The United Nations' Montreal Protocol is undoubtedly one of the world's most successful environmental pacts by phasing out chemicals that eat the world's protective ozone layer high in the atmosphere.

If the world hadn't acted, the ozone hole would have expanded, greatly increasing the risk to life on the planet from the Sun's ultraviolet radiation.

But there is a downside. While nations agreed to phase out ozone-destroying chlorofluorocarbons, or CFCs, their replacements called hydrofluorocarbons, or HFCs, are highly potent greenhouse gases and contribute to climate change.

HFCs, like CFCs, are used as refrigerant gases in air-conditioners and refrigerators and a large percentage of the gases leaks out over time.

There are different types of HFCs and some are more than 10,000 times more powerful at trapping heat in the atmosphere than carbon dioxide and can last years in the air. Green groups say it is misleading for HFCs to be labelled as environmentally friendly.

The UN says HFCs are responsible for only a fraction of global greenhouse gas emissions, but their concentrations are rising.

The good news: Governments are also trying to address HFC emissions under the Montreal Protocol.