The second largest ozone hole formed in 2003 over Antarctica and covered 17.8 million square kilometres.

Credit: NASA

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GENEVA, 21 August 2006: The recovery of the Earth's protective ozone layer, which was ravaged by chemicals in the 20th century, will take five to 15 years longer than predicted, scientists say.

"The delayed recovery is a warning that we cannot take the ozone layer for granted and must maintain and accelerate our efforts to phase out harmful chemicals," said Achim Steiner, executive director of the United Nations Environment Programme.

Some 250 scientists concluded in a report released on Friday that there were more amounts of damaging chemicals - some of them contained in refrigerators - still available or being produced than previously estimated, UNEP and the World Meteorological Organisation said.

Most but not all ozone-depleting substances are banned or being eliminated under the 1989 Montreal Protocol, one of the world's few successful environmental treaties.

"We have an unfinished job which bears directly on human health and wider environmental concerns," Marco Gonzalez, executive secretary of the Montreal Protocol, told journalists.

The layer in the upper atmosphere, which protects life on Earth from excessive solar radiation, should start to recover over inhabitated northern and southern mid-latitude areas of the world by 2049, instead of 2044, according to the report.

The ozone layer over the Antarctic will only be completely replenished 15 years later than predicted, by 2065.

The hole that opens up nearly once a year over the region is expected to occur regularly for another two decades, due to the particular "supercold, superfast winds" high above the South Pole, the agencies added.

Another study published in the British scientific journal Nature in May had also warned that earlier estimates failed to take into account the damage caused by volcanic eruptions, solar storms and other natural phenomena.

Research in 2000 and 2002 found that there had been no acceleration in ozone loss above any region for the previous decade, and had pointed to a recovery in the early 21st century.

"The early signs that the atmosphere is healing demonstrate that the Montreal Protocol is working," said Steiner.

Geir Braathen, a senior scientist with the World Meteorological Organisation, confirmed that the amounts of ozone depleting chemicals in the upper atmosphere were still declining. But that decline was expected to be slower than expected.

"The delay is not related to non-compliance with the Montreal Protocol, but caused by new estimates of production and emission of certain ozone depleting substances," he told journalists.

The report revised estimates of stocks of types of banned chlorofluorocarbons (CFCs), which are used mainly for refrigeration and plastic foam, in existing machinery.

But it also cautioned that production levels of a key substitute chemical, known as HFCF-22 (hydrochlorofluorocarbons), were higher than expected.

The substance is regarded as far less noxious for ozone than CFCs and is not banned, but it still causes some ozone depletion.