RIO DE JANEIRO, Brazil — The epic battle over whether to build the world’s third-largest dam is often billed as a fight between those who love the land and those who love development.

But a brief scientific report released today suggests something more basic may be wrong with the project known as Belo Monte, set to be built in the eastern Amazon. It pins Brazil’s energy future on an expensive hydropower installation in a region where water may be increasingly scarce.

The paper, published in the journal Science, makes no mention of the dam. Instead, it documents how for the second time in just five years, the Amazon basin has seen a once-in-a-century drought, arguing the shift matches predictions for how climate change will affect the region.

“This new research adds to a body of evidence suggesting that severe droughts will become more frequent,” said Simon Lewis, a forest ecologist at the University of Leeds, and a lead author on the paper. “It’s worrying because it fits with projections from some of the most sophisticated [climate] models — and those results were published before these two droughts had occurred.”

And some scientists fear Belo Monte is moving forward without taking these potentially game-changing climate trends into account. While much of the high-profile opposition has focused on the dam’s environmental impact — “Avatar” director James Cameron famously called Belo Monte an ecological disaster — less visible critics are asking whether the investment will even deliver the energy it promises.

“There is a danger that we may be building some extremely expensive white elephants,” said Foster Brown, environmental scientist at the federal university in the Brazilian state of Acre.

Brown and other scientists say extreme weather has been hitting the Amazon more often. The last five years have seen records both for rains and droughts. Both extremes can affect energy production, Brown said. Rain surges can increase how fast sediment builds behind hydroelectric dams, he said, while droughts can cause power outages.

Creating a system that can deal with these extremes would likely increase the cost of the electricity the dam produces, and could be enough to make the project a bad investment. “What people haven’t been asking,” Brown said, “is whether these projects are actually viable from a technical perspective.”

The pressure to bring more energy online is huge. Brazil aims to lift millions out of poverty, and it desperately needs better infrastructure to feed a growing economy. Officials with the state-run energy company Eletrobras, which oversees hydroelectric development, say big hydro projects are the greenest way to achieve that on a large scale, and Belo Monte is an essential part of the push.

“In the current circumstances, there is no better project,” Eletrobras president Jose Antonio Muniz Lopes said in a recent newspaper interview.

Eletrobras declined to make anyone available to answer questions about the new climate data. In an emailed statement, a spokesperson for the agency said projections regarding climate change are not a part of viability studies for hydroelectric projects, though the Eletrobras does consider the impact of “extreme events.”

Critics, however, say Belo Monte’s design makes it particularly vulnerable to extremes.

The reasons date back to the government’s earliest attempts to address environmental concerns about the project. The dam was initially proposed in the 1970s under Brazil’s military dictatorship. At the time, plans called for a series of dams that would flood vast swaths of forest. Authorities shelved the idea under a hail of local and international opposition.

The plan’s final incarnation was more modest: just one dam with a much smaller lake behind it. But some engineers say the smaller reservoir won’t be able to deliver on promises of cheap, abundant power.

“The more that huge dams with relatively small reservoirs are built in the Amazon, the more insecure the electric system will be in the face of extreme climatic events,” said engineer Pedro Bara Neto, head of infrastructure strategy for the environmental organization WWF-Brasil.

According to plans the Brazilian government has made public, the Belo Monte plant will have 11,000 megawatts of production capacity, but it will only be able to operate at that level for a few months out of the year, when the river is high.

Several scientists who have studied the project say the plant will produce about one-tenth that much energy during the dry season, even in non-drought years.

“The problem with Belo Monte is that it only becomes viable if a whole complex of dams is built,” said Celio Bermann, professor at the Institute of Electrotechnics and Energy of the University of Sao Paulo.

He and others say the government has underestimated the true scope and costs of the project. Belo Monte will need a network of upstream dams to guarantee it a consistent water supply, he said. Once the first dam is in place, it becomes much easier for authorities to dust off 1970s-era plans and begin building more of them upstream.

“These other predicted plants would effectively inundate an enormous area,” he said.

The government has stated it has no plans to build these dams, and says it has factored in seasonal differences in electricity production.

But as it stands, the one-dam plan doesn’t make financial sense, said Wilson Cabral de Sousa Jr., a professor of environmental engineering at one of Brazil’s top engineering schools, the Instituto Tecnologico de Aeronautica.

“This undertaking, from an economic perspective is unviable, unfeasible,” Cabral said, and more frequent and severe droughts would only make it less so.

According to his analysis, the costs of building the hydropower plant will likely be higher than the income from selling the energy. And because government agencies are both financing much of the construction, and buying most of the energy produced, he says, that loss will be borne by the public.

“We proved that the organization of the project is such that public money is responsible for more than 90 percent of the cost,” Cabral said. “Society is going to pay for this, one way or another.”