas in the face of erosion and sea level rise. Some key deltas, such as the Nile, have now lost more than 90% of their sediment supply and are now sinking and shrinking. Thus, hydropower dams can have major impacts on key resources across large river basins, including globally important food supplies, but, too often, environmental review of hydropower projects focuses primarily on local impacts.

Fish passage around dams has rarely mitigated the negative impacts of dams on migratory fish. Fish passage, such as fish ladders or even elevators, is a common mitigation requirement for dams. Fish passage was originally developed on rivers that had powerful swimming and leaping fish species, such as salmon, but passage structures are now being added to dams on large tropical rivers—such as the Mekong or tributaries to the Amazon—although there is very limited data or examples of how fish passage works in these rivers. A 2012 review of all peer-reviewed studies on fish passage performance found that fish passage worked far better for salmon than for other types of fish; on average, structures have a 62% success rate for salmon swimming upstream. That number may seem high, but most fish must navigate multiple dams in a row; even with the relatively high success rate of 62% at each dam, less than a quarter of salmon would successfully pass three dams. For non-salmon, the success rate was 21% – even with just two dams, only 4% of migrating fish will be successful (see below). Further, most fish also require downstream migration, at least for larval or juvenile fish, and the downstream passage rate is often even lower.

Hydropower is no longer the lowest cost renewable generation technology. In the past decades, the cost of wind has dropped by about one-third and the cost of solar has dropped by 90% – and these reductions in cost appear likely to continue. Meanwhile, the average cost of hydropower has increased somewhat over the past decade, such that onshore wind has now become the lowest average cost among renewables. Although its average cost is still slightly higher than hydropower, solar projects now consistently set the record for lowest cost energy project.

Hydropower does have the highest frequency of delays and cost-overruns among large infrastructure projects. A study by EY found that 80 percent of hydropower projects experienced cost overruns with an average overrun of 60 percent. Both of these proportions were the highest among the types of large infrastructure projects in their study, including fossil and nuclear power plants, water projects and offshore wind projects. The study also found that 60 percent of hydropower projects experienced delays with an average delay of nearly three years, exceeded only by coal projects which had slightly longer average delays.

Hydropower can provide firm energy generation or storage in support of variable renewables such as wind and solar….

Wind and solar are already the leading form of new generation added each year and forecasts envision low-carbon grids where wind and solar are the dominant forms of generation. But stable grids will need more than wind and solar, they will also need some combination of firm generation and storage that will balance grids during periods—from minutes to weeks—when the availability of those resources drops. In many grids, hydropower is among the technologies that can provide firm energy. One type of hydropower—pumped storage hydropower (PSH)—is currently the dominant form of utility-scale storage on grids (about 95%). In a PSH project, water is pumped uphill when power is plentiful and stored in an upper reservoir. When power is needed, the water flows back downhill to the lower reservoir, generating electricity for the grid.

…but these services can often be provided without further loss of free-flowing rivers. Research focused on options for grid expansion has shown that countries can often meet future demand for electricity with low-carbon options that avoid new dams on free-flowing rivers, either through greater investment in wind and solar to substitute for hydropower with large negative impacts or through careful siting of new hydropower that avoids dam development on major free-flowing rivers or in protected areas. Further, the two reservoirs of a pumped storage project can be built in locations away from rivers and cycle the water back and forth between them. Researchers from the Australian National University mapped 530,000 locations around the world with the appropriate topography to support off-channel pumped storage, with only a small fraction needed to provide sufficient storage for renewable-dominated grids around the world. Existing reservoirs or other features such as abandoned mining pits can also be used in pumped storage projects.

Not all global scenarios consistent with climate targets include a doubling of hydropower. Although, several prominent organizations (e.g., IEA and IRENA) that model how future power systems can be consistent with climate targets include a doubling of global hydropower capacity, not all such scenarios do. For example, while the IEA and IRENA models include at least 1200 GW of new hydropower capacity by 2050, among the scenarios used by the Intergovernmental Panel on Climate Change (IPCC) that are consistent with the 1.5° C target, approximately one-quarter of them included less than 500 GW of new hydropower. Similarly, the One Earth Climate Model, also consistent with the 1.5° C target, includes only about 300 GW of new hydropower by 2050.

Hydropower generation can expand without new dams Power systems can add hydropower generation without adding new hydropower dams in two primary ways: (1) retrofitting existing hydropower projects with modern turbines and other equipment; and (2) adding turbines to non-powered dams. A study by the U.S. Department of Energy found that, with the right financial incentives in place, those two approaches could add 11 GW of hydropower to the U.S. hydropower fleet, an increase of 14% from today’s capacity. If similar potential were available in other countries around the world, that represents more than half of the additional global hydropower capacity included in the One Earth Climate Model by 2050. Further, adding “floating solar” projects on the reservoirs behind hydropower dams, covering just 10% of their surface, could add 4,000 GW of new capacity, capable of generating approximately twice as much power as is generated from all hydropower today.

Hydropower is vulnerable to climate change, emphasizing the value of diversified grids. I was lead author on a study that found that, by 2050, 61 percent of all global hydropower dams will be in basins with very high or extreme risk for droughts, floods or both. By 2050, 1 in 5 existing hydropower dams will be in high flood risk areas because of climate change, up from 1 in 25 today. A study in Nature Climate Change predicted that up to three-quarters of hydropower projects worldwide will have reduced generation due to climate-driven shifts in hydrology by the middle of this century. Countries that are highly dependent on hydropower are vulnerable to drought and, in many regions, this risk will increase. For example, hydropower provides nearly all of the electricity for Zambia and a 2016 drought in southern Africa caused Zambia’s national electricity generation to decline by 40%, causing immense economic disruption and losses. This vulnerability emphasizes the value of diversified sources of generation within grids.

Hydropower is not always contentious, common ground can be found. While conservation organizations and the hydropower sector have often had a contentious relationship, common ground can be found. For example, in the United States, representatives of the hydropower sector, including the National Hydropower Association (NHA), and several conservation organizations formed an “Uncommon Dialogue for Hydropower” (full disclosure: I represented my organization, World Wildlife Fund-US, in this dialogue). Participants in the Uncommon Dialogue agreed that hydropower had a key role in a sustainable energy future and that protection and restoration of rivers in the U.S. should be a priority. The Uncommon Dialogue participants supported legislation consistent with that shared vision and the Infrastructure Bill, signed into law last year, included US$2.3 billion for increasing hydropower capacity without adding new dams (through retrofits and powering non-powered dams) and for removal of aging dams to restore rivers and improve public safety.