Even in a world of stringent climate policies and clean power generation, the continued use of fossil fuels in industry, transportation and buildings could still cause enough carbon dioxide emissions to endanger the climate targets agreed upon by the international community. That’s the conclusion of an international team of researchers who have just published a study which is the first to focus specifically on residual fossil fuel emissions from these hard-to-decarbonize sectors.

"We wanted to decipher what really makes the difference in terms of carbon budgets and residual emissions,” explained Shinichiro Fujimori, a researcher from the National Institute for Environmental Studies (NIES) and Kyoto University in Japan. “We focused on the role of fossil fuel emissions that originate in industries like cement or steel making, fuel our transport sector from cars to freight to aviation and goes into heating our buildings… these sectors are much more complicated to decarbonize than our energy supply, as there are no such obvious options available as wind and solar electricity generation."

The Paris targets include keeping global warming between 1.5° C to below 2° C, which implies a tight limit on cumulative CO2 emissions until 2100. How tight? Well, if current trends continue, 4,000 gigatons of CO2 will be emitted over the remainder of the century. But to meet the 1.5° C goal, that number would need to be as low as 200 gigatons — a 95 percent reduction.

There are “negative CO2 emissions” technologies designed to pull greenhouse gases out of the atmosphere, such as bioenergy plantations and carbon capture and storage (CCS). But these are uncertain and potentially risky. More tried-and-true mitigation efforts that have been pledged are inadequate to sufficiently reduce emissions.

“We found that even with enormous efforts by all countries… our calculations show that residual fossil carbon emissions will remain at about 1000 gigatons of CO2," said study lead author Gunnar Luderer from the Potsdam Institute for Climate Impact Research (PIK). "This seems to be a lower end of what can be achieved with even the most stringent climate policies, because much of the residual emissions are already locked into the system.”

As a result, Luderer concludes, those negative emissions strategies are not an option — they’re a necessity. The researchers also note that not strengthening nationally determined contributions (NDCs) before 2030 would not only increase near-term emissions, but also hurt longer-term emission reduction potentials. That’s because it locks in even more investments into fossil-based infrastructures.

"Climate mitigation might be a complex challenge, but it boils down to quite simple math in the end: If the Paris targets are to be met, future CO2 emissions have to be kept within a finite budget," said the PIK’s Elmar Kriegler. "The more the budget is overrun, the more relevant will carbon dioxide removal technologies become, and those come with great uncertainties.”

The study appears in the journal Nature Climate Change.