New Study Says World Must Cut Short-Lived Climate Pollutants as Well as Carbon Dioxide to Meet Paris Agreement Goals

Climate policies that only focus on decarbonization will not be able to limit atmospheric warming below 2°C. In fact, they would encourage further warming, according to a study published Monday by the Proceedings of the National Academy of Sciences. 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 concluded that decarbonization-only initiatives would result in more warming in the short term. This is because the carbon dioxide and sulfates that are produced by burning fossil fuels cause warming. Sulfate particles reflect sunlight into space, but they only stay in the atmosphere for a few days. This is in contrast to carbon dioxide which heats the planet and stays there for centuries.

Dreyfus stated that the planet’s current warming is offset by the continual release of sulfurates from fossil fuel combustion. She said that the short-term temperature reduction caused by sulfate emissions will be quickly eliminated by transitioning to renewable energy. The planet will continue to heat up for several decades before the longer-term cooling effect of cutting carbon dioxide emissions takes place.  

Dreyfus stated that decarbonization can reduce near-term and long term warming if emissions of methane and HFCs, soot, and nitrous oxide are not simultaneous.  

The current study isn’t the first to highlight the need for short-lived climate pollutants to be addressed along with carbon dioxide emission reductions in order to limit 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. 

Working Group I’s first report was about climate change science. It stressed the need for short-term reductions in climate pollutants. The report stated that almost half of the warming experienced by the planet to date has been caused by greenhouse gases other than carbon dioxide.  

Dreyfus and co-authors stated that Working Group III, which produced its final report, was too focused on climate policy and too much on the long-term effects of carbon dioxide.  

“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.” 

The U.S. and other nations have been focusing on short-lived climate pollutants in recent policy initiatives. More than 100 countries pledged last year to reduce methane emissions by 30% by 2030. These reduction targets are voluntary and it is not clear how the U.S. or other countries will achieve them.

More than 100 countries have also ratified an agreement international to reduce HFC production. The Kigali Amendment of the Montreal Protocol is expected to reduce as much as half a degree additional warming by 2100.

The EPA has recently passed regulations to reduce HFCs in accordance with the Kigali Amendment. However, the United States has yet to ratify 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 stated that it is crucial to continue efforts to reduce methane and HFCs, as well as other climate pollutants, now in order to limit warming in the future.

“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.”