Emissions hit a record high in 2023. Blame hydropower.

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Hydropower is a staple of clean energy-the modern version has been around for over a century, and it's one of the world's largest sources of renewable electricity.

But last year, weather conditions caused hydropower to fall short in a major way, with generation dropping by a record amount. In fact, the decrease was significant enough to have a measurable effect on global emissions. Total energy-related emissions rose by about 1.1% in 2023, and a shortfall of hydroelectric power accounts for 40% of that rise, according to a new report from the International Energy Agency.

Between year-to-year weather variability and climate change, there could be rocky times ahead for hydropower. Here's what we can expect from the power source and what it might mean for climate goals.

Drying up

Hydroelectric power plants use moving water to generate electricity. The majority of plants today use dams to hold back water, creating reservoirs. Operators can allow water to flow through the power plant as needed, creating an energy source that can be turned on and off on demand.

This dispatchability is a godsend for the grid, especially because some renewables, like wind and solar, aren't quite so easy to control. (If anyone figures out how to send more sunshine my way, please let me know-I could use more of it.)

But while most hydroelectric plants do have some level of dispatchability, the power source is still reliant on the weather, since rain and snow are generally what fills up reservoirs. That's been a problem for the past few years, when many regions around the world have faced major droughts.

The world actually added about 20 gigawatts of hydropower capacity in 2023, but because of weather conditions, the amount of electricity generated from hydropower fell overall.

The shortfall was especially bad in China, with generation falling by 4.9% there. North America also faced droughts that contributed to hydro's troubles, partly because El Nino brought warmer and drier conditions. Europe was one of the few places where conditions improved in 2023-mostly because 2022 was an even worse year for drought on the continent.

As hydroelectric plants fell short, fossil fuels like coal and natural gas stepped in to fill the gap, contributing to a rise in global emissions. In total, changes in hydropower output had more of an effect on global emissions than the post-pandemic aviation industry's growth from 2022 to 2023.

A trickle

Some of the changes in the weather that caused falling hydropower output last year can be chalked up to expected yearly variation. But in a changing climate, a question looms: Is hydropower in trouble?

The effects of climate change on rainfall patterns can be complicated and not entirely clear. But there are a few key mechanisms by which hydropower is likely to be affected, as one 2022 review paper outlined:

• Rising temperatures will mean more droughts, since warmer air sucks up more moisture, causing rivers, soil, and plants to dry out more quickly.
• Winters will generally be warmer, meaning less snowpack and ice, which often fills up reservoirs in the early spring in places like the western US.
• There's going to be more variability in precipitation, with periods of more extreme rainfall that can cause flooding (meaning water isn't stored neatly in reservoirs for later use in a power plant).

What all this will mean for electricity generation depends on the region of the world in question. One global study from 2021 found that around half of countries with hydropower capacity could expect to see a 20% reduction in generation once per decade. Another report focused on China found that in more extreme emissions scenarios, nearly a quarter of power plants in the country could see that level of reduced generation consistently.

It's not likely that hydropower will slow to a mere trickle, even during dry years. But the grid of the future will need to be prepared for variations in the weather. Having a wide range of electricity sources and tying them together with transmission infrastructure over wide geographic areas will help keep the grid robust and ready for our changing climate.

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