Study Reaffirms Importance of CO2 as Driver of Long-Term Climate

Short-lived Climate Pollutants Most Important Through 2030

A new study published in the Proceedings of the National Academy of Science USA, reaffirms that carbon dioxide (CO2) emissions are the primary driver of long-term temperature, and underlines the importance of immediate and parallel action to address emissions of both CO2 and short-lived climate pollutants (SLCPs) to effectively combat near- and long-term climate change.

The study highlights linkages between mitigation measures, noting that many measures such as reducing emissions from the burning of fossil fuels can eliminate emissions of both CO2 and some SLCPs, such as black carbon. According to the study, as much as 65% of energy-related black carbon emissions are linked to CO2-emitting fossil fuel sources, potentially allowing for dual reductions from CO2 mitigation measures from these sources. However, the study notes that linkages do not exist in all cases, such as with hydrofluorocarbons (HFC) refrigerants, which need to be addressed separately.

“The science has always told us that we need to immediately address both short-lived and long-lived climate pollutants to avoid the worst impacts of climate change,” stated Durwood Zaelke, President of the Institute for Governance & Sustainable Development. “Parallel strategies to quickly cut both groups of pollutants are critical for success; however, measures to reduce short-lived and long-lived climate pollutants often exist as independent political opportunities with different near- and long-term benefits, economics, and politics. Some SLCP mitigation measures, such as those undertaken by the Climate and Clean Air Coalition to Reduce Short-Lived Climate Pollutants, are often politically easier to implement and rapidly scale-up, due to their multiple benefits for human health, ecosystems, and the climate.”

“It is important to remember that the success of SLCPs builds the political momentum we need to ultimately succeed with CO2 and win the battle against climate change,” added Zaelke. “This study explains how action on CO2 will reduce SLCPs, but it’s important to remember that the reverse is also true: that reducing SLCPs reduces CO, even in the case of HFCs. Past phaseouts of refrigerants that the HFCs are now replacing have catalyzed energy efficiency improvements in air conditioners and other appliances on the order of 30 to 60%, providing significant CO2 mitigation, given that up to half of all electricity in the hottest countries goes to air conditioning.”

SLCPs include black carbon, methane, tropospheric ozone, and HFCs, which have a lifespan in the atmosphere of 15 years or less. Previous research over the past several decades concludes that cutting SLCPs could significantly cut the current rate of warming through 2050, and because many SLCPs are also powerful air pollutants, reductions can also prevent more than two million deaths a year from air pollution and avoid around 50 million tonnes of crop losses annually.

While the new paper questions how much the SLCPs can help beyond 2030, Zaelke notes that “the world will win or lose the climate game before 2030 by setting off uncountable feedback mechanisms, and in this critical window it’s essential to cut SLCPs, along with CO2. The IPCC’s new Summary for Policymakers makes it clear that we’re running out of time.” Climate negotiators are aiming for a new climate treaty by the end of 2015, to go into effect in 2020.

Professor V. Ramanathan, at Scripps Institute of Oceanography, University of California, San Diego, a leading authority on SLCPs since the 1970s, added the following quotes on the new paper:

“The reduction in warming obtained by the Rogelj et al. study due to reductions in concentrations of short-lived climate pollutants (SLCPs) is about 1°C by the end of the century, which is consistent with the findings of the earlier Ramanathan and Xu (2010) study. Thus reduction of SLCPs, including HFCs, will have a substantial long-term cooling effect.

The issue raised by the paper is not about the need to reduce SLCPs, but more about how to mitigate SLCPs. Rogelj et al. argue CO2 measures will accomplish most of the needed reductions in SLCPs, assuming that we can succeed in immediately reducing fossil fuel CO2 emissions. This recommendation requires careful scrutiny. For example, black carbon and other particulates from the transportation sector can be cut by more than 90% by off-the-shelf technologies (ultra low sulphur diesel fuel with diesel particulate filter) as demonstrated by California and other states. Mitigating the emission of one ton of black carbon from diesel vehicles has the same effect as mitigating 1000 to 2000 tons of CO2 (on a 100 year time scale). In addition, emissions of particulates and ozone precursors are important sources of ambient air pollution, which is responsible for about 4 million premature deaths annually. They are also major sources of ozone, which destroys over 100 million tons of crops each year. Rogelj et al, perhaps came to their conclusion, because of the neglect of co-benefits of SLCPs to human health and food security. Reductions in SLCPs should be considered as complementary to reductions in CO2 emissions.

A ‘no-regrets’ policy would take advantage of political opportunities to cut SLCPs as soon as possible to save the millions of lives that are at stake and hundreds of millions of tons of crops that are damaged each year by SLCPs — losses that could feed over 500 million people living in extreme poverty while slowing down climate change. Just this week, a study in PNAS concluded that approximately 36% of wheat crops are damaged every year by SLCPs in India.”

Joeri Rogelj, et al., Disentangling the effects of CO2 and short-lived climate forcer mitigation, PNAS (2014) is here.

Jennifer Burney and V. Ramanathan, Recent climate and air pollution impacts on Indian agriculture, PNAS (2014) is here.

IGSD’s Primer on Short-Lived Climate Pollutants is here & IGSD Primer on Hydrofluorocarbons is here.