Soot from car exhaust and cookstoves, sulfates from coal-fired power plants, methane leaked during oil and gas production, and hydrofluorocarbons (HFCs) from air conditioning are all greenhouse gases that trap heat within the Earth's atmosphere for a short while before decaying into less virulent chemicals.

Cutting emissions of such "short-lived climate pollutants," or SLCPs, will not have much impact on long-term climate change, finds a new study published yesterday in the Proceedings of the National Academy of the Sciences.

The study reaffirms strongly that, as far as climate change goes, the gas that truly matters is carbon dioxide. Unlike its shorter-lived cousins, CO2 sticks around in the atmosphere for decades to centuries, wreaking climate havoc.

"It has become very clear that if you want to stabilize warming at any level, you have to start talking about phasing out CO2," said Joeri Rogelj, a research scholar at the International Institute for Applied Systems Analysis and lead author of the study. "Reducing other climate pollution can help in different ways and for different things, but in climate stabilization terms, it's noise on the fact that you have to phase out CO2."

The study comes at a time when nations are discussing a global climate treaty to be signed in Paris next year. An emphasis during the talks has been the mitigation of CO2 as well as a basket of SLCP gases. There is a broad belief in policy spheres that tackling SLCPs in the short run would create a time buffer for nations to begin tackling CO2 in the future.

The study suggests that is not strictly true. Using modeling of the interactions between CO2 and SLCPs, Rogelj and colleagues showed that reducing SCLP emissions would help limit global temperature rise up to 2030 but would have little impact beyond that.

Focus on CO2 has to be immediate
To have a sustained, long-term effect on climate change up to 2100, nations would have to focus on CO2 beginning immediately, the study finds. This would limit warming to 2 degrees Celsius (3.6 degrees Fahrenheit) above preindustrial levels by 2100.

"There is a huge energy system infrastructure behind CO2 emissions," Rogelj said. "Any period that you delay these [CO2] emissions reductions will result in a further lock-in of this infrastructure that is very CO2-intensive."

Targeting CO2 would, at the same time, reduce SLCP emissions, since both are emitted by similar sources: cars, energy extraction and power plants.

"So if you were to phase out coal-fired power generation, then you by default also phase out the air pollution emissions that would have come from that plant," Rogelj said.

The study is unlikely to be the final say on the topic. Piers Forster, a professor of physical climate change at the University of Leeds who is unaffiliated with the scientists, praised the study, saying, "I think its science is very good."

At the same time, Forster said that SLCP mitigation would be beneficial for other reasons. For instance, reducing soot and black carbon would benefit the health of women in poorer nations who use cookstoves.

"As far as climate mitigation goes, it doesn't have to be either/or," Forster said via email. "Everything helps."

Durwood Zaelke, president of the Institute for Governance and Sustainable Development, was skeptical of the study's findings. Given the present gridlock over climate action, all victories ought to be pursued, he said.

"Addressing climate change requires that you do the pieces you can do politically as fast as you can possibly do them," he said. "Countries are preserving their economic positions by going slow on CO2, but they don't have that same challenge on HFCs, or methane or black carbon. Those are all pieces that can be done, and no one will even notice that they've been done."

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500