Towns and cities around the world are turning out the lights for an hour, to highlight the threat of climate change.
Towns and cities around the world are turning out the lights for an hour to highlight the threat of climate change.
"Earth Hour" began in Sydney at 2000 local time (0900 GMT), when lights went out on landmarks like the Opera House and Harbour Bridge.
"Earth Hour" began in Sydney at 2000 local time (0900 GMT), when lights went out on landmarks like the Opera House and Harbour Bridge.
Bangkok, London and Chicago are among cities due to follow suit when clocks reach 2000 local time.
Bangkok, Toronto, Chicago and Dublin are among 27 other cities officially due to follow suit at 2000 local time.
The initiative began in Sydney last year but now involves 35 countries.
But critics have dismissed the event as a gimmick that will not make any difference, a BBC correspondent notes.
"Earth Hour is about everyone and every organisation, from individuals to global companies, joining together to own a shared problem - climate change," organiser Andy Ridley told AP news agency.
The initiative began in Sydney last year when an estimated two million residents took part, cutting energy usage by more than 10% for the hour.
He said he had not expected the event to grow in scale so quickly.
Organisers expect hundreds of towns and cities across the world to take part in the event and hope some capitals not officially involved, such as London and Rome, will mark the event by dimming lights on some landmarks.
"What's amazing is that it's transcending political boundaries and happening in places like China, Vietnam, Papua New Guinea. It really seems to have resonated with anybody and everybody."
In its own contribution, the Google web search engine is putting a dark background on its homepage.
More than two million residents in Sydney reportedly participated in last year's event - cutting energy usage by more than 10% for the hour.
Top emitter
Observers say droughts and water shortages have drawn Australians' attention to the problem of global warming.
Organisers insist the aim of Earth Hour is to show that communities care passionately about climate change and want to keep up the pressure on governments to act decisively.
Per capita the vast continent is the world's largest producer of carbon dioxide and other gases that most scientists believe are helping to warm the Earth's atmosphere.
Andy Ridley of the World Wildlife Fund, which is behind the initiative, says interest has been immense.
"We're aware of villages in Norfolk in England that are doing Earth Hour and we're aware of the big cities like Chicago and Sydney that are doing it," he told the BBC.
Celebrations in Australia to mark Earth Hour include traditional Aboriginal torchlight performances, environmentally friendly dinner parties and special candlelit evenings for single people, the BBC's Phil Mercer reports from Sydney.
Some pubs are spending the evening without the lights on while many Australians are marking the occasion quietly in the darkness at home.
Australia is one of the world's worst per capita emitters of greenhouse gases and many believe recent droughts and floods are the result of man's destabilising influence on the climate, our correspondent says.
New Prime Minister Kevin Rudd has made the environment one of his priorities, signing up to the Kyoto Protocol on tackling climate change soon after he took office.
New Prime Minister Kevin Rudd has made the environment one of his priorities, signing up to the Kyoto Protocol on tackling climate change soon after he took office.
The Google web search engine is marking the event by putting a dark background on its homepage.
Will you be switching your lights off? What do you think of this idea? Send us your comments using the form below:
Will you be switching your lights off? What do you think of this idea? Send us your comments using the form below:
