Little-studied manmade gases have big warming potential

Chlorofluorocarbons (CFCs) — commonly used in refrigerators and air conditioners — garnered public attention for their role in creating a hole in the ozone layer over Antarctica. As these chemicals were phased-down thanks to international agreements limiting their use, they were replaced by other synthesized gases that can still be harmful to the ozone layer and are greenhouse gases that contribute to climate change.  Despite this, synthetic greenhouse gases (SGHGs) beyond the CFCs have received relatively little attention from the research community — until now. 

“Left unchecked, synthetic greenhouse gases have the potential to cause significant further warming,” says Ronald Prinn, co-director of the MIT Joint Program on the Science and Policy of Global Change and a co-author of a study estimating the warming impact of the 25 most abundant SGHGs, to be published this month in Geophysical Research Letters. The authors have estimated that, without additional limits on SGHG use, the resulting increase in warming could outweigh the climate benefits gained thus far from phasing down CFCs.

The researchers used measurements of SGHG levels from the Advanced Global Atmospheric Gases Experiment (AGAGE), a global observing system developed by Prinn and colleagues and sponsored by NASA and other agencies. The study team headed by Matthew Rigby, lead author and a research fellow at the University of Bristol, analyzed observed atmospheric levels of SGHGs from 1978 to 2012, and then used these measurements to predict the impact these gases could have on global warming through 2050.

In response to the phase-down of CFCs through the 1987 Montreal Protocol, the researchers discovered that the use of other synthetic gases as refrigerants — such as hydrofluorocarbons (HFCs) — has risen. HFCs had been limited in the now-defunct 1997 Kyoto Protocol, but there is currently no agreement restricting their use. So using HFCs as a test case, the researchers examined the effect of phasing down HFCs by amending the Montreal Protocol to include these gases.

“We could avoid adding the equivalent of up to another three years of carbon dioxide emissions into the atmosphere if these gases were being phased down,” Rigby says.

HFCs are particularly strong greenhouse gases, so even relatively small levels in the atmosphere can contribute to warming. “Per ton of emissions, HFCs are much more potent greenhouse gases than carbon dioxide, and are very good at trapping the radiation that heats the Earth,” Rigby says.

While HFCs are currently not a major driver of climate change compared to carbon dioxide or even other SGHGs, Prinn points out that if unabated they may contribute significantly to future warming. “Addressing HFCs, and other SGHGs, now will ensure that they don’t contribute significantly to warming in the future,” Prinn says.  

Meanwhile, the researchers note that due to extensive use, CFCs will continue to warm the planet for years to come.

“CFCs have contributed the most among the synthetic greenhouse gases to warming,” Prinn says. “Their use peaked and levels are now declining, but these gases will remain in the atmosphere for many years. This is likely the trend we will see with most SGHG gases, so it is important that we address these gases now before they do more severe damage.”