Confronting the role of non-CO2 pollutants in global warming

Swift action on other greenhouse agents could solve the “fast half” of the climate problem, researchers say.

AbstractFull Text HTMLHi-Res PDF[2037 KB]PDF w/ Links[86 KB]Noreen Parks
Environ. Sci. Technol., Article ASAP
Publication Date (Web): November 18, 2009
Copyright © 2009 American Chemical Society

Aggressively reducing emissions of non-CO2 climate drivers could forestall abrupt climate change for up to 40 years, according to a recent study in the Proceedings of the National Academy of Sciences (2009, DOI 10.1073/pnas.0902568106). Without such efforts, even drastic cuts to CO2 emissions will fail to put the brakes on planetary warming soon enough to avoid climate tipping points, the authors warn.

Hydrofluorocarbons (HFCs), black carbon (soot), ground-level ozone, and methane together represent an estimated 40−50% of the warming caused by human activities. “We’re on track for a 2 °C warming that will put us in the danger zone, and current research shows it’s coming faster than anticipated,” says study coauthor Durwood Zaelke of the Institute for Governance and Sustainable Development. “Restricting CO2 emissions is absolutely critical, but it won’t be enough. So the question is how quickly we can deliver cooling on the non-CO2 side.”

Implementing “fast-action” mitigation strategies based on available technologies would jump-start this effort, the researchers say. One key step would be to phase down the production and use of HFCs, which are now known to act as long-lasting greenhouse gases. Use of HFCs has been growing because of the rising demand for air-conditioning and refrigeration in developing countries. Current projections indicate that by mid-century the impact of HFCs on the climate could be approximately 20% of that from CO2 emissions, if the current trends continue unabated. “The Montreal Protocol (MP) has already delayed climate change by 7−12 years,” lead author Mario Molina of the University of California San Diego noted in a prepared statement. “We have to take advantage of the proven ability of this legally binding treaty to quickly phase down HFCs.” North American leaders recently submitted a proposal to start this process for consideration at the MP annual meeting in November.

Soot now ranks as the second or third biggest contributor to climate change. However, soot’s short life span offers opportunities for comparatively quick fixes—such as particulate filters for vehicles and clean-burning or solar-powered stoves—that could yield significant climate savings, the authors say. Likewise, the means for slashing levels of ozone precursors such as carbon monoxide and nitrogen oxides are within reach. Research shows that rigorous enforcement of air-pollution technologies and regulations could cut these emissions by more than half, dramatically decreasing tropospheric ozone. “We know how to curb air pollution; we just need to do it better and faster, and get the solutions applied in developing countries. We can borrow from and utilize working international agreements to do this,” Zaelke emphasizes.

Comparing the overall greenhouse contributions of climate drivers reveals only part of their potential for limiting global warming, Michael MacCracken of the Climate Institute noted in a recent paper. Also critical are their atmospheric lifetimes, which range from centuries to millennia for CO2 and HFCs and from days to weeks for black carbon. “Steep, immediate reductions in soot would eliminate its warming influence over the entire 21st century,” he explains. Similarly, swift cutbacks in emissions of methane and ozone-producing pollutants would yield sharp and enduring declines in their warming influence.

Because much of the non-CO2 pollution originates in developing countries, those nations can play a substantial role in combating climate change, MacCracken stresses. “We don’t want climate negotiations to fail because we can’t get agreement on everyone cutting CO2 emissions immediately. Developing nations can do a lot to offset their ongoing CO2 output by going after these other pollutants—at the same time improving public health and energy efficiency. This would allow for their continued development, while cost-effective, climate-friendly energy technologies evolve, and demonstrate the necessary commitment from all nations while recognizing the equity imbalance created by very different per capita emissions.”

The study is intended as a call to action, Zaelke says. “Speed matters. We have to move forward on all fronts now, using existing governance structures, without waiting for a climate treaty to get started. We need optimism and energy to solve this problem, and these fast-action strategies can help provide more confidence that we can do it.”