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Carbon dioxide emissions from burning fossil fuels must nearly completely cease in the next couple decades to have a chance at keeping climate warming within a relatively safe level for humanity.

As last month’s IPCC report made clear, even if global emissions move to a very low carbon track, we’ll likely hit 1.5ºC of surface warming by midcentury, passing a target that global leaders have set as a goal to limit the worst effects of climate change. But, in a near future world of net-zero emissions, this could be a temporary “overshoot”, and we might be able to wrangle the atmosphere back within that boundary by 2100. 

So, what do these warnings mean for the actual reserves of these hydrocarbons—coal, oil, and natural gas—that countries sit on top of? A new study, published Wednesday in Nature, estimates how much of fossil fuel reserves must be left in the ground for a 50 percent chance of limiting warming to 1.5ºC. “This is a very important study because it provides updated estimates which are specific to the [1.5ºC] target of the Paris agreement,” Mathew Barlow, a climate scientist at the University of Massachusetts Lowell who wasn’t involved with the paper, told PopSci in an email. 

Right now, the majority of the world’s energy is still harvested from hydrocarbons, though their use seems to be leveling off in some cases. Global coal production peaked in 2013 and coal plants are increasingly retiring. Oil extraction seems to have plateaued in recent years. But fossil gas (AKA “natural gas”) has yet to crest (apart from a pandemic-related dip in 2020), having become a major supply to meet increased energy demand and a fuel viewed by the industry as a “transition fuel”—or perhaps a way to buy time as the climate crisis grows ever more pressing. 

According to the most recent UN Emissions Gap Report, the climate mitigation policies that countries around the world have pledged are not enough to meet the international Paris Agreement goals of 1.5 or, less ideally, 2ºC warming. The fossil fuel production that countries have planned is 120 percent higher than what’s acceptable for staying within 1.5ºC of warming.

[Related: The 4 biggest lessons from the latest IPCC climate report]

To further illustrate how much reserves we should be leaving in the ground, the Nature study authors, all researchers at University College London, used a global energy system model. By working backward from an emissions trajectory that results in keeping temperatures within the 1.5ºC target, they calculated what proportion of current identified fossil fuel reserves could be mined within that emissions budget. They found that, globally, 58 percent of oil, 59 percent of fossil gas, and 89 percent of coal needs to stay in the ground for a 50-50 shot at limiting warming this century to 1.5ºC. (In a press briefing, coauthor James Price, an energy and climate change researcher, said that 50-50 chance is “essentially at the very limit of what our model can solve for” given assumed constraints such as global energy demand).

Their analysis included economic factors that determined the most cost-effective approach countries could take. That means that nations in which it’s both expensive and carbon-intensive to mine hydrocarbons would need to leave a greater amount in the ground. In Canada, for example, where oil sands represent an abundant but difficult and dirty resource to extract, would need to leave 82 percent of its reserves alone. This global cost-benefit analysis could be helpful for thinking about how countries could collaborate for the greater good of limiting carbon dioxide concentration in the atmosphere, says Jean-François Lamarque, a climate scientist with the National Center for Atmospheric Research. “That’s one of the issues, the degree of cooperation between countries.”

To reach the level of reduced extraction required, many countries would need to peak fossil fuel production in the next few years. Fossil fuel production would need to decline by a global average of 3 percent every year to 2050.

After 2050, in the model, only a few hard-to-decarbonize sectors would continue to use hydrocarbons. These include the petrochemical industry, for uses like manufacturing plastics. Airplane engines may also prove tricky to switch to zero carbon fuels. 

“This has wide implications for businesses and countries who are not accounting for these large needed changes and highlights the stark contrast between what needs to be done to meet the 1.5C target and what is actually being done,” says Barlow. “For instance, any new or continued development of fossil fuel infrastructure like oil or gas pipelines is clearly not consistent with limiting warming to 1.5ºC or 2ºC and is very likely a huge financial risk as well, as such projects will likely become “stranded assets” in the near future.”

“Further investment in fossil fuel extraction is not compatible [with mitigating climate change], as shown by this research,” coauthor and energy systems researcher Steve Pye said in the press briefing. “And this is particularly important for climate leaders who are looking at projects in their own backyard and overseas.” Countries that have historically benefitted from fossil fuel extraction should take the lead on leaving their remaining reserves in the ground, write the authors in the paper, and support other countries that rely on hydrocarbon production for their economy and have limited ability to transition.

[Related: Offshore wind has huge potential. Here’s how it could change the US.]

The authors also emphasize that—while they’ve made a thorough effort to estimate what needs to stay in the ground—it’s probably an underestimate. For one, we’re talking about a 50-50 chance; if we want better odds, we have to scale down production even faster. There’s also the looming uncertainty around earth system feedback loops that could lead to accelerated warming, as well as the scalability of technologies like direct carbon capture, which is currently very expensive. 

The window for this action is small, but still technically feasible. “It really is a case of having the political will to resist the temptation of extracting every last bit of fossil fuels and focusing on really pushing hard on the low carbon economy,” Price said in the press briefing. “It really comes down to the politics of the situation.”

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