The Sydney Opera House and Harbour Bridge went dark for an hour Saturday, kicking off a global campaign that will see landmark buildings around the world dim their lights to raise awareness about the impacts of climate change.
Earth Hour, which started in Australia in 2007, is being observed by millions of supporters in 187 countries, who are turning off their lights at 8.30pm local time in what organisers describe as the world's "largest grassroots movement for climate change".
"It aims to raise awareness about the importance of protecting the environment and wildlife," Earth Hour organiser WWF Australia chief Dermot O'Gorman told AFP.
Images from across Asia showed buildings including Kuala Lumpur's Petronas Towers, as well as the famous harbour skylines of Hong Kong and Singapore, being plunged into darkness to mark the occasion.
Other global landmarks that will take part include Beijing's Bird's Nest Olympic stadium, the Eiffel Tower in Paris, the Pyramids of Egypt and New York's Empire State Building.
With global temperatures the highest on record, O'Gorman said this year's theme was the impact of climate change on biodiversity and plant and animal species.
"More than half of plant and animal species face local extinction in some of the world's most naturally rich areas in biodiversity by the turn of this century if we continue along the current path that we are trending in terms of global warming," he said.
Species at risk include Australia's green turtles, black-flanked rock wallabies and koalas, as well as the Adelie penguin colonies in Antarctica, the conservation group said in a report it commissioned that was published in the science journal Climatic Change.
The analysis, released last week, said key biodiverse sites around the world projected to be most affected by localised extinction include the Amazon, the plant's largest tropical rainforest, and southern Africa's Miombo Woodlands.
While the lights-off event is a symbolic gesture, Earth Hour has led successful campaigns over the past decade to ban plastics in the Galapagos Islands and plant 17 million trees in Kazakhstan.
Sydneysider Dianna Ali, who was having dinner with family as the lights went off in the city, said the initiative had made her more aware of the impact of her lifestyle on the planet's health.
"Since Earth Hour started, it's made me more conscious of how much power I'm using," she told AFP.
"I think... about how much one individual can make a difference."