By its very success, the Montreal Protocol underscores the failures of another UN-brokered attempt to prevent environmental catastrophe: the negotiations on how to cope with climate change.

As it turns out, the two are linked in ways unforeseen 25 years ago when scientists first sounded the ozone alarm: Patching up the ozone layer will help reduce the carbon dioxide emissions that drive global warming, according to the UN Environment Programme (UNEP).

A key meeting of signatories starting Monday in Montreal will decide just how quickly that should happen.

Trimming 10 years off the timetable for phasing out the chemicals that eat up atmospheric ozone -- a protective blanket of oxygen molecules 25 kilometres (15 miles) above the Earth's surface -- could cut CO2 emissions by 35 billion tonnes, according to Sylvie Lemmet, an official in the UNEP's technical division.

That is 15 times greater than the carbon dioxide reductions targeted by the 1997 Kyoto Protocol -- the troubled global treaty for reducing greenhouse gases -- between 2008 and 2012.

The ozone layer is a filter for solar radiation, and when part of that filter is removed, it causes additional warming of the Earth's surface. Ozone-depleting chemicals also contribute to global warming through the direct emission of potent greenhouse gases.

"If governments adopt accelerated action on HCFCs, we can look forward to not only faster recovery of the ozone layer, but a further important contribution to the climate change challenge," said UNEP head Achim Steiner.

Hydrochlorofluorocarbons are the stop-gap compounds that replaced even more damaging substances known as chlorofluorocarbons (CFCs).

Both families of chemicals -- used in products ranging from hair spray to fire retardants to refrigerants -- deplete the stratosphere's ozone. Switching to newer, and more energy efficient, technologies would also help curtail global warming.

"For the European Union, the schedule for eliminating HCFCs must be pushed up by 10 years -- that will be the benchmark for deciding if the negotiations are successful," said French Environment Minister Nathalie Kosciusko-Morizet ahead of the meeting.

"We have the agreement of the United States" for the amended calendar, she said.

The current calendar calls for developed countries to stop using these compounds in 2030, and developing nations in 2040. Some 88,000 tons of ozone-depleting substances are still produced every year, 85 percent of them in the industrialized world. Experts estimate that an additional 10,000 to 15,000 tonnes are produced illegally.

Stratospheric ozone provides a natural filter against harmful ultraviolet rays from the Sun, which can damage plant DNA and cause sunburn, cataracts and skin cancers.

The first warnings came in 1984, when scientists said a gaping hole had opened over the Antarctic, and that 30 to 40 percent of the layer had already disappeared.

In October 2006 the ozone layer's open wound spanned a record 29.5 million square kilometers (10.81 million square miles) and showed a loss of 40 million tonnes, exceeding the previous record of 39 million tonnes set in 2000, according to the European Space Agency (ESA).

There is already so much of the slowly degrading pollutant in the atmosphere that large ozone holes -- which open in August and close again in December -- are expected to persist for decades.

But barring a string of extremely cold winters, the ozone depletion will stop and fully reverse itself, returning to normal sometime around 2065, says the World Meteorological Organization (WMO).

Since the Montreal Protocol was adopted on September 16, 1987, 95 percent of targets for CFCs elimination by 2010 have already been met.

Experts estimate that there would have been an additional 100 million cases of cancer around the world had the Protocol not been adopted.

Former UN head Kofi Annan described the treaty as "perhaps the single most successful international agreement to date."

In the stratosphere, ozone is protective. At ground level, however, it is a pollutant -- created by a chemical reaction between exhaust fumes and sunlight -- that can be dangerous for people with bad respiratory or heart problems.