The world’s attention fell on a Japanese power plant yesterday, as technicians turned on a nuclear reactor for the first time since it was shut down following the catastrophic 2011 Fukushima disaster. It’s scheduled to be the first of dozens to start up again, despite public opposition to reviving nuclear power in the earthquake-prone nation.

To many in the energy and climate world, however, Japan’s nuclear issue is just a sideshow. A bigger concern is Japan’s growing role in keeping alive an energy source that President Barack Obama and Japan’s allies elsewhere have been fighting to bury: coal.

After the Fukushima disaster shut down nearly all of the country’s nuclear capacity, which had represented 29 percent of Japan’s electricity generation, the nation filled the gap by turning back to fossil-fuel power, burning more natural gas, oil, and a massive influx of coal.

This might seem like a short-term response to the nuclear crisis. It’s not. A recent trip to Tokyo, Nagoya and Yokohama found that Japanese utilities are in the midst of replacing many of their aging coal power units with more efficient new ones, a decision that locks in carbon dioxide emissions for decades. According to a tally kept by the Kiko Network, a Kyoto-based environmental group, there are more than 40 new coal plants slated for construction domestically. By comparison, the coal-rich U.S. has only one going up: Southern Co.’s Kemper project, an effort to demonstrate new carbon-capture technology.

Even more worrysome to environmentalists, however, Japan’s power industry and government have turned their attention overseas, hoping to sell profitable, high-tech Japanese plants to other coal-burning nations, particularly in the developing world.

Japan is now the planet’s top public financer of overseas coal plants, technology and mining, according to a report by the Natural Resources Defense Council, Oil Change International and the World Wide Fund for Nature released in June. The report found that Japan poured more than $20 billion into coal projects between 2007 and 2014—roughly a quarter of total international support for coal power. Since 2011, two of Japan’s main public financing arms have backed more than 22 gigawatts of new coal power from Vietnam to India to Chile. They’ve also put about $5 billion into coal mining projects around the world over the same period.

“They’re not only the biggest financer of coal, but they’re also frankly the most active still,” said Jake Schmidt, NRDC’s director of international programs, who worked on the joint report.

In 2013, by contrast, Obama dropped American support for public financing for new coal plants overseas, except where a plant uses carbon-capture technology or where a developing country has no other choice. Especially with a major international climate negotiation on the horizon later this year, the divergence in policy has become an open diplomatic wound between the two countries.

“Coal financing is an area where the two governments have firmly agreed to openly disagree,” Jane Nakano, a senior fellow with the Center for Strategic and International Studies, said in an email.

GEOLOGY WAS NOT kind to Japan, at least where energy is concerned. What reserves the country has in fossil fuels is extremely limited or caught up in territorial disputes with China; in 2002, Japan gave up on domestic coal production altogether. With a fuel-hungry industrial economy to feed, Japan had been the world’s biggest importer of coal for at least three decades, overtaken by China only in 2011, according the Energy Information Administration. It’s also the world’s largest importer of liquefied natural gas.

One legacy of that long history of energy dependence is Japan’s mastery of fossil-fuel burning technology.

“There’s a long tradition in this country of top engineers devoting their sweat and time and all efforts, generation after generation, to produce efficient coal-burning power plants,” said Mutsuyoshi Nishimura, a former chief climate negotiator for the Japanese government. “Coal-burning technology is one great pillar in our ‘Tech Japan.’”

Where the efficiency of coal plants in India lie around 27 percent and their American brethren hover around 36 percent, Japan’s fleet crept above 40 percent long ago, according to an analysis by Dutch consulting firm Ecofys. J-Power, a utility run by the government until 2004, burns more coal than any other power company in the country and claims to have the most world’s most efficient coal plant — pushing close to 45 percent.

Today, Japanese officials and companies see big opportunities in other Asian nations. As those countries grow, so do their energy needs, and many—like Japan—are still heavily dependent on coal.

Tetsuya Watabe, general manager of Chubu Electric’s 4,100-megawatt Hekinan Thermal Power Station, Japan’s largest coal plant, said in an interview that within the last year alone, power plant operators and government officials from Thailand, China, Myanmar, India and Indonesia have visited the site as they consider building new coal plants in their countries.

The high price of the newest Japanese plants puts them largely out of reach of poorer coal-burning countries, but even Japan’s second- or third-tier coal technology is still more efficient that what’s running in the developing world. The pitch from Japanese power companies and government officials alike is pretty straightforward: While you pay more in capital costs, the investment will pay off over the plant’s 40-year or so lifespan, and the higher efficiency means you can power households or factories with less coal. And Japan offers a network of public financing, like the Japan Bank for International Cooperation — its version of the U.S. Export-Import Bank — to sweeten the price.

Although the idea of coal plants proliferating across Asia might – and does – horrify environmentalists, Japan also makes a serious argument that it’s a good thing for the world overall, both strategically and environmentally. Officials are quick to point out that if they didn’t sell this technology, China would step in to sell cheaper, dirtier plants.

“The Chinese are now marketing its coal power plants to developing Asia. The Japanese and Chinese are competing head to head in this sense,” CSIS’s Nakano said.

Toshikazu Okuya, director the Energy Supply and Demand Policy Office at Japan’s Ministry of Economy, argued in an interview at the agency’s headquarters that coal use was still inevitable, and it might as well be Japan that supplies the plants.

“Asian and Eastern European countries will introduce coal thermal plants — no question about it. We cannot stop it. Then who will supply coal thermal technology to them?” he said. “We can’t make all countries stop using coal…because coal is very cheap.”

Japan has also pitched its coal technology as a tool in addressing gaps in energy security beyond Asia — including as a partial fix for Europe’s energy dependence on Russia.

Before the G7 meeting in Germany earlier this year, Prime Minister Shinzo Abe traveled to Ukraine and met with President Petro Poroshenko to discuss, among many other issues, replacing that country’s fleet of Soviet-era coal power plants with new Japanese technology in order to ease its consumption of Russian gas. (American policymakers, meanwhile, have pressed Ukraine to explore its own gas reserves using hydraulic fracturing—a technology that Americans just happen to be very good at.)

“The coal thermal plant is not just business,” said Okuya. “This is strategic, too.”

JAPAN’S EMBRACE OF coal power has attracted concern around the world, notably among Western and Japanese environmentalists, but it has also attracted a chorus of domestic critics.

Nishimura, the former government climate negotiator, is now a senior fellow at the Japan Institute of International Affairs; he opposes his country’s proliferation of carbon-burning technology, and sees it as a step in the wrong direction as much of the world moves away from coal—one that could hurt Japan’s standing in international talks.

“Today, we’re talking about de-carbonization. That’s the key word,” he said in an interview at the Institute’s Tokyo offices. “It’s not okay if you reduce your emissions. We must decarbonize, we must phase out our emissions. So even with the best technology, which will emit the least amount of CO2, is not good as long as it going to be used in other countries.”

“I really don’t believe our prime minister will play any leading role in the Paris climate talks because of our very frugal ambition.” Nishimura said. “In my view, the only solution is for outside pressure, global pressure, to be applied to this country in the name of de-carbonization.”

For much of the international environmental community, Japan’s embrace of coal technology has been a profound fall from grace.

“Among the developed countries, they’ve gone from being quite progressive and a leader on the climate change issue to really forfeiting that position now,” said Taryn Fransen, director of the Open Climate Network, an independent climate policy-tracking coalition.

Japan’s position on coal has turned the country into a frustrating obstacle rather than a consistent ally going into the international climate talks in Paris set for later this year. Schmidt said that a broad view among those following the climate negotiations is that a credibility gap has emerged when it comes to Japan. A more aggressive Japanese emissions target and a commitment to rein in its coal financing might have made it easier for U.S. officials and others nations to press more developing countries in the region to put more on the negotiating table.

Japan’s defense of using coal financing as a means of addressing climate change has also turned greater attention to next month’s meeting of the Organization for Economic Cooperation and Development, where European leaders are hoping to broker a compromise on the issue before the Paris climate talks. The last meeting, held in June, ended in a stalemate with Japan leading the resistance to efforts aimed at restricting support for coal projects.

As the US doubles down on its push against high-carbon fuels, meanwhile, this new commitment to coal by its chief Asian ally is creating an inescapable point of tension.

“I believe that the U.S. government has been making its position very clear to the Japanese government both in the bilateral manner as well as at multilateral platforms like the World Bank and OECD export credit meetings,” CSIS’s Nakano said about coal financing.

“The Japanese climate commitment is not a bad one, especially in light of their nuclear power supply uncertainty,” she added, “but this disagreement hinders the two governments from making climate change as one of the highlighted areas of strong bilateral partnership.”