It has been decades since we recognized the threat of climate change, yet very few governments have instituted policies that address the threat. The first strong international agreement was only established very recently. Meanwhile, the companies that supply fossil fuels continue to push exploration for new supplies.

Under those circumstances, it's fair to consider what would happen if the burning of fossil fuel continued unabated. The Intergovernmental Panel on Climate Change typically considers a scenario in which fossil fuel use continues along its current trends until the end of the century. But a new study examines what would happen if the burning of fossil fuels continues for centuries and we gobble up a conservative estimate of everything that's left to extract. That study suggests that the future is going to be significantly warmer than we might have expected.

Carbon dioxide's warming influence doesn't go up in a linear fashion as its concentration increases. Once an infrared photon gets absorbed, it can't be absorbed again, no matter how many additional carbon dioxide molecules are present. So, as concentrations go up, the warming influence tails off. This is generally approximated by saying that each doubling of the gas' concentration has about the same warming effect.

Does that approximation hold up under real-world conditions? Our current emissions are on the path to allow us to find out. But it would be nice to know before we got there. So, the researchers behind the new work, all based in British Columbia, set up some whole-Earth climate models to explore how the planet would respond to lots of additional carbon—5,000 gigatonnes, roughly equal to the amount currently in conservative estimates of our fossil fuel reserves. The models were run out to the year 2300.

Things got complicated once the researchers started looking into how much of the ensuing emissions end up in the atmosphere. Right now, the ocean and land ecosystems absorb a lot of our emissions. Because carbon dioxide promotes plant growth, researchers expect the land will soak up carbon until about the year 2100, at which point the land will become saturated. Extreme temperatures in the tropics will limit the amount of plant growth. The oceans will not stop absorbing our emissions between now and 2300, but their rate of absorption will slow a bit after 2100.

So, in the future, more of the carbon dioxide we emit will stay in the atmosphere. And what ends up in the atmosphere does more warming than we might have expected. "The ratio of warming to cumulative emissions continues to behave approximately linearly even up to cumulative emissions of [5,000 gigatonnes]," they write.

How is that possible? Although part of the answer is the carbon feedbacks mentioned above, the authors find that the issue might be with the ocean. In most of their models, heat transport into the deep ocean doesn't continue to rise with the rising temperatures. As a result, more of the energy remains stuck in the atmosphere and upper ocean. Smaller-scale climate models, which are generally used for longer-term simulations, simply don't recapitulate these details and therefore show less warming on these timescales.

What does this mean for the planet? By 2300, the models suggest that the global warming will be somewhere between 6.4 and 9.5 degrees Celsius. In the Arctic, however, the warming will reach at least 15 degrees Celsius, and possibly close to 20 degrees Celsius. Precipitation will also change dramatically, with some regions seeing a four-fold increase.

The work highlights how approximations in some of the less computationally intense models lead to different results over the long term. But it mostly highlights the huge disconnect between policymakers and businesses. While governments are talking about keeping warming below 2 degrees Celsius, energy companies are currently looking for other places to extract hydrocarbons. At this point, it's not clear which of these futures we're going to get.

Nature Climate Change, 2016. DOI: 10.1038/NCLIMATE3036 (About DOIs).