Small island nations gained North America's powerful backing Tuesday for a plan to convert the UN ozone treaty into a tool for phasing out some of the world's most powerful climate-warming gases.

The Obama administration announced the United States, Canada and Mexico now support using the treaty to require cuts in powerful greenhouse gases known as hydrofluorocarbons, or HFCs.

The treaty, called the Montreal Protocol, is aimed at fixing the ozone layer. It has been signed by 196 nations, with East Timor announcing its decision to sign early Wednesday. That makes the ozone treaty the first global environmental agreement to achieve universal ratification.

UN officials say they believe there would be millions more cases of cancer and eye cataracts without the treaty. But the treaty's success is not yet assured; even with such global co-operation, UN officials say, the damaged ozone layer is expected to take another 40 to 50 years to fully recover.

In its current version, the treaty encourages using HFCs in refrigerators and air conditioners to replace ozone-depleting chlorofluorocarbons, or CFCs, which have now been virtually eliminated.

But in April, Micronesia and Mauritius proposed formally amending the treaty to phase out use of the coolants. They argued that other chemicals would be better for the climate while still helping the ozone layer.

The U.S. State Department called that plan Thursday "a significant down payment" on efforts to reach a new global climate pact in Copenhagen, Denmark in December. U.S. President Barack Obama is attending a UN climate summit next week to build momentum.

"Once adopted, the proposal would make great strides to achieve President Obama's call to reduce U.S. greenhouse gas emissions 80 per cent by 2050 as well as contribute to multilateral efforts to reduce global emissions 50 per cent by 2050," the State Department said.

HFC share expected to grow

The U.S. said the proposal "calls on all countries to take action to reduce consumption and production of HFCs, although developed countries would take the lead in this effort, as they have consistently under the Montreal Protocol."

Countries will consider whether to alter the Montreal Protocol at a meeting in Egypt in November.

Though HFCs account for only about two per cent of the world's climate-warming gases, their share is expected to grow by up to a third of all greenhouse gases by mid-century — mainly because of their promotion under the ozone treaty.

A global fund affiliated with the treaty has invested billions in creating new markets for HFCs and other chemicals that do not harm the protective ozone layer above the Earth.

But Micronesia has called that promotion "irresponsible," since HFCs, like CFCs, are powerful climate-warming chemicals — up to 10,000 times more so than carbon dioxide — and other climate and ozone-friendly alternatives are available for use.

Proposal 'sends clear signal'

The Associated Press first reported in early May that the Obama administration considered HFCs "a very significant threat" to climate change and probably would seek to use the ozone treaty to dramatically reduce HFCs, but not phase them out entirely.

Senate foreign relations chair John Kerry applauded the Obama administration Tuesday and described the decision as a sign of things to come from the U.S.

"Anyone who doubted the intentions of the new administration should pay close attention to this announcement and to those of us in the Senate who urged this ambitious action under the Montreal Protocol," Kerry said. The proposal, he said, "sends another clear signal to the global community that the United States will not remain on the sidelines and will lead efforts to achieve a strong agreement in Copenhagen.'"'

It is uncertain whether the Senate will pass climate legislation in time for the Copenhagen climate talks, but Kerry said the ozone treaty was another appropriate tool for tackling the urgency of planetary overheating.

In April, Kerry and another leading Democrat, Senate environment committee chair Barbara Boxer, wrote Obama calling for using the ozone treaty to phase down HFCs by 85 per cent by 2030.

"The growth projections of HFCs alone better be a wakeup call to anyone still left in Congress who doubts the urgent need to address climate change," Kerry said. "If allowed to grow, this extremely potent greenhouse gas could counteract global efforts to reduce carbon dioxide."