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The floodlit cream shells of the famed Opera House dimmed Saturday as Sydney became the world's first major city to plunge itself into darkness for the second worldwide Earth Hour, a global campaign to highlight the threat of climate change.

From the Great Pyramids to the Acropolis, the London Eye to the Las Vegas strip, nearly 4,000 cities and towns in 88 countries planned to join in the World Wildlife Fund-sponsored event, a time zone-by-time zone plan to dim nonessential lights between 8:30 p.m. and 9:30 p.m.

Involvement in the effort has exploded since last year's Earth Hour, which drew participation from 400 cities after Sydney held a solo event in 2007. Interest has spiked ahead of planned negotiations on a new global warming treaty in Copenhagen, Denmark, this December. The last global accord, the Kyoto Protocol, is set to expire in 2012.

Despite the boost in interest from the Copenhagen negotiations, organizers initially worried enthusiasm for this year's event would wane with the world's attention focused largely on the global economic crisis, Earth Hour executive director Andy Ridley told The Associated Press. "Strangely enough," he said, "it's seemed to have the opposite effect."

"Earth Hour has always been a positive campaign; it's always around street parties, not street protests, it's the idea of hope not despair. And I think that's something that's been incredibly important this year because there is so much despair around," he said. "On the other side of it, there's savings in cutting your power usage and being more sustainable and more efficient."

In Australia, people attended candlelit speed-dating events and gathered at outdoor concerts as the hour of darkness rolled through the country. Sydney's glittering harbor was bathed in shadows as lights dimmed on the steel arch of the city's iconic Harbour Bridge and the nearby Opera House. Earlier Saturday, the Chatham Islands, a group of small islands around 500 miles east of New Zealand, officially kicked off Earth Hour by switching off its diesel generators. Soon after, the lights of Auckland's Sky Tower, the tallest man-made structure in New Zealand, blinked off.

Forty-four New Zealand towns and cities participated in the event, and more than 60,000 people showed up for an Earth Hour-themed hot air ballooning festival in the city of Hamilton.

At Scott Base in Antarctica, New Zealand's 26-member winter team resorted to minimum safety lighting and switched off appliances and computers.

China was participating in the campaign for the first time, with Beijing turning off the lights at its iconic Bird's Nest Stadium and Water Cube, the two most prominent venues for the Olympics, according to WWF. Shanghai was also cutting lights in all government buildings and other structures on its waterfront, while Hong Kong, Baoding, Changchun, Dalian, Nanjing and Guangzhou were also participating, WWF said.

Later Saturday, Thailand Prime Minister Abhisit Vejjajiva planned to press a button to turn off the lights at Khao San Road, a famous haven for budget travelers in Bangkok that is packed with bars and outdoor cafes.

"Lights were to go down at the Grand Palace and other riverside monuments, and businesses along some of the Thai capital's busiest boulevards were also asked to participate," the Bangkok Metropolitan Administration said in a statement.

The capital hoped to reduce electricity consumption in the city of more than 8 million people by at least 30 percent - or 1,400 megawatts - during the event. Earth Hour organizers say there's no uniform way to measure how much energy is saved worldwide.

Earth Hour 2009 has garnered support from global corporations, nonprofit groups, schools, scientists and celebrities - including Oscar-winning actress Cate Blanchett and retired Cape Town Archbishop Desmond Tutu.

McDonald's Corp. planned to dim its arches at 500 locations around the Midwest in the United States. The Marriott, Ritz-Carlton and Fairmont hotel chains and Coca-Cola Co. also planned to participate.