From Sydney Harbour Bridge, Buckingham Palace and the Brandenburg Gate to the Burj Khalifa tower, the Empire State Building, the Taj Mahal and Table Mountain, some of the world's greatest landmarks will briefly darken on Saturday night for Earth Hour, a campaign now becoming a broader vehicle for green activism.
Earth Hour originated in Sydney in 2007 with an appeal to people and businesses to turn off their lights for an hour to heighten awareness about climate change, driven by carbon-emitting fossil fuels.
The annual switch-off is now being followed in locations in three-quarters of the world's nations and has the potential to touch hundreds of millions of lives, say its organisers.
Promoted through social media, Earth Hour has also mutated in some countries into a potent tool to lobby on local issues unconnected with global warming, they add.
Switch-off events this Saturday at the local time of 8:30 pm are planned in more than 150 countries, including for the first time the Palestinian territories, Tunisia, Galapagos, Suriname, French Guiana, St. Helena and Rwanda.
Newcomers to the campaign include Copenhagen's Little Mermaid, the Statue of David in Florence and Cape Town's Table Mountain, which in 2011 joined a list of the "New Seven Wonders of Nature".
"Last year, Earth Hour was followed in 7,000 communities, an increase of about 30 percent over 2011," said Andy Ridley, co-founder of the project launched by WWF.
"The biggest area of growth has been the Asia-Pacific, the economic engine of the planet, where wherever you go now, people are living with the problems of environmental damage," Ridley said in a phone interview from Singapore.
"If you're in a big city in China, you may well be going to work wearing a face mask (against air pollution), or if you are in the Philippines, you and your family may well have been affected by a super-typhoon."
Ridley added: "What we are finding is that environment issues may be perceived differently in the big cities of Asia compared with how they are perceived in Europe or the States. They may not be perceived as exclusively green issues. They are seen as issues that are damaging your lifestyle and potentially threatening your family."
In Japan, where floodlights will be turned off on Saturday at the capital's signature Tokyo Tower, campaigners are intertwining Earth Hour with remembrance of the March 2011 mega-quake and tsunami, which unleashed a nuclear disaster at Fukushima.
In Russia, activists last year harnessed the success of Earth Hour to secure 100,000 signatures for a petition for a law to protect sea areas around oil exploration sites, says WWF.
Uganda created the first "Earth Hour forest" of 2,700 hectares (10.4 square miles) of cleared land, which greens plan to fill with half a million trees.
Campaigners in Argentina are hoping that this year's event will provide traction to protect a 3.4-million-hectare (13,100-square mile) marine zone.
As it grows in visibility, Earth Hour is also becoming a target, with critics saying it is little more than token, encourages smugness about combating climate change and ignores the needs of development.
"India observes on average eight Earth hours a day," tweeted VictorTango@airkatana, a reference to the country's notorious power outages. More than 400 million Indians still lack access to mains electricity, according to the World Bank.
Ridley said no figures existed for the overall amount of energy saved by Earth Hour. But the brief switch-off, he argued, encouraged many people to do more to curb waste and think of the environment.
Leading French climate scientist Jean Jouzel said Earth Hour at least reminded people of the crisis posed by carbon emissions, which each year scale new peaks while UN talks for curbing them mark time.
"Does Earth Hour encourage self-satisfaction? Are its results limited? I would say 'yes' to both," Jouzel said.
"But if it's a toss-up between doing nothing and doing something that is not perfect but is still something very visible, I think the answer is: do something. In fact, we need more action of this kind."