Combine harvests corn stover.
A combine harvests corn stover (corn leaves, stalks, and cobs), one of the chief fuels in bioenergy production. Credit: Idaho National Laboratory Bioenergy Program, CC-BY-2.0

Through the 2015 Paris Agreement, nearly 200 state parties collectively aspired to limit global warming to 1.5°C above preindustrial levels. Even compared to 2°C of warming, meeting this goal would significantly curtail the extent of heat waves and other extremes induced by rising temperatures. But by 2017, the world had already reached 1°C above preindustrial levels and is projected to hit 1.5°C in 2040 with the current pace of warming.

How can we meet such an ambitious target?

The United Nations formally turned to the Intergovernmental Panel on Climate Change (IPCC) to get input from scientists. In response, the IPCC presented in 2018 a special report that included about 50 scenarios that could limit warming to 1.5°C. These mitigation scenarios modify variables including population, consumption of goods and services (including food), economic growth, behavior, technology, policies, and institutions. A new study, published in Environmental Research Letters, finds a problem with these scenarios: Only half can be realistically achieved, and all require the world to take a wide array of very bold actions.

We Need All Options

The study assessed how reasonable the IPCC scenarios were based on the extent to which they include five actions: reducing fossil fuel use, reducing energy use, planting more trees, reducing greenhouse gas emissions besides carbon dioxide (CO2), and removing CO2 from the air to store deep underground.

Shifting away from fossil fuels is vital, but “we have to do something more than that in terms of structural changes, behavior, energy demand, and land demand.”

Through their appraisal, the authors found that the only realistic scenarios to limit warming to 1.5°C are ones in which all five options are pursued at full throttle. “We do not have the luxury to discard a few and just rely on the others,” said Elmar Kriegler, Professor for Integrated Assessment of Climate Change at the University of Potsdam, Germany, and a coauthor of the study.

The work is “highlighting some points that people are missing” from the IPCC special report, said Natalie Mahowald, a professor of atmospheric science at Cornell University in Ithaca, N.Y. Mahowald was a lead author of the special report but was not involved in the new study. Shifting away from fossil fuels is vital, she said, but “we have to do something more than that in terms of structural changes, behavior, energy demand, and land demand. I feel like people didn’t really understand that” when the special report was published.

For example, the required reductions in global energy use “are profound,” Mahowald said. “We did not do them under COVID,” she pointed out, when global CO2 emissions dropped by only around 6%.

“Questionably Optimistic” Assumptions

Many modeled scenarios rely too much on bioenergy with carbon capture and storage.

In the paper, the authors identify several scenarios that include “questionably optimistic” technology deployments and behavioral shifts. “The underlying assumptions which have been made in the report are not always realistic or feasible,” said Daniela Jacob, director of the Climate Service Center Germany, a coauthor of the study, and a coordinating lead author of the special report. Most prominently, she said, many modeled scenarios rely too much on bioenergy with carbon capture and storage, or BECCS. This strategy entails growing crops (or harvesting scraps) to burn for energy, trapping the resulting CO2, and strategically storing it deep underground. Because the plants pulled CO2 from the air as they grew, the net effect is long-term CO2 removal.

At the time of the special report, the scenarios—most of which were created using integrated assessment models—“had difficulty limiting energy demand and dropping CO2 emissions quickly enough,” said Heleen de Coninck, a professor at Eindhoven University of Technology, Netherlands, and a special report coordinating lead author who was not involved in the new study. “There was this option available: BECCS that was producing electricity and it was giving you negative emissions. The models were overusing it, some models more than others,” she said. Alternative technologies like direct air capture of CO2 will likely take some of the spotlight from BECCS in future IPCC reports as the science advances.

More to Consider

Although the paper has value in “narrowing down the options,” said de Coninck, it avoids some important conversations that limit its scope. Notably, competition between strategies like reforestation and BECCS over land resources is left out, a factor that would likely further reduce feasibilities.

Still, the paper makes bold, yet warranted, conclusions that the IPCC, with its stance to be “policy-neutral, never policy-prescriptive,” cannot. Jacob agrees that the IPCC should not be policy prescriptive, but “it’s very clear that we have to act now,” she said. Outside the IPCC, “we cannot shy away from absolute statements on feasibility and on urgent needs.”

These absolute statements aim to motivate substantive debate about how to meet the Paris Agreement’s target. But that debate needs to happen quickly if the world hopes to limit warming to 1.5°C. After a couple more years of current emission rates, Kriegler said, “our attainability statement that it’s still possible is going to disappear.”

—Jordan Wilkerson (@JordanPWilks), Science Writer


Wilkerson, J. (2021), Half of the IPCC scenarios to limit warming don’t work, Eos, 102, Published on 07 July 2021.

Text © 2021. The authors. CC BY-NC-ND 3.0
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