Icons including the Great Pyramids, the Eiffel Tower and China's Forbidden City will be plunged into darkness on Saturday as millions take part in 'Earth Hour', a rolling grassroots movement aimed at tackling climate change.
Now in its fourth year, the global campaign promises to be the biggest ever, organisers said, with thousands of cities and towns in 125 countries pledging to take part, despite last year's failed climate talks.
Hopes for a binding treaty to halt global warming were dashed when December's summit of world leaders in Copenhagen only yielded a general agreement on limiting warming to two degrees Celsius (3.6 degrees Fahrenheit).
Seen even by its supporters as a disappointment and a chaotic failure by critics, Copenhagen was expected to have scuttled people's hopes for meaningful action on climate change, explained Earth Hour founder Andy Ridley.
"But the craziest thing this year is that rather than that happening, the reaction to Earth Hour has been immense. It is so much more than last year," Ridley told AFP.
"There appears to be some fatigue to the politics around it... But people are far more motivated this year than they were last year," when 88 countries took part, he added.
Earth Hour began in Sydney in 2007 when 2.2 million people switched off the lights in their homes, offices and businesses for 60 minutes to make a point about electricity consumption and carbon pollution.
The campaign went global the following year, and this Saturday, March 27, more than 1,200 of the world's best-known landmarks will kill their lights at 8.30pm local time in what organisers describe as a "24-hour wave of hope and action."
A raft of major multinational corporations including Google, Coca Cola, Hilton, McDonalds, Canon, HSBC and IKEA have endorsed Earth Hour 2010 and pledged to darken their offices worldwide in support.
Sydney's iconic Harbour Bridge and Opera House will help kick off the energy-saving marathon, with Egypt's Great Pyramids and Sphinx, the Trevi Fountain and Tower of Pisa in Italy and all major landmarks in Paris to take part, led by a five-minute blackout of the Eiffel Tower.
Beijing's Forbidden City -- the figurative and geographic heart of the world's largest emitter of greenhouse gases -- will go dark, along with the so-called "Bird's Nest" Olympic stadium.
Elsewhere in Asia, where 3.3 million people have registered to take part, the world's biggest observation wheel, the Singapore Flyer, will extinguish its main lights, while official buildings will be blacked out in Seoul.
WWF Indonesia said around 200 buildings in Jakarta would be taking part in the blackout.
Japan's heritage-listed Hiroshima Peace Memorial, one of few buildings to survive America's 1945 atomic bomb attack, will take part, while major companies including Sony, Sharp and Asahi were to switch off across Tokyo.
Some 30 US states and municipalities were to take part, with an hour of darkness at sites including Mount Rushmore, the Empire State Building, the Golden Gate Bridge and Seattle's Space Needle.
And in Dubai, the world's tallest skyscraper, the Burj Khalifa tower, will also dim its lights.
Residents of Norway's Longyearben, the world's northernmost town, are set to brave an influx of curious polar bears normally deterred by lights after voting -- for the first time -- that participating was worth the risk.
"Earth Hour is meant to cross geographic, economic, country boundaries," said Ridley, admitting that it was mostly a symbolic act.
"It's one hour, one day, one year. We're not saving the planet by turning the lights off for one hour."
But, he explained: "What you are doing is adding your voice to a global call for action."