Global pollution from electricity generation was set to fall last year, thanks to the growth of renewable energy. Then came the droughts.

Hydropower, the biggest source of renewable energy in the world, was crippled by lack of rain in several countries last year, driving up emissions as countries turned to fossil fuels to fill the gap. To cope with the electricity shortfall, China and India turned to coal plants, and Colombia to natural gas.

A recent report by the International Energy Agency showed that hydropower’s decline last year pushed countries to use dirtier sources of energy that produced an extra 170 million metric tons of carbon dioxide. That’s like turning on an extra 42 coal-fired power plants for a year. In China, the worst-hit country, hydroelectricity generation saw the steepest fall in the past two decades, according to the I.E.A.

This year, the dip in hydropower has continued in some countries, including Ecuador and Turkey, as temperatures continue to shatter records. Because its giant hydroelectric dams didn’t have enough water, Canada imported more electricity from the United States than it had done in over a decade, as my colleague Ivan Penn wrote this week.

But even in rainier years, hydropower comes with a catch.

Today, I want to explain why this century-old technology is struggling, why it may not be as clean as many people think and also why, despite all this, experts believe it still has an important role to play.

As Joe Bernardi, who tracks the industry for Global Energy Monitor, told me: “Hydropower remains a key piece of the global transition away from fossil fuels.”