Last year, 50 million people worldwide switched off their home and office lights for an hour. Golden Gate Bridge in San Francisco, the Colosseum in Rome, the Sydney Opera House and the Coca Cola billboard in Times Square all went dark.

With the darkness moving around the globe, people were trying to demonstrate to world leaders their support for tackling climate change.

This Saturday, at 8:30 PM local time, the event, sponsored by WWF, will take place again.

Peter Parkes reports on the main Skype blog: "This year, WWF has an even more ambitious target. To help them reach it, join Skype and vote earth. The result of the vote will be presented to the United Nations Climate Change Conference http://unfccc.int/2860.php in Copenhagen later this year, and WWF are hoping to get 1 billion votes for Earth."