"Earth Hour is a visual mandate that allows people to vote on this issue of climate change. Do we act or do we not," Earth Hour executive director Andy Ridley said in a phone interview from Sydney, Australia, where the event began two years ago.
Paris is one of 28 French cities and towns to join the World Wildlife Fund Earth Hour initiative. Belize agreed to participate after South Korea joined.
Ridley believes that "2009 is probably the most important year in terms of climate change" because the Copenhagen negotiations on climate are set for this December.
"No country on its own can act on its own to deal with climate change," Ridley said. "It has to be everyone acting and everyone making that commitment to lower emissions."
Last year, more than 50 million people around the world flipped the switch. Other landmarks set to go dark include the CN Tower, Rio de Janeiro's Christ the Redeemer statue and Sydney's Opera House.