What would happen if the lights went off all over the world?
Major cities and global landmarks have been plunged into darkness as millions of people switched off lights for an hour to protest against climate change.Earth Hour is sponsored by the World Wildlife Fund. The Washington Post reports that "Eighty-four countries pitched in, including tourist icons such as the Eiffel Tower, the Sydney Opera House, Times Square, the Pyramids, Beijing's National Stadium and the Las Vegas Strip."
The initiative, Earth Hour, was begun in Sydney two years ago by green campaigners keen to cut energy use.
Correspondents say the aim is to create a huge wave of public pressure to influence a meeting in Copenhagen later this year to seek a new climate treaty.
You can see a video from the BBC at the link above.
Visit Earthhour.org for pictures from around the world and more information.
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