THE biggest turn-off in human history will start at 8.30pm today in Chatham, a tiny South Pacific island with only 12 street lamps.

Almost 25 hours later, but at 9.30pm the same day, it will finish on the other side of the international dateline in the Galapagos Islands, where scientists at the Charles Darwin Research Station will share a candle-lit dinner with several hundred locals and environmental activists.

In between, Earth Hour — the annual worldwide call for action against climate change — will spread darkness across all seven continents, drawing in 120 nations, 1,700 municipalities, hotels and restaurants and hundreds of millions of people, including supermodels, archbishops, and footballers.

The event, now in its fourth year, is organised by the World Wide Fund for Nature and has spread an ever-widening expanse of hour-long darkness.

Two million people took part at the first switch-off in Sydney in 2007. Last year, hundreds of millions participated in 88 countries. This year, organisers expect the figure to be close to a billion.

Beijing's Forbidden City and London's Houses of Parliament, the Eiffel Tower, the Hiroshima Peace Memorial, the Empire State Building, Rio de Janeiro's Christ the Redeemer statue, and the world's tallest building, Burj Khalifa will all plunge into darkness.

Nations that have signed up for the first time this year include Saudi Arabia — long seen as resistant to climate change action — Mongolia, Nepal and the Czech Republic.

Another debutant is a first group of participants from Antarctica the Davis Research Station, which is home to several dozen scientists who presumably will not be also switching off their heaters in — 10 degrees Celsius.

James Leape, the director general of the WWF, said the worldwide event was particularly important after the disappointment of the climate conference in Copenhagen last year.

Leape is in Beijing to attend a darkening ceremony at the Forbidden City, the 600-year-old symbol of Chinese power.

— The Guardian, London