Hydropower's carbon footprint
Hydropower is a low-carbon source of renewable energy and a reliable and cost-effective alternative to electricity generation by fossil fuels.
Hydropower generates more than 4,000 terawatt hours of electricity globally every year, enough to supply over 1 billion people with clean energy.
Independent research suggests that use of hydropower instead of fossil fuels for electricity generation has helped to avoid more than 100 billion tonnes of carbon dioxide in the past 50 years alone. That’s roughly equivalent to the total annual carbon footprint of the United States for 20 years.
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's (IPCC) Fifth Assessment Report noted that only wind and nuclear power have lower median lifecycle greenhouse gas emissions than hydropower. However, the panel cautioned that few studies had assessed the net emissions of freshwater reservoirs, accounting for pre-existing natural sources and unrelated human emissions.
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 accounts for local emissions both before and after reservoir construction, calculating the net emissions resulting from its creation.
Read more about carbon emissions from hydropower reservoirs.
G-res Tool
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 evaluates the land's pre-impoundment state, including natural emissions and those from human activities over the reservoir's lifespan. It also apportions the net GHG footprint to various freshwater services provided by the reservoir, such as water supply, flood and drought management, navigation, fisheries, and recreation.
Developed by IHA and UNESCO with researchers from the University of Quebec at Montreal, SINTEF, and LUKE, the research received support from the World Bank and hydropower sector sponsors. After over a decade of development, the G-res Tool was launched at the World Hydropower Congress in Addis Ababa, Ethiopia, in May 2017.
Read more: Climate change
