Climate policies that rely on decarbonization alone are not enough to hold atmospheric warming below 2 degrees Celsius and, rather than curbing climate change, would fuel additional warming in the near term, a study published Monday in the Proceedings of the National Academy of Sciences concludes. The study found that limiting warming in coming decades as well as longer term requires policies that focus not only on reducing emissions of carbon dioxide, but also of “short-lived climate pollutants”—greenhouse gases including methane and hydrofluorocarbons (HFCs)—along with black carbon, or soot.

“We’re simultaneously in two races to avert climate catastrophe,” said Gabrielle Dreyfus, chief scientist for the Institute for Governance & Sustainable Development and lead author of the study.  “We have to win the sprint to slow warming in the near term by tackling the short-lived climate pollutants, so that we can stay in the race to win the marathon against CO2.”

The study used climate models to assess how the planet would respond if countries addressed climate change solely through decarbonization efforts—namely transitioning from fossil fuels to renewable energy—without reining in methane and other short-lived but potent climate pollutants.

The authors found that decarbonization-only efforts would actually result in increased warming over the near term. This is because burning fossil fuels emits both carbon dioxide and sulfates. Unlike carbon dioxide, which warms the planet and remains in the atmosphere for centuries, sulfate particles reflect sunlight back into space but only remain in the atmosphere for several days, so they have a powerful, but short-lived cooling effect.

The continual release of sulfates through the ongoing burning of fossil fuels currently offsets roughly half a degree of warming that the planet would otherwise experience from the carbon dioxide emissions of fossil fuel combustion, Dreyfus said. Transitioning to renewable energy will quickly remove the short-term curb on warming provided by sulfate emissions, and the planet will continue to heat up for a couple of decades before the longer-term cooling from cutting carbon dioxide emissions takes hold, she added.  

If, however, emissions of methane, HFCs, soot and nitrous oxide occur at the same time as decarbonization, both near-term and long-term warming can be reduced, Dreyfus said.  

The current study is not the first to identify the need to address short-lived climate pollutants along with carbon dioxide emissions reductions to rein in climate change. In 2018, the U.N.’s Intergovernmental Panel on Climate Change noted that reductions of short-lived climate pollutants were essential to any effort to limit warming to 1.5 C.   

However, more recent reports, such as the IPCC’s sixth assessment, a three-part report published in 2021 and early this year, sent mixed messages about the need to reduce short-lived climate emissions, Dreyfus and her co-authors said. 

The first report of the series, produced by Working Group I, focused on the science of climate change and underscored the need to reduce short-lived climate pollutants. The report noted that nearly half of all warming that the planet has experienced to date comes from greenhouse gases other than carbon dioxide.  

However, Working Group III, which produced the final report of the series and focused on climate policy, placed too much emphasis on the long-term impacts of carbon dioxide and focused too little on rising temperatures in the near term, Dreyfus and her co-authors said.  

“If you’re going to pass one and a half degrees in 10 years, and then you are going to pass two degrees in about 25 years, that’s what we need to focus on,” said Veerabhadran Ramanathan, an atmospheric and climate sciences professor at Scripps Institution of Oceanography and a co-author of the study. “We need to cut the short-lived pollutants so that there are no short-term catastrophes in the next 25 years, without losing track of the long term.”

David Doniger, the Natural Resources Defense Council’s climate and clean energy program director, who was not part of the current study, agreed. 

“Until recently, you might say that CO2 sucked all the oxygen from the room in international negotiations and domestic policy making,” he said. “Now we know we must rapidly curb the extremely potent, short-lived non-CO2 heat-trapping pollutants to meet the near term challenge, as well as curb CO2 itself for the longer run.” 

In recent policy efforts, the U.S. and other countries have begun to target short-lived climate pollutants alongside carbon dioxide. Last year more than 100 countries pledged to collectively reduce methane emissions by 30 percent by 2030. Those reduction targets are voluntary, however, and it remains unclear how the U.S. and other countries will meet their goals.

Similarly, more than 100 countries have ratified an international agreement in recent years to phase down HFC production and use. The agreement, known as the Kigali Amendment to the Montreal Protocol, is expected to eliminate as much as half a degree of additional warming by 2100.

The EPA recently passed regulations to phase down HFCs that align with reductions called for in the Kigali Amendment, but the U.S. has not yet ratified the international agreement. The amendment, which enjoys rare bipartisan support as well as the support of industry, passed a vote in the Senate’s Foreign Relations Committee earlier this month; a full Senate vote has not yet been scheduled.

Dreyfus said following through on efforts to reduce methane, HFCs and other short lived climate pollutants now will be crucial to curbing warming in the coming decades.

“We know what levers to pull to slow that warming in the near term, “she said, “we just need to intentionally make it part of our strategy.”