A new study calculates that replacing chemicals widely used as refrigerants and propellants with alternatives, as the U.S. and many other countries have proposed under the Montreal Protocol, can avoid up to 0.5° C (0.9° F) of warming by 2100.
“Our calculations show that controlling hydrofluorocarbon (HFC) growth can avoid a significant amount of warming in this century, at least comparable to 50 percent of CO2 mitigation by 2100,” said Scripps Institution of Oceanography, UC San Diego, graduate student Yangyang Xu, lead author of the study.
HFCs are a small contributor to global warming today, but they are the fastest-growing greenhouse gas in many countries, including the United States, the European Union, China, and India. Many of the HFCs are, molecule-for-molecule, thousands of times more powerful at causing warming than CO2. They are in a class of climate change agents known as short-lived climate pollutants. These pollutants include methane, black carbon (a major component of soot), and ozone in the lower atmosphere in addition to HFCs. They are so-called because they are potent contributors to global warming but last in the atmosphere for periods of several days for black carbon to about 15 years in the case of HFCs. In contrast, CO2 remains in the atmosphere for more than a century.
The study appears in the June 26 edition of the journal Atmospheric Chemistry and Physics.
“It is still possible to avert disastrous climate changes including extreme sea-level rise,” said study co-author Scripps Distinguished Professor of Climate and Atmospheric Sciences Veerabhadran Ramanathan. “We have to simultaneously cut down CO2 and short-lived climate pollutants including HFCs, methane, tropospheric ozone, and black carbon. HFC mitigation emerges as an attractive low-hanging fruit for mitigating warming.”
Ramanathan’s discovery of the greenhouse effect of chlorofluorocarbons (CFCs) in 1975 paved the way for this study as well as others that have identified the greenhouse effect of other non-CO2 pollutant gases including HFCs and methane. The Montreal Protocol, a landmark international treaty, phased out the use of CFCs to protect the stratospheric ozone layer. CFCs have now been replaced by HFCs as refrigerants and propellants that have no direct negative effects on the ozone layer. However, many HFCs, like CFCs, are super-climate warmers. If allowed to grow at the current rates they can add another 0.5ºC warming by 2100. Replacement compounds such as some hydrocarbons or some HFCs with negligible global warming potentials are available.
“This timely paper shows how important reducing high-global warming potential (GWP) HFCs can be for avoiding future warming,” said A. Ravishankara, director at the Chemical Sciences Division of the NOAA Earth System Research Laboratory. “It confirms the U.N. Environment Programme-World Meteorological Organization assessment on avoided warming from black carbon and methane and tropospheric ozone, then extends the analysis by adding the avoided warming from reducing high-GWP HFCs, concluding that this will avoid another 0.1°C of warming by 2050, and up to 0.5°C by the end of the century.”
“We've known for several decades that some of the HFCs developed to replace ozone-depleting CFCs are powerful climate change agents, but this is the first calculation showing how much global warming we can avoid by reducing emissions of these chemicals, helping us to address the challenge in the near-term as well as through the end of the century,” said Mario Molina, a distinguished professor of chemistry and biochemistry at UC San Diego who shared the Nobel Prize in Chemistry in 1995 for his work on the impact of CFCs on the stratospheric ozone layer.
Earlier this month at the first-ever summit between President Barack Obama and Chinese President Xi Jinping, the two presidents agreed to work together to phase down HFCs under the framework of the Montreal Protocol. More than 100 other parties to the Montreal Protocol have expressed support for reducing HFCs. A formal discussion group was formed this week during the Montreal Protocol Open-ended Working Group meeting in Bangkok, Thailand to discuss how best to deal with HFCs, an important milestone in the effort to reach consensus on the amendment.
“The metric of ‘avoided warming’ in a given time frame may be the most relevant for climate policymakers today,” said co-author Guus Velders of the National Institute for Public Health and the Environment in The Netherlands. “This paper now adds the avoided warming from limiting HFCs growth to the earlier work of UNEP and WMO, which calculated how much warming can be avoided by cutting black carbon, methane, and tropospheric ozone. HFCs in the atmosphere are growing at a high rate throughout the world, 10 to 15 percent per year, making them a vital target for climate mitigation.”
“The prominence that the president gave HFCs in his speech and his climate plan makes it clear that he will be continuing his personal diplomacy to ensure a swift victory under the Montreal Protocol,” added report co-author Durwood Zaelke, president of the Institute for Governance & Sustainable Development. “Phasing down HFCs will be seamless, and won't even be noticed by consumers.”
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