At 8:30pm local time this evening (March 25), the lights will be switched off in 7,000 cities across 172 countries for one hour. Iconic buildings across the world, including the Empire State Building in New York, the Sydney Opera House, the Burj Khalifa in Dubai, London’s Big Ben, the Eiffel Tower in Paris, the Colosseum in Rome, Istanbul’s Blue Mosque, Moscow’s Kremlin, and the Pyramids of Egypt, will go dark.

The dramatic gesture will mark the 10th annual Earth Hour, an initiative organized by the World Wildlife Fund (WWF) Australia, to shine a light on climate change. The group also encourages businesses and ordinary people to turn of their lights for the hour, and almost 40,000 have signed up on the website so far to participate.

Before and after.

While critics say Earth Hour is an empty symbolic gesture, the organizers told Reuters that it’s not about energy saved during the hour (they do not keep track of that), but rather the awareness raised.

“Movements matter, and the Earth Hour movement is a global reminder that people are leading the transformation to a more prosperous and renewable future,” said Lou Leonard, the senior vice president of climate and energy at the World Wildlife Fund.

This year, when the lights go out, the group suggests passing the time by tuning in to an online concert by musicians around the world.