Hydropower reservoirs are considered to be major sources of low-carbon electricity that can be used to cut greenhouse gas emissions to combat climate change. But those same reservoirs emit global warming gases, and a team of researchers has now tallied just how much those gases impact the climate.
Globally, reservoirs are responsible for about 1.3 percent of the world’s man-made greenhouse gas emissions each year, or about the same as Canada’s total emissions, according to a study to be published in the journal Bioscience next week. The findings come from a team of researchers led by Washington State University-Vancouver and the Environmental Protection Agency.
That emissions estimate may not sound like much, but the study says reservoir emissions can contribute significantly to carbon budgets countries use to meet their climate goals. Reservoirs emit mostly methane, a greenhouse gas about 35 times as potent as carbon dioxide in its potential to warm the atmosphere over the course of a century.
Knowing how reservoirs contribute to global warming is critical because there is a dam-building boom occurring worldwide. Previous research shows that about 3,700 major hydropower dams were planned or under construction globally as of 2014 as countries scramble to generate renewable energy and find new ways to store water during drought.
The study’s greenhouse gas estimates include reservoirs built for all uses, including hydropower, drinking water, farm irrigation, and flood control, among others.
Reservoirs used for hydropower are often considered climate friendly because they are a renewable electricity source that does not burn fossil fuels. Most U.S. states include electricity generated at large hydropower dams as part of their strategy to use more renewables to cut greenhouse gas emissions and combat climate change.
Scientists have long known that reservoirs emit some greenhouse gases into the atmosphere because decomposing plant matter submerged under water releases methane, carbon dioxide and other gases. But until recently, scientists had too little data to estimate the total emissions from all the world’s reservoirs put together.
To do that, the research team synthesized the results of dozens of recent studies of more than 200 reservoirs across the globe, estimating that all the world’s reservoirs put together emit between 0.5 and 1.2 gigatons of carbon dioxide equivalents annually.
The study shows that reservoirs that grow more algae and other microbes tend to produce more greenhouse gases, and reservoirs in northern latitudes emit more methane than previously thought.
The single largest source of greenhouse gas emissions globally is electricity generation (about 25 percent of global greenhouse gas emissions), and that’s because fossil fuels are the dominant source of electric power. Even though reservoirs emit methane and nitrous oxide, they still have an overall lower greenhouse gas footprint than burning coal for electricity.
The study’s authors suggest that policymakers should weigh the global warming implications of reservoirs as they consider building new ones, but Townsend-Small said they still play a vital role in reducing emissions from fossil fuels.
— source climatecentral.org