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Cuts Needed in Both CO2 and Short-Lived Climate Pollutants


Parallel strategies needed for both near-term and long-term protection

Short-lived climate pollutants more than short-term fix, benefits extend beyond 2200

Pressure must remain on CO2 to spur technology innovation

Washington, DC, 12 December 2013 - A paper to be published today in Science describes the importance of reducing both long-lived CO2 and short-lived climate pollutants (SLCPs) to achieve near- and long-term climate goals, along with other benefits for health and food security from reductions of the SLCPs. SLCPs include black carbon particulates, methane, tropospheric ozone—the main component of urban smog—and hydrofluorocarbons, or HFCs, used primarily as coolants.

The authors include two Harvard professors, Julie Shoemaker and Daniel Schrag, and two professors from the University of California, San Diego, V. Ramanathan and Nobel Laureate Mario Molina. They explain that cuts to SLCPs should not be traded off for increased emissions in CO2, and that parallel strategies to cut both CO2 and SLCPs are needed to keep warming to an acceptable level of no more than 2.0°C above pre-Industrial levels through the end of the century:

“Reducing emissions of SLCPs is an essential component of any comprehensive climate action plan for addressing both near- term and long-term climate change impacts. There are real opportunities to reduce emissions of SLCPs without distracting from other mitigation efforts focused on CO2.”

SLCPs cause a third or more of today’s warming. They stay in the atmosphere for only a short time— hours to days for tropospheric ozone, days to weeks for black carbon, about a decade for methane, and about 15 years for HFCs. This means that fast reductions of SLCP produce fast climate benefits.

“Cutting SLCP is the best way to reduce impacts over the next 50 years and beyond,” said Durwood Zaelke, President of the Institute for Governance & Sustainable Development. “Maximum mitigation of SLCPs, using existing technologies, can cut the rate of warming in half and sea level rise by a quarter,” added Zaelke. “The ability to slow the rate of warming makes fast mitigation of SLCPs one of the best adaptation strategies for vulnerable societies and ecosystems.”

Cutting SLCPs provides considerable long-term mitigation according to the new paper, well beyond the current century and even beyond the next century. See Fig. 1 below.

The paper notes that CO2, on the other hand, has a very long lifetime, with more than 20% of emissions remaining in the atmosphere for thousands to tens of thousands of years. This means that the bulk of the benefits from slowing CO2 emissions will be observable only over the longer term, beyond the next few decades. A recent study calculates that aggressive mitigation of SLCPs could prevent as much as 0.6°C of additional warming by 2050, compared to 0.1°C from CO2 cuts alone. A parallel strategy that cuts both CO2 and SLCPs can avoid as much as 2.6°C by 2100. This combined mitigation provides the greatest chance of keeping global temperature increases below 1.5°C over pre-Industrial temperatures for the next 30 to 40 years and below 2°C through 2100. (See IGSD’s press release on this study.)

The new Science paper notes that pressure to reduce CO2 is the key driver for developing climate friendly technologies, and that reducing such pressure can slow innovation and lead to higher emissions and a warmer climate.

According to Ramanathan, an author of the study and Distinguished Professor of Atmospheric and Climate Sciences at Scripps and UNESCO Professor at TERI University, New Delhi:

“The message is clear: We need to mitigate both CO2 and SLCPs. SLCPs contribute about 40% of the total mitigation even at 2200, about two centuries from now. Had we included HFCs mitigation, the SLCPs mitigation would be even larger.”

Some steps to reduce CO2 will also reduce SLCPs, including for example, black carbon and methane reductions from cuts in fossil fuel production and use. Today’s paper notes that “efforts to reduce BC emissions can be undertaken through air pollution measures whose main focus is on public health, such as regulations on diesel exhaust or the promotion of cleaner cooking technologies. HFCs can be regulated through the Montreal protocol. Such strategies have already proven to be effective.”

The paper cautions that SLCPs should not be traded off for CO2, as is currently the case in the climate treaty, where reductions in methane and HFCs can be traded for credit toward achieving CO2 emissions reduction goals. “Widespread trading between different greenhouse gases, especially when it may affect markets for low-CO2 technologies, risks committing our children and grand-children to even greater climate impacts in the more distant future.”

“In the long run, given the significant amount of CO2 that remains in the atmosphere for thousands to tens of thousands of year, even aggressive strategies to reduce emissions of both SLCPs and CO2 won’t be sufficient,” said Zaelke. “We’ll also have to remove a good deal of the CO2 already in the atmospheres on a decades to century time scale, using carbon removal strategies, starting with the protection and expansion of forests, grasslands, wetlands, and other ecosystems that use photosynthesis to store carbon in biomass and soil.”

See IGSD’s Primer on Short-lived Climate Pollutants here

See IGSD’s Primer on Hydrofluorocarbons here

Figure 1

Climate temperature response to reductions in emissions of CO2, SLCPs, and both.