Global landmarks from Sydney's Opera House to the Forbidden City and the glittering Las Vegas Strip will be plunged into darkness today as activists try to reinvigorate the climate change fight.

Hundreds of millions of homes in scores of cities scattered around some 125 countries will join the great Earth Hour switch-off, which comes just months after disappointing UN climate talks in Copenhagen.

Iconic structures like the Empire State Building and Egypt's Pyramids will heed the WWF conservation group's call to turn off the lights in the name of environmentalism, creating a wave of darkness that will roll across the world.

Earth Hour started in 2007 in Sydney and now enjoys widespread support both from the public and big business, including Google, Coca-Cola and McDonald's.

United Nations chief Ban Ki-moon called the campaign, now in its fourth year and expected to attract record participation, "both a warning and a beacon of hope".

"Climate change is a concern for each of us. Solutions are within our grasp and are ready to be implemented by individuals, communities, businesses and governments around the globe," Ban said.

Sydney's Opera House and Harbour Bridge will informally kick off proceedings when they go dark at 8:30 pm (local time) along with millions of Australian households for one hour.