U.N. is close to accord banning HFCs

WASHINGTON — When negotiators from nearly 200 countries gathered outside Paris in December for the United Nations summit meeting on climate change, they reached the first agreement to take action on curbing their planet-warming pollution.

This weekend in Vienna, with far less attention, negotiators from those same countries are nearing a deal that many environmentalists have called the most significant action this year to reduce global warming.

While the Paris agreement aims to reduce the use of coal and oil, which produce the carbon dioxide emissions that are the chief cause of global warming, negotiators in Vienna are working on a deal to ban the use of hydrofluorocarbons, chemicals used in air-conditioners and refrigerators.

Although they contribute only a small percentage of the world’s greenhouse gases, these chemicals, known as HFCs, can trap heat in the atmosphere at levels a thousand times higher than carbon dioxide can, according to published scientific studies.

The deal would be an amendment to the Montreal Protocol, the landmark 1989 environmental treaty designed to close the hole in the ozone layer by banning ozone-depleting coolants called chlorofluorocarbons, or CFCs.

In response, chemical companies developed HFCs, which do not harm the ozone.

But the substitute had the wholly unexpected side effect of increasing heat trapped in the atmosphere, which worsened climate change.

The Montreal treaty allows nations to amend it to ban substitute chemicals that have negative environmental effects even if they do not harm the ozone.

And U.S. chemical companies such as Dow, DuPont and Honeywell have already begun to patent climate-friendly HFC substitutes.