"In 2007 we, the same group of authors, published a paper on the climate benefits of the Montreal Protocol," Guus Velders of the Netherlands Environmental Agency told environmentalresearchweb. "This triggered policymakers to accelerate the phaseout of HCFCs – transitional substitutes for chlorofluorocarbons (CFCs). As a consequence other chemicals are likely to be used. HFCs are likely candidates; they don't deplete the ozone layer, but are strong greenhouse gases."

HCFCs were introduced as a transitional substitute for CFCs as they are less damaging to the ozone layer, but HFCs are better still, at least with respect to ozone. While HFCs are covered by the Kyoto Protocol, HCFCs are not.

Velders says that the previous long-term HFC scenarios were developed more than a decade ago so the team thought it was time to update them. Working together with colleagues from the US National Oceanic and Atmospheric Administration, DuPont Fluoroproducts, US, and the US Environmental Protection Agency, Velders found that global HFC emissions are likely to significantly exceed previous estimates after 2025.

By 2050 HFC emissions could be three to four times larger than previously estimated and "could equal (on a CO2-equivalent basis) between 9 and 19% of the emissions of carbon dioxide, in business-as-usual scenarios". And, if atmospheric levels of carbon dioxide were stabilised to 450 ppm, the greenhouse gas effect of the HFCs could equal 28–45% of that of the carbon dioxide.

What's more, emissions from developing countries could be up to 8 times those in developed countries by 2050. "The large growth in use and emissions of HFCs is especially foreseen in the developing world (mainly Asia), linked with their growing economies and large populations," said Velders. "The climate benefits of the Montreal Protocol could thereby be significantly offset by large growth in HFC use and emissions. We hope that [our study] will inform policymakers that HFCs, if nothing is done, could contribute significantly to future climate change."

HFCs can be up to 11,000 times stronger greenhouse gases than carbon dioxide but are much more short-lived in the atmosphere. The researchers suggest that a global cap on HFC emissions followed by a 4% per year reduction in consumption would cause warming from HFCs to peak and begin to decline before 2050.

"For climate mitigation is it important that actions are taken on several fronts," said Velders. "We will therefore closely follow what the countries decide in their discussions on the Montreal Protocol and in the climate negotiations in Copenhagen later this year. Our goal is to inform the public and policymakers with good scientific information, in support of possible actions."

Set up in 1987, the Montreal Protocol on Substances that Deplete the Ozone Layer is claimed to have avoided five to six times as many greenhouse gas emissions as the Kyoto Protocol.

The researchers reported their work in PNAS.