Record-high global temperatures aren’t driven only by well-known greenhouse gas culprits.
These other emissions, unlike carbon dioxide, don’t have a direct warming effect on their own. Instead, they trigger reactions in the atmosphere that create more greenhouse gases or make the gases stick around longer.
A new paper published Thursday in the journal Science suggests that 15 percent of human-driven global warming has come from these indirect interactions. None of these pollutants appear on the international climate treaty list that forms the basis for nations’ pledges to cut back—and the authors say it’s time for that to change.
“We’re emitting things into the atmosphere that don’t directly warm the planet, but they increase the amount of the greenhouse gases that do directly warm the planet,” said the paper’s lead author, Ilissa Ocko, a former climate advisor for the U.S. Department of State. She’s now senior climate scientist at Spark Climate Solutions, a nonprofit that aims to identify and mitigate sources of unmanaged climate risk.
Carbon monoxide and non-methane volatile organic compounds are the major players named in the paper, indirectly accounting for most of that 15 percent of warming. Black carbon, commonly known as soot, also contributes.
These drivers of global warming are not addressed by the 1997 Kyoto Protocol, the treaty that set the standard for which gases are tracked in climate policy agreements. Indirect contributions to warming were already being studied at the time, but there wasn’t enough detail to form a basis for policy commitments.
The paper’s authors, who work at groups including the Environmental Defense Fund and include a former U.S. deputy special envoy for climate, think there’s now enough information to act. Their work synthesized data, including from the Intergovernmental Panel on Climate Change’s Sixth Assessment Report, which was released in 2021.
But integrating carbon monoxide and the other contributors into policy will be an uphill battle.
“The argument for including these gases in a climate accounting framework has been made since the late 1990s,” Vaishali Naik, one of the authors of the 2021 IPCC report and a scientist at the National Oceanic and Atmospheric Administration, wrote in a statement she shared with Inside Climate News. Despite the progress in the generation since then, she notes that “persistent scientific and political challenges remain.”
One is that there is still work to be done on quantifying the emissions from specific sources and tracing them to their effects on the climate, Naik said.
“The political climate in many countries is not for adopting even stronger climate rules,” added Michael Gerrard, founder of Columbia University’s Sabin Center for Climate Change Law. It’s enough of a struggle to meet current emission reduction goals that he doubts countries would want to add more pollutants to the list.
But despite the challenges, the paper’s finding “highlights an important missing piece of the climate regulatory picture,” Gerrard said. “It demonstrates that these pollutants that are not regulated under the climate regime still have some significance.”
The collective impact of the indirect climate pollutants tops all but two of the seven greenhouse gases on the Kyoto list, the study found.
The upside is that unlike No. 1 on the list, carbon dioxide—which stays in the atmosphere for centuries—their contributions are short-lived. Reducing them could slow the rate of global warming in the near future.
These potential benefits are more important than ever. “We’re already seeing damages, so anything we can do to shave off extra fractions of a degree is critical,” said Ocko.
While the indirect climate pollutants have been largely under the radar in climate talks, they’re regulated as health-harming air pollutants in multiple countries, including the United States. Carbon monoxide, for instance, contributes to smog.
Ocko, who thinks their impacts shouldn’t be siloed, is optimistic that policy work on the pollutants can slow the rate of global warming while also improving air quality.
“I’m excited to see where all of this goes,” she said, “and hopefully we can uncover new mitigation opportunities to address climate change.”
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Nina Sablan is a Pennsylvania-based fellow at Inside Climate News with a passion for local journalism, and environmental justice and law. She was previously a reporting intern for Seven Days in Vermont and is a current student at Swarthmore College, majoring in philosophy and minoring in peace and conflict studies in the honors program. Her work has appeared in The Philadelphia Inquirer, as well as campus publications at Oxford University and Swarthmore College.