Earth Hour kicks off in Sydney (PHOTOS)

Sydney's iconic Harbor Bridge and Opera House went dark on Saturday as Australians were the first to switch off their lights for an hour as part of a global effort to shine a spotlight on climate change, according to Zee News. Hundreds of landmarks around the world, including Washington's National Cathedral, London's Clock Tower, the Great Wall of China and Tokyo Tower will plunge into darkness when their clocks strike 8:30 p.m. local time.

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Thousands of cities will take turns flicking off switches in some 150 countries and territories, reported the Montreal Gazette. And in a twist to this year's Earth Hour, Dutch astronaut André Kuipers will observe all the countries around the world as their lights go out from the International Space Station. He'll also be posting his photos of the world going dark on his blog for everyone to see.

Said to be the largest environmental event in history, Earth Hour began in Sydney in 2007, with the idea of sending a powerful message for action on climate change and to celebrate the planet, wrote Australia's 9News.

"We are living beyond our means. That is not sustainable," said Andy Ridley, co-founder and executive director of Earth Hour, according to CNN. "We want to unite people around the world to build a sustainable future."