The Sydney Opera House and Harbour Bridge went dark for an hour yesterday to kick off a global campaign raising awareness about the impacts of climate change.

Earth Hour, which started in Australia in 2007, is set to be observed by millions of supporters in 187 countries, who will turn off their lights at 8.30pm local time in what organisers describe as the world's "largest grassroots movement for climate change".

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.