Rethinking the Carbon Footprint of Hydropower
Hydropower is often presented as one of the most reliable forms of renewable energy, valued not only for producing electricity but also for stabilising the grid through storage and rapid-response generation. Yet its climate impact is more complex than commonly assumed. According to research commissioned by the U.S. Department of Energy and conducted by Oak Ridge National Laboratory, current methods for measuring greenhouse gas emissions from hydropower may be insufficient and, in some cases, misleading.
At the core of the issue is how reservoirs alter natural carbon cycles. All inland waters emit some greenhouse gases, but the creation of hydropower reservoirs changes both carbon storage and release. When land is flooded, submerged vegetation and organic material decompose, releasing carbon dioxide and methane. These gases reach the atmosphere through multiple pathways, including surface diffusion, bubbling from sediments, plant-mediated transport, and degassing during turbine operations. Methane is particularly significant due to its high short-term warming potential, even though it remains in the atmosphere for a shorter period than carbon dioxide.
At the same time, reservoirs are not simply sources of emissions. The same DOE-backed research highlights that they can also act as substantial carbon sinks. Reservoirs often trap organic material in sediments at rates far exceeding natural lakes, and in some cases—particularly in temperate climates—they may store more carbon than they emit. This creates a complex balance in which conditions that favour carbon burial can also promote methane production, making it difficult to determine whether a reservoir is a net source or sink of emissions.
One of the most important findings of the research is that current accounting approaches fail to capture this complexity. Most existing studies estimate emissions based on the surface area of reservoirs, assigning an average emission rate per square kilometre. However, reservoirs are part of broader watershed systems and accumulate carbon from upstream sources, meaning emissions do not scale linearly with surface area. Factors such as depth, shape, sedimentation rates, and inflows of organic matter play a critical role but are often overlooked in simplified models.
Data limitations further undermine current estimates. Existing global assessments rely on relatively small datasets collected using inconsistent methodologies. Many measurements are taken from limited locations within reservoirs or over short time periods, failing to capture seasonal variation. As a result, widely cited estimates are often based on extrapolations from incomplete and non-representative samples. Notably, there is still no comprehensive national inventory of hydropower reservoir emissions in the United States.
Another key issue identified by the U.S. Department of Energy study is the reliance on gross emissions rather than net emissions. Current approaches typically measure total greenhouse gas output from reservoirs without accounting for what emissions would have occurred in the absence of the dam. This contrasts with standard practices in other areas of carbon accounting, where net changes—before and after an intervention—are used to assess impact. Without this baseline comparison, the climate footprint of hydropower may be overstated.
Time scale is another overlooked factor. Carbon stored in reservoir sediments can remain buried for centuries or even millennia, particularly in low-oxygen environments. However, most accounting frameworks do not fully account for long-term storage. At the same time, climate change introduces additional uncertainty. Warmer temperatures may accelerate decomposition and increase emissions, while also altering aquatic ecosystems in ways that affect carbon storage. Extreme events such as floods and wildfires may further shift carbon between land and water systems.
The research also highlights a broader issue: hydropower is rarely assessed in the context of the full electricity system. Because hydropower can provide flexible, dispatchable power, it often displaces fossil fuel generation—particularly natural gas, which is commonly used to stabilise grids. However, these avoided emissions are not consistently included in hydropower’s carbon footprint. This omission can distort comparisons with other energy sources, especially when fossil fuel assessments often exclude upstream emissions such as exploration and extraction.
To address these shortcomings, the study calls for a more comprehensive and system-level approach. Reservoirs should be analysed within the context of entire watersheds, accounting for upstream carbon flows and landscape characteristics. Larger, more representative datasets are needed, ideally collected through consistent, long-term monitoring. Improved models should incorporate uncertainty and better reflect the variability of real-world systems.
Crucially, future assessments should shift toward net emissions accounting, comparing conditions before and after reservoir construction. Greater attention should also be given to emissions directly linked to hydropower operations, such as methane released during turbine use. Finally, hydropower should be evaluated within the broader energy mix, taking into account its role in enabling renewable integration and reducing reliance on fossil fuels.
As the global energy transition accelerates, hydropower will continue to play a key role in providing reliable, low-carbon electricity. But as this research makes clear, understanding its true climate impact requires more sophisticated methods than those currently in use. Accurately measuring that impact is essential—not only for assessing hydropower itself, but for making informed decisions about the future of clean energy systems.
