Solar and wind power get all the attention these days, but the global hydropower frenzy that's currently underway could end up being just as consequential for the planet — for better or for worse.

Hydropower can provide a source of renewable energy — but also wreak havoc on river habitats

Hydroelectric dams are still the biggest source of renewable energy around, generating 16 percent of the world's electricity. And, according to a recent study in Aquatic Sciences, more than 620 large hydroelectric dams are now under construction, largely in Latin America and Asia — with thousands more in various stages of planning. At the high end, if most of the planned dams were built, global hydropower production could double to 1,700 gigawatts, the study's authors predict. This would also reduce the number of large free-flowing rivers on the planet by another 20 percent.

There are upsides and downsides here. If built, these dams could provide electricity for millions of poor people who don't have it. But dams can also be extremely controversial. Some projects can end up displacing thousands of people and destroying river habitats — something the United States learned the hard way last century. What's more, recent research has questioned whether hydropower is as climate-friendly as once thought.

The worldwide dam-building frenzy

The new study, by researchers at the University of Tübingen in Germany, compiled a database of 3,700 large dams either under construction (in blue) or in various stages of planning (in red). Many planned dams may never get built — especially if the plans are hazy — but there are still at least 629 major dam projects underway right now.

The biggest dam-building booms are occurring in the Amazon and La Plata basins in Brazil, the Ganges-Brahmaputra basin in India, and the Yangtze basin in China. (China remains the leader with 31 percent of the world's hydropower capacity.)

Hydropower's share of electricity could rise from 16% to 18%

If most of the planned dams were built, the Aquatic Sciences paper notes, hydropower's share of global electricity could rise from 16 percent to 18 percent. That seems tiny, but only because the world's demand for energy is growing so massively and other energy sources are also expanding. It actually represents a lot of hydroelectricity.

One major benefit of these dams is that they can help provide electricity to some of the 1.2 billion people around the world who still don't have it. A recent report from the International Energy Agency found that hydropower in Africa would need to grow significantly (alongside natural gas and solar power) between now and 2040 in order to boost energy access for those without.

But what about global warming? In theory, hydroelectric dams should produce fewer greenhouse-gas emissions than, say, coal plants. But there's a huge caveat here. Some recent research has suggested that decaying vegetation in stagnant dam reservoirs may end up producing lots and lots of methane, another potent greenhouse gas.

For many dams, this can actually offset a big chunk of the climate benefits, especially in tropical countries like Brazil. Researchers are still trying to calculate the size of these emissions, but hydropower isn't always as green as it might seem. (There are ways to build lower-emitting dams, but these methods aren't always followed.)

Why dams are often controversial

As noted, however, it's far from certain that all of the planned dams will be built. For one, large hydroelectric plants are expensive and prone to cost overruns. One recent review in Energy Policy found that large dams ended up costing 96 percent more, on average, than initially planned.

If all these dams were built, the number of large, free-flowing rivers would decline by 20%

Large hydroelectric dams can also generate a lot of public backlash. The proposed Belo Monte Dam in northern Brazil, which would be one of the world's largest if completed, has been held up since the 1970s over a variety concerns. Opponents have argued that the dam will disrupt fishing and other downstream ecosystems and could force thousands of indigenous people from their lands. (Brazil's newly re-elected president, Dilma Rousseff, is determined to see the dam built.)

It's easy to find similar dam controversies all around the world. The gargantuan Three Gorges Dam in China displaced 1.3 million people during its construction. Laos is planning to build more than 30 new dams in the coming years — but farmers and fishermen downstream are suing to block it, arguing that the dams will degrade the quality of the river and deplete the supply of fish. Ethiopia's massive Renaissance Dam, currently under construction, has set off a diplomatic feud with Egypt over control of the Nile River's water.

Learning from past mistakes in the US

That's not to say all dams are a bad idea. But it's worth learning from past mistakes and finding ways to minimize the damage. One country that has experience here? The United States.

The US is now tearing down more than 1,000 of its dams

The US went on its own dam-building frenzy between the 1920s and 1960s — before many of the full impacts of dams were well understood. Today, there are more than 79,000 dams across the country, and some have caused major problems. So, in recent decades, the US has removed about 1,000 of them, often at great expense.

The reasons varied. Some dams, by turning free-flowing rivers into stagnant reservoirs and disrupting flow and sediment regimes, ended up destroying whole ecosystems. The Elwha River in the Pacific Northwest, for instance, lost 90 percent of its salmon after being dammed up. Other dams turned out to be extremely expensive to maintain over the years, and many simply weren't worth the cost.

Obviously no one's going to tear down the Hoover Dam, which still provides electricity for millions of people in Nevada, Arizona, and California. But many smaller dams are now getting dismantled — including the 108-foot Elwha River dam. (The hunt for carbon-free electricity, however, has complicated this task. Back in 2013, Congress passed a bill to help eke out more power from small rivers and streams, particularly by adding generating capacity to existing dams.)

"I'll be honest with you, we made some mistakes," said then-Secretary of State Hillary Clinton at a meeting in Cambodia back in 2012 over plans to dam up the Mekong River. "We started more than a hundred years ago, so we've learned some hard lessons about what happens when you make certain infrastructure decisions." Among other things, the US was providing aid to Cambodia to study how dam construction would affect the river's flow.

The latest Aquatic Sciences paper, by providing a comprehensive database of the 3,700 largest dams being contemplated, aims to provide a useful resources to planners hoping to address some of these concerns. "Clearly," the authors note, "there is an urgent need to evaluate and to mitigate the social, economic, and ecological ramifications of the current boom in global dam construction."

Further reading

The United States isn't building too many more massive dams — apart from a few planned projects in Alaska. But smaller dams are a possibility. Back in 2013, Congress passed a bill to promote hydropower in smaller rivers and streams, particularly by retrofitting existing dams with generating capacity.

Speaking of dams, here's a fun New York Times piece on the growing interest in using beavers to help restore river ecosystems.