If hydropower was replaced with burning coal to generate electricity, the International Hydropower Association (IHA) estimates that more than 4 billion metric tonnes of additional greenhouse gases would be emitted annually, and global emissions from fossil fuels and industry would be at least 10 per cent higher.
Greenhouse gases caused by renewables
All energy sources, even renewables, produce carbon emissions in their lifecycle, due to the emissions caused by their manufacture, construction or operation. Hydropower assets have a very long lifespan, meaning that emissions associated with construction can be amortized over a much longer time (and associated generation) when compared to technologies that have shorter life spans.
The greenhouse gas footprint of hydropower has long been questioned in both scientific and policy spheres, especially with regard to emissions caused by the creation of a reservoir. In certain conditions, a reservoir will release greenhouse gases due to the decomposition of flooded organic material. Emissions from reservoirs therefore tend to be highest in the first 10 to 20 years immediately following reservoir impoundment, then decrease over time. In other conditions, a reservoir may act as carbon sink: absorbing more emissions than it emits.
Emissions relating to the construction and operation of a reservoir vary depending on its type, size and location. Once filled, factors such as a reservoir’s depth and shape, the amount of sun reaching its floor, and wind speed, affect the different biogeochemical pathways by which CO₂ and CH₄ are created and released to the atmosphere.
Hydropower's low global carbon footprint
The Intergovernmental Panel on Climate Change (IPCC), in its Fifth Assessment Report published, noted that only wind and nuclear power have lower median lifecycle greenhouse gas emissions than hydropower. However the panel cautioned that few studies had appraised the net emissions of freshwater reservoirs, allowing for pre-existing natural sources and sinks and unrelated human emission sources.
The IPCC states that hydropower has a median greenhouse gas (GHG) emission intensity of 24 gCO₂-eq/kWh - this is the grams of carbon dioxide equivalent per kilowatt-hour of electricity generated allocated over its life-cycle. By comparison, the median figure for gas is 490 gCO₂-eq/kWh.
Analysis by the International Hydropower Association of almost 500 reservoirs worldwide using the G-res Tool (see below) confirms this low carbon footprint. The results published in Water Security and Climate Change: Hydropower Reservoir Greenhouse Gas Emissions (2021) found the median value of lifecycle GHG emissions for hydropower to be 23 gCO2-eq/kWh, which is consistent with the IPCC findings. The G-res Tool takes into account local emissions not just after construction, but also before construction reservoir impoundment and calculates the net emissions due to the creation of the reservoir.
The GHG Reservoir (G-res) Tool was devised to enable companies, investors and researchers to more accurately estimate the net change in greenhouse gas (GHG) emissions attributable to the creation of a specific reservoir.
The G-res Tool takes into account the state of the land pre-impoundment, considering naturally occurring emissions and emissions related to other human activities over the lifetime of the reservoir. It also provides a method for apportioning the net GHG footprint to the various freshwater services that a reservoir provides, such as water supply for irrigation and cities, flood and drought management, navigation, fisheries and recreation.
The G-res Tool was developed by IHA and UNESCO in cooperation with researchers from the University of Quebec at Montreal (UQÀM) in Canada, the Norwegian Foundation for Scientific and Industrial Research (SINTEF) and the Natural Resources Institute of Finland (LUKE).
This research was supported by the World Bank and sponsors from the hydropower sector. After more than a decade in development, the G-res Tool was launched at the World Hydropower Congress in Addis Ababa, Ethiopia, in May 2017.