But many environmental groups argue that the dam’s potential harm to the newly identified Tapanuli species of orangutan, along with other endangered wildlife in the area, far outweighs any benefit in reduced emissions.

The project, which is already under construction and scheduled for completion in 2022, would initially take up about 1,600 acres in the orangutan’s historic habitat. Once the work is finished and the area reforested, its final footprint would be about 300 acres, according to North Sumatera Hydro Energy , whose name uses an alternate spelling of Sumatra.

With the flooding of the steep Batang Toru gorge, about 150 acres of the site would be underwater, an area smaller than the reservoirs of typical hydropower dams.

The Bank of China, a major Chinese state-owned bank, is financing the project. Mr. Anton said it was not part of the Belt and Road Initiative, China’s ambitious program to finance infrastructure projects around the world, which has been criticized for saddling developing countries with debt.

Planning for the dam project had been underway for a decade when scientists determined in 2017 that orangutans living in the Batang Toru ecosystem constituted a distinct species, which they named the Tapanuli after the area where it lives.

There are only about 800 Tapanuli, making it the most endangered of the three orangutan species. Sumatran orangutans number about 14,600 and Borneo orangutans about 105,000, experts say.