Slowdown in warming may be due to water vapour: study

The slowdown in global warming in the last few years might have been caused by a decline in water vapour in the stratosphere, a new report suggests.

While climate warming is continuing the decade of 2000 to 2009 was the hottest on record worldwide the increase in temperatures was not as rapid as in the 1990s.

Balloon and satellite observations show the amount of water vapour in a layer about 16 kilometres high declined after 2000. The stratosphere extends from about 13 to 48 kilometres above the Earth's surface.

The reason for the decline is unknown, according to researchers led by Susan Solomon of the National Oceanic and Atmospheric Administration in Washington, D.C. They report their findings in Thursday's online edition of the journal Science.

Water vapour is a potent greenhouse gas, and its decline in the stratosphere would reduce the rate of global warming expected from other gases such as carbon dioxide, the researchers said.

According to the researchers, water vapour enters the stratosphere primarily from air rising in the tropics.