In the abstract, moving away from fossil fuels sounds relatively straightforward. But the closer you look, the more complex the challenge appears. How quickly do we have to kick the fossil fuel habit? Should developing nations forego the artery-clogging feast of dirty energy that other nations used to fuel their development? And how much coal, oil, and gas will each nation have to leave in the ground—potential profits notwithstanding?

In a new study, University College London researchers Christophe McGlade and Paul Ekins examine that last question. The latest Intergovernmental Panel on Climate Change report concluded that between 2011 and 2050, we can only emit around 1,000 gigatons of CO 2 if we want to limit warming to 2°C above preindustrial temperatures, which governments have pledged to do. Current fossil fuel reserves—the known amount of fossil fuels that can be produced at a reasonable profit today—equal almost 3,000 gigatons of CO 2 . Adding in the fossil fuels that are not economically viable today but probably could be eventually brings that number up in the neighborhood of 11,000 gigatons.

That means a whole lot of fossil fuels need to stay in the ground. The researchers ran simulations with economic models to find the mix of fossil fuels that maximizes the economic benefit to each country while still staying below 1,000 gigatons of CO 2 . They took into account the costs of producing various types of coal, natural gas, and oil as well as the cost of bringing it to market in each region. In the end, they produced estimates of how much of each region's reserves should be considered “unburnable” in this economically optimized scenario.

They present the amount of fossil fuels that can’t be produced as percentages of current reserves, but a small amount of currently non-economic resources end up being extracted instead of reserves. So it’s slightly more complicated than saying “This chunk of reserves can be produced and the rest can’t,” but the general point is the same.

Leave 80 percent of unmined coal in the ground

Globally, the authors find that one-third of oil reserves, about half of the natural gas, and over 80 percent of coal must be left in the ground in 2050 to stay below 2°C warming.

Breaking the world apart into 10 regions, things get even more complex. Looking at oil, the US could produce over 90 percent of its oil, while Canada would have to leave 75 percent in the ground (largely because Canada’s oil sands are more costly). While the Middle East would only be leaving 38 percent of its oil—just above the global average—that works out to be 263 billion barrels, accounting for about half of the total global “unburnable” oil.

The US and Europe could produce most of their natural gas as well, while China, India, and the Middle East could only produce about 40 percent of theirs. Again, the Middle East would pitch in about half the world’s restraint. And the shale gas revolution would remain primarily a US phenomenon, with the US capturing about half the global total of unconventional gas.

The proportions of “unburnable” coal would be a little more even. The regions richest in coal are the US, China and India, and the former Soviet Union states. The US and former Soviet Union would have to leave over 90 percent untouched. Rapidly developing China and India could use a little more— roughly 30 percent of their coal.

These numbers vary a bit depending on whether carbon capture and sequestration technology is available, but it’s not a huge impact given that the model assumes such efforts don’t begin until 2025 and expand relatively slowly. Still, without carbon capture, coal faces an even bigger disadvantage; with it, a little more has to swap out for cleaner-burning natural gas.

The fact that we can’t even use all of our current reserves calls into question the value of discovering or developing new sources of fossil fuels. No new production of oil and gas takes place in the Arctic, for example, in the simulations that meet the total emissions target. In an associated article about the study, Potsdam Institute for Climate Impacts Research scientists Michael Jakob and Jérôme Hilaire note, “Fossil-fuel companies must therefore ask themselves whether they should continue to invest in exploration for, and processing of, oil, gas, and coal, or risk losing billions of dollars of stranded assets.”

“To conclude,” the study’s authors write, “these results demonstrate that a stark transformation in our understanding of fossil fuel availability is necessary. Although there have previously been fears over the scarcity of fossil fuels, in a climate-constrained world this is no longer a relevant concern: large portions of the reserve base and an even greater proportion of the [total] resource base should not be produced if the temperature rise is to remain below 2°C.”

Jakob and Hilaire also point out the practical implication of this research: “Importantly, [this study] clearly highlight[s] the distributional challenge of climate policy: imposing a limit on the use of fossil fuels transfers economic benefits (known as rents) from resource owners to those who obtain the right to use the remaining burnable reserves. Hence, successful climate policy will crucially hinge on the question of whether this ‘climate rent’ can be shared in an equitable way that also ensures resource owners are compensated for their losses.”

Nature, 2015. DOI: 10.1038/nature14016, 10.1038/517150a (About DOIs).