I've written before about the global coal renaissance — the single biggest energy and climate story of the past 15 years. Since 2000, countries like China, India, Indonesia, and Vietnam have been building coal-fired power plants at a torrid pace:

The coal boom has had undeniable benefits, helping poor countries climb out of poverty. But it also has serious downsides: Carbon dioxide emissions accelerated in the 2000s, and if coal continues to be the world's leading source of electricity, we'll cook the planet.

So the biggest, most important climate question for the next 15 years is: How long will this global coal boom last? Or, put another way, when will the rise of clean energy finally stop coal's growth for good?

One invaluable data source here is an annual report from three environmental groups: CoalSwarm, the Sierra Club, and Greenpeace. Each year, the authors document all the new coal plants that have been announced, permitted, or are currently being built around the world.

In their 2016 "Boom and Bust" report, they find the equivalent of 1,500 new coal plants in the pipeline worldwide. That's a staggering number. If even one-third of these plants get built and operate for their full lifetime, we'll likely bust through the 2°C global warming threshold that world leaders have promised to stay below. Even 3°C could be tough to avoid.

But there's a major asterisk here: It's not yet certain all these plants will actually get completed. Since 2010, two-thirds of proposed coal projects have gotten scrapped. China, which today accounts for half the world's planned capacity, has seen its coal appetite wane in the last few years and is tacking toward cleaner energy sources. India is another big question mark. So is Southeast Asia. Let's take a closer look.

Globally, there are enough coal plants planned to bust through the 2°C threshold

As of January 2016, the report's authors found, there were nearly 338 gigawatts of coal power capacity under construction worldwide and another 1,085 gigawatts of capacity in various phases of permitting or planning. This is the equivalent of 1,500 large coal plants:

China, India, Vietnam, Indonesia, and South Korea are leading the current coal rush. Japan is also contemplating dozens of new coal-burning units to replace its nuclear fleet, which has been shuttered indefinitely after Fukushima. But it's not just Asia. There are coal plants in the works just about everywhere, save for the United States.

This is a huge deal, environmentally. If we want decent odds of staying below the 2°C global warming threshold, then humanity can only emit another 765 gigatons of CO2 into the atmosphere, give or take. (A gigaton is a billion tons.) We currently emit about 35 gigatons per year from existing infrastructure, so our "carbon budget" is rapidly dwindling.

If all the new coal plants now under construction were built and operated for their full lifespans, they'd pump an extra 58 gigatons of CO2 into the atmosphere. If all the additional planned/permitted plants were built, that would emit another 186 gigatons of CO2. There are a lot of variables at play here — since you also have to factor in oil and gas infrastructure — but it looks extraordinarily difficult to stay below 2°C if even one-third of these planned coal plants get built.

(Note: This is assuming all coal plants last their full lifespans, usually 40 years or more, and run at average capacity. It's entirely possible that some plants could get retired early, though that would mean billions in lost revenue. Alternatively, we could figure out how to retrofit existing coal plants so as to capture their CO2 emissions and store them underground, but that technology is not yet widely commercialized.)

China accounts for half the planned coal plants — but could still cancel many of them

Now, it's not yet guaranteed that all of these proposed coal plants will get built. As the report's authors — Christine Shearer, Nicole Ghio, Lauri Myllyvirta, Aiqun Yu, and Ted Nace — point out, two out of every three proposed plants since 2010 have been canceled due to economic headwinds, local opposition, or growing competition from cleaner energy sources. The question is how many get scrapped going forward.

Take China, which currently has 193 gigawatts under construction and 515 gigawatts in various stages of planning — roughly half of the proposed coal capacity worldwide:

As I've reported before, there's a reasonable argument that many of these proposed coal plants are superfluous. China's coal consumption has shrunk the past two years as its economy has slowed down, and the country is trying to transition away from heavy industry. If China manages this high-wire rebalancing act successfully, it's possible that future electricity demand will be much lower — and can mostly be filled by aggressive plans to expand nuclear, hydro, wind, and solar.

That's the optimistic view, at least, and it's why some analysts think China has either hit peak coal or will very soon. But with China, nothing is ever simple. Despite record-low utilization rates at existing plants, many of China's mining provinces are still pushing new coal plants as an economic development strategy. The central government, for its part, has ordered a halt to some of these plants, but not all. Beijing also has to be mindful of rising unemployment in these areas.

So there's a fierce tug of war here, and the outcome is uncertain. China arguably doesn't need all of these coal plants, but it may end up building a sizable fraction regardless. If it does, that would make it harder to expand clean energy and push down CO2 emissions in the future.

India has major plans to expand coal capacity — but has run into serious obstacles

India is another focal point. The country has 72 gigawatts of coal capacity under construction and another 218 gigawatts in various planning stages — more than any country save China:

India has plenty of reasons to want more coal capacity. It hasn't yet gone through the same rapid coal-driven industrialization that China did in the 2000s. There are still more than 300 million people in the country without electricity. And though the country is scaling up clean energy to address climate change, this has limits. Wind and solar, while growing fast, still have intermittency issues. And nuclear power has struggled to gain a foothold. That's why Prime Minister Narendra Modi has called for a near doubling of coal production by 2020.

Yet this isn't a sure thing. India faces a slew of obstacles to expanding coal capacity. Nearly 390 gigawatts worth of projects have already been put on hold since 2010. The authors of the "Boom and Bust" report break down some of the hurdles:

Barriers include financially strapped electricity distribution companies; the difficulty of ramping up mining of domestic coal; the high cost of coal imports relative to the constraints on the ability of India’s distribution companies to charge their customers for power; and grassroots opposition against new coal plants and mines. Finally, given the inadequacies of India’s electrical grid, there is the fundamental question of whether building more coal plants is actually a feasible way to deliver power to the estimated 300 to 400 million people in India lacking access to electricity.

(There's more detail in the report if you're curious. Or see the IEA's special report on India for a deep dive.)

These obstacles shouldn't be entirely comforting to climate hawks. Ideally, what we'd want is for India to turn away from coal and toward cleaner sources of energy because the latter is cheaper and presents a plainly superior path for development.

But the India story is more complicated than that. Many of the barriers standing in the way of a national coal binge are things like economic and regulatory dysfunction — barriers that many people would rightly prefer to tear down. We don't want to pin the fate of the planet onto hopes that India's rail infrastructure will remain a hopeless mess forever.

Beyond India, the report also delves into coal aspirations for all the other regions of the world. The section on Southeast Asia is especially noteworthy: demand seems robust, but there is growing local opposition. Vietnam, for instance, is grappling with severe air pollution, and the government has recently ordered a halt to new coal construction while it carries out a review.

Is the world facing a coal bubble?

One striking aspect of the "Boom and Bust" report is its argument that most of these planned coal plants are unneeded and amount to a "capacity bubble" — in which investors risk squandering $981 billion.

That's certainly contentious. There are places like China where a major expansion of coal capacity is arguably overkill and new plants may prove money-losing white elephants. Likewise, Japan would no doubt be better off restarting its nuclear fleet than massively ramping up coal consumption. But the picture is murkier for poorer countries like India.

Most developing countries are building coal plants because they need access to low-cost electricity to light up their homes, provide an alternative to indoor wood burning, bolster industry, and lift people out of poverty. For all its downsides — including deadly air pollution — coal has a proven track record of enabling economic growth. The only way countries will turn away from coal is if cleaner alternatives become available that can accomplish all those goals.

That's the real challenge here. The report's authors argue that renewables like solar and wind can bring electricity to the world's poor, particularly in off-grid rural areas. In some cases, yes. But those sources still have a ways to improve, and right now they can't provide the sort of 24/7 power needed to run factories and heavy industry — the path to development that every other rich country has taken to date.

What's clear is that the world desperately needs to deply low-cost viable alternatives to coal if we want to halt global warming. In some places, that may involve more natural gas, which has half the CO2 footprint of coal (though methane leaks are a concern). Elsewhere, it will mean nuclear power. Or hydropower dams. Or wind plus solar plus storage. Or perhaps someday we'll figure out carbon capture for coal. (This Allam cycle technology may prove useful. MIT scientists have also proposed a method for cutting CO2 from coal plants in half and sequestering the waste.)

This is the major energy task of the next two decades. If we can't stop the global coal boom, then we better prepare for a serious rise in temperatures.

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