Long considered a “clean” energy source, hydropower can actually be bad for climate

A new EDF study published this week in Environmental Science and Technology shows that hydropower — the leading renewable energy technology projected to grow rapidly — is not always as good for the climate as broadly assumed. Moreover, continuing to assume that it is could mean that projects meant to reduce greenhouse emissions will unintentionally increase them instead.

Motivated by pervasive misconceptions of the climate impacts of hydropower, we assessed the warming impacts over time of sustained greenhouse gas emissions estimated from nearly 1,500 existing hydropower plants around the globe. We also looked at the implications of future hydropower development.

If minimizing climate impacts are not a priority in the design, construction and geographic placement of new hydropower facilities, we could end up generating electricity that yields more warming — especially in the near-term — than fossil fuels.

How hydropower can be worse for the climate than fossil fuels

Hydropower is produced when water stored behind a dam is released, using the power of gravity to spin turbines, which generate electricity. There is no fossil fuel burning or smokestacks involved. Which is why, on average, electricity from a hydropower facility has a smaller carbon footprint per unit of energy than electricity generated from fossil fuels.

Long considered a “clean” energy source, hydropower can actually be bad for climate Click To Tweet

But the reservoirs where water is stored also produce both carbon dioxide and methane (an even more potent greenhouse gas, with over 80 times the warming power of CO2 for the first 20 years after it’s released). Both carbon dioxide and methane are released when vegetation decomposes under water. And here, there are enormous differences from facility to facility due to a range of varying reservoir features and meteorological characteristics.

Some hydropower reservoirs are actually carbon sinks, taking in more carbon through photosynthesis by organisms living in the water than they emit through decomposition, while others have carbon footprints equal to or greater than, fossil fuels. In fact, of the nearly 1,500 plants worldwide that we examined and account for half of global hydropower generation, more than 100 facilities have greenhouse gas emissions that cause more warming than fossil fuels.

Further, some regions, such as Africa and India, have proportionally more plants with high greenhouse gas emissions from hydropower compared to the rest of the world. Unfortunately, these also happen to be hotspots for future hydropower growth. For example, electricity generation from hydropower in India is projected to increase by 230% between 2015 and 2040.

Timeframes matter

It is also important to note that if we are building new hydropower facilities with the expectation of climate benefits, those benefits will be significantly smaller in the near-term than over the long haul. This is due to methane emissions’ powerful near term impacts, and also the large amount of carbon dioxide released from newly-flooded reservoirs.

For example, after 50 years of operation, a hydropower facility could cause less than 40% of the warming that would be caused by a coal-fired plant. But in the first decade after the hydro facility is built, it could cause more warming than a coal-fired power plant. Our study finds that over 200 existing hydropower facilities across the globe potentially cause more warming in the near-term than fossil fuel plants.

Why it matters and what to do

Despite the rapid growth of wind and solar, hydropower accounts for two-thirds of renewable energy generation worldwide. Thousands of new hydro facilities are either planned or under construction globally. The International Energy Agency predicts that hydropower could grow by nearly 80% by 2040 as society works to displace fossil fuels.

Our research shows why it is important to ensure that future hydropower projects don’t hurt, but help, the climate. For example, if new hydropower plants in India have greenhouse gas emissions properties similar to existing Indian hydro plants, they could be worse for the climate than emissions from average natural gas plants over the first 50 years of operation, due to methane emissions as well as carbon dioxide emissions from reservoir creation.

There are things that can be done when planning a new hydropower project that can help keep emissions down.
For example, if a facility draws from a large reservoir, but generates a relatively small amount of electricity – it’s likely to produce disproportionate greenhouse gas emissions. Keeping this ratio in mind is a valuable part of the planning process as this “power density” has been found to be the best indicator of overall greenhouse gas emissions from hydropower facilities.

Temperature of the reservoir water can also play a role in the production of methane emissions, and therefore avoiding development in extremely hot places could also lower emissions.

Overall, we must dispel the myth that hydropower is universally low-carbon, and be mindful of the potential for high emissions when developing new facilities. This paper gives us a more effective way to assess these projects so we can ensure new developments deliver climate benefits and not disbenefits.

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