Earlier this week, I wrote about the global coal renaissance — arguably the most important climate story in the world right now. Since 2000, developing countries like China, India, Vietnam, and Indonesia have been constructing coal-fired power plants at a staggering pace:

The good: coal power is helping these countries climb out of poverty. The not-so-good: the coal boom has led to a surge in carbon-dioxide emissions, and if coal continues to be the world's leading energy source, we'll have little hope of avoiding severe global warming.

Which brings us to our next question: How long will this coal boom continue?

For that, I'd recommend this March 2015 report from two environmental groups, CoalSwarm and the Sierra Club. The authors documented, in painstaking detail, all the new coal plants that have either been proposed, permitted, or are currently being built around the world.

There's a tremendous amount of coal capacity being planned worldwide, some 2,177 plants in all. Not all of these coal plants will actually get finished: many are getting sunk by local opposition or economic headwinds. But if even one-third of the planned plants get built, we run a high risk of busting through the 2°C global warming threshold. And right now, we're on track to do just that.

There are more than enough coal plants in the pipeline to bust through the 2°C threshold

Let's start with the first point — there are at least 2,177 coal units currently on the drawing board around the world. Of those, 557 are actually under construction. The rest are in various planning stages:

This is a huge deal, environmentally: The authors calculate that if even one-third of these 2,177 projects get built, the world will use up nearly all of the carbon budget needed to stay below 2°C of global warming — the widely agreed-on limit in international climate talks.

(Note: This is assuming all the coal plants last their full lifespans. It's possible that some plants could get decommissioned or retired early, though that would cost billions in lost revenue. Alternatively, we could figure out how to retrofit existing coal plants so as to capture their carbon dioxide emissions and store them underground, though that technology is not yet widely commercialized.)

Below is a breakdown of the proposed plants by generating capacity. Between 2005 and 2013, the world added about 626 gigawatts of coal capacity, on net. Going forward, there's another 276 gigawatts under construction, and another 1,000 gigawatts in various stages of planning:

Note that China, India, Vietnam, and Indonesia account for the vast majority of current construction. But it's not just those countries. There are coal plants going up just about everywhere save for the United States (where coal is declining due to cheap natural gas and EPA air-pollution rules) and Western Europe.

On the flip side, proposed coal plants are getting bogged down in places like India

There is a twist here. It's not yet guaranteed that all these proposed coal plants will actually get built. The report's authors — Christine Shearer, Nicole Ghio, Lauri Myllyvirta, and Ted Nace — point out that plans for many plants have fallen through in recent years, due to local opposition, competition from renewables, or various economic forces.

Take, for example, South Asia, which accounts for roughly one-third of all proposals for new coal plants. Right now, for every one proposed coal plant that actually gets built, another six proposals get scuttled due to protests or other complications:

The authors break down some of the steep hurdles facing new coal investments in India:

The paltry amount of capacity actually being implemented is strong evidence that banks and other financial gate-keepers have become hesitant to finance coal projects in India. This hesitancy derives from a confluence of negative factors, including the following: (1) citizen opposition, which appears to be effective in blocking projects even at advanced stages of development, (2) ongoing coal shortages due to Coal India’s inability to meet production targets, (3) very low operating efficiency for India Railways in actually freighting coal on time and as specified, (4) the Coalgate scandal, (5) upheaval in the international coal markets, (6) competition from renewables, and (7) a hugely ineffective transmission and distribution grid (with electricity loss rates regularly exceeding 25%).

This isn't entirely comforting, though. 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 story is a little more complicated than that. Some of the obstacles standing in the way of India's coal boom are things like economic and regulatory dysfunction — things that many people would (rightly) prefer to fix. And we don't want to pin the fate of the planet onto hopes that India's rail infrastructure will remain a hopeless mess forever.

Ultimately, countries like India are building coal plants because they need access to low-cost electricity to help light up homes, offer an alternative to indoor wood-burning, power heavy industry, and lift people out of poverty. The only way they'll shift away from coal is if cleaner alternatives become available that can help these countries accomplish those goals. That's the real challenge here. Renewables like solar and wind can sometimes help, particularly in off-grid rural areas, but they've still got a long ways to go.

(By the way, the report isn't just focused on India. It also delves into coal plans for all the other regions of the world. There's a section on the debate over whether China is starting to curb its appetite for coal — a question I explored here. There's also a section on the coal booms in Southeast Asia, which looks fairly robust, though there is local opposition here and there.)

Bottom line: "Stronger efforts are needed to curb new coal capacity"

The big climate-related takeaway from the report is pretty clear. There are currently 2,177 coal plants either being proposed, being developed, or being built worldwide. If just one-third of those plants get built and end up burning CO2 into the air for their full lifespans, we'll run a high risk of exceeding the carbon budget needed to stay below the 2°C global warming limit — with everything that means for sea-level rise, extinctions, extreme weather, and the like.

And, right now, that's where we're heading. The CoalSwarm/Sierra Club report calculates that since 2010, for every coal proposal that actually gets built, two are getting canceled. Which means we're on track to build roughly one-third of those 2,177 plants.

So there are humanity's options. Either current plans for new coal plants get scaled back in favor of cleaner alternatives. We shut down and decommission existing plants prematurely, leaving behind stranded assets worth billions. We figure out how to retrofit these coal plants to capture their CO2 emissions and store them underground. Or, barring any of those steps, we get ready for a significant rise in temperatures.

Read more: The global coal renaissance is the most important climate story today