If you only focused on the United States, you might think coal's days were numbered.

The dirtiest of all fossil fuels once provided more than half of America's electricity. That has since dropped to 39 percent, thanks to competition from cheap natural gas, a tireless campaign by the Sierra Club to shutter old coal plants, and strict new air pollution regulations. Add in the Obama administration's upcoming crackdown on carbon-dioxide emissions from power plants, and US coal will keep waning in the future.

But that's not true globally. Far from it. According to data from BP's Statistical Review of Energy, coal consumption has actually been accelerating worldwide since the end of the 1990s:

It's tempting to think this global coal boom is mainly a one-time blip due to China, where coal use has surged since 2000 amid frenetic economic growth but has since leveled off as the country tries to transition away from heavy industry. But as it turns out, that's not true either.

The coal renaissance isn't just about China — it's global

According to an important study in The Proceedings of the National Academy of Sciences, we're in the midst of a global "renaissance of coal" that's not confined to just a few countries like China or India. Rather, coal is becoming the energy source of choice for a vast array of poorer and fast-growing countries around the world, particularly in Southeast Asia. "This renaissance of coal," the authors write, "has even accelerated in the last decade."

Why is coal so widely popular? The authors of the PNAS study — Jan Christoph Steckel, Ottmar Edenhofer, and Michael Jakob — argue that coal is often the cheapest energy option in many parts of the world, relative to other sources like oil, gas, nuclear, or renewables.

What's interesting is that countries no longer need their own domestic mines to take advantage of coal power. International coal markets have become so robust, with exports surging in mining countries like Australia and Indonesia, that it's become much easier for a wide variety of countries to build coal-fired power plants. (Notably, the authors say, the price of coal itself, rather than the capital costs of building power plants, seems to be the important economic driver here.)

It's impossible to tackle climate change without tackling the coal boom

Suffice to say, this has huge implications for global warming. Coal now accounts for roughly 30 percent of the world's primary energy use. That fraction, the authors say, is simply incompatible with "ambitious scenarios of climate-change mitigation."

The chart below, from the PNAS paper, offers one way to visualize this. In order to fend off significant climate change, we'd want global CO2 emissions to stay somewhere in the red zone — falling dramatically by midcentury. To do that, coal's share of the world's primary energy would need to drop from 30 percent today to below 10 percent in the next two decades:

Right now, that task looks daunting. Yes, China is taking proactive steps to curtail its coal use, but many other poorer and fast-growing countries aren't. Vietnam, India, and Indonesia are all embarking on their own coal sprees. And once a country builds a new coal-fired power plant, that plant generally lasts for 30 to 50 years or more, spewing carbon all the while. So the more coal plants the world builds, the harder it gets to reduce coal's share — because of "lock-in" effects.

"If that lock-in is to be avoided," the authors warn, "international climate policy must find ways to offer viable alternatives to coal for developing countries." Not impossible, but not easy either.

Is there any way to halt the global coal renaissance?

The authors of the PNAS paper consider a variety of options for reining in the coal boom. Some people have suggested that policymakers try to clamp down on coal supplies and shutter coal infrastructure to increase the relative price of coal and make it less appealing. (US environmental groups have had some success blocking export infrastructure in the Pacific Northwest.) One economist at Northwestern, Bård Harstad, has even suggested that wealthy countries buy up coal reserves — and not burn them — in order to drive up the price.

The authors of the PNAS paper caution that "supply-side" restrictions on coal, on their own, are likely to prove fairly painful for poorer countries — at least compared with policies to reduce demand, such as a global climate agreement that provides explicit aid to help developing countries to shift away from fossil fuels. But it's also possible to imagine a mix of the two strategies: "Integrating a coal moratorium in a mix of other policy instruments, including lower-than-optimal carbon prices and support for low-carbon technology, seems to be more promising," they write.

The authors also point out that some of the most successful policies in developing countries to transition away from coal have tended to focus on "co-benefits" like health or air quality rather than emphasizing climate change per se. Examples include China's efforts to get its urban smog under control, or Vietnam's push to reform costly fossil-fuel subsidies.

Ultimately, though, the authors don't sound terribly optimistic about the prospects of poorer countries shifting away from coal in the short term. So, as a last resort, they suggest measures to ensure that, at the very least, any new coal plant built today can be retrofitted with carbon capture and storage technology down the road. (One problem here is that it's far from certain carbon capture will ever become viable.) They call this a "second-best alternative to measures such as carbon pricing that may be more politically contentious."

Whatever the policy approach, they add, we'll need to figure something out — and soon: "Developing economies now account for such a large share of global energy use that the trend toward higher carbon intensity in these countries cancels out the effect of decreasing carbon intensities in industrialized countries. If the future economic convergence of poor countries is fueled to a major extent by coal — i.e., if current trends continue, ambitious mitigation targets likely will become infeasible."

Update: Here's my follow-up story on the future of the coal renaissance: "There are 2,100 coal plants being planned worldwide — more than enough to cook the planet."

Further reading

-- Clean energy is growing fast — but it's not yet winning the race against fossil fuels.

-- "China's emissions have been plummeting lately. What's going on?" A closer look at China's efforts to curb its appetite for coal and cut its CO2 emissions. Note that China is the world's biggest coal consumer, so even if it's not the entire story, it's a big one.

-- "Inside the war on coal." An excellent piece by Michael Grunwald looking at the various reasons for coal's decline in the United States.