The UN environmental Program (UNEP) on Sunday welcomed a decision reached by China and U.S. on phasing down a group of synthetic chemicals in order to combat climate change.

In a statement issued in Nairobi, UNEP Executive Director Achim Steiner said the declaration made by U.S. President Barack Obama and Chinese President Xi Jinping to target Hydroflucorcarbons (HFCs) demonstrated a transformational chapter in international cooperation on climate change.

Washington and Beijing agreed to work together and with other countries through multilateral approaches that include using the expertise and institutions of the Montreal Protocol to phase down the production and consumption of HFCs.

HFCs are potent greenhouse gases used in refrigerators, air conditioners, and industrial applications. While they do not deplete the ozone layer, many are highly potent greenhouse gases.

Their use is growing rapidly as replacements for ozone- depleting substances that are being phased out under the Montreal Protocol on Substances that Deplete the Ozone Layer.

Left unabated, HFC emissions growth could grow to nearly 20 percent of carbon dioxide emissions by 2050, a serious climate mitigation concern.

According to Steiner, it is widely recognized that securing a meaningful treaty and keeping an average global temperature rise under 2 degrees this century will require all hands on deck.

"What however must not be overlooked or sidelined is the urgency to also tackle the principle greenhouse gas, carbon dioxide, as part of negotiations underway under the UN climate convention," Steiner said.

He said while HFCs are ozone-layer friendly they are however powerful greenhouse gases: if taken up by industry over the next few years and decades they are likely, by 2050 to amount to emissions equivalent to 3.5 to 8.8 Gigatonnes (Gt) of carbon dioxide.

According to Steiner, that is comparable to total current annual emissions from transport, estimated at around 6-7 Gt annually according to a UNEP-coordinated study from 2011.

"Along with a variety of recent signals from several key countries including China and the United States, this one on HFCs by these two key economies is welcome as the world moves towards a universal UN treaty on climate change by 2015 -- certainly allowing the market for HFCs to grow will only aggravate the challenge of combating climate change," Steiner said.

UNEP in partnership with over 60 countries and organizations is also working to phase down some HFCs and other so called short- lived climate pollutants such as black carbon or "soot" and methane under a one year old initiative called the Climate and Clean Air Coalition.

The voluntary coalition is promoting reduction of short lived climate pollutants to tackle air pollution, bring widespread benefits for health and agriculture as well as to lead to near term climate benefits.

The U.S., Canada, and Mexico have for the past four years, proposed an amendment to the Montreal Protocol to phase down the production and consumption of HFCs.

The amendment would gradually reduce consumption and production and control byproduct emissions of HFCs in all countries, and require reporting in these areas.