Switzerland's Chillon Castle glows pastel in a picture taken March 31, 2012, just before much of the world went dark for Earth Hour.
This year's Earth Hour will take place on March 23 at 8:30 p.m. local time, when people in more than 7,000 cities and 152 countries and territories will turn off their lights as a show of solidarity against global warming, according to the Earth Hour website.
Palestine, Tunisia, Galapagos, Suriname, French Guyana, St. Helena, and Rwanda are among those joining the movement for the first time in 2013.
Billed as the largest symbolic mass participation event in the world, Earth Hour began more humbly in Sydney, Australia, in 2007 with about two million participants. The voluntary one-hour "fade-out" has since grown into an international event involving hundreds of millions of people. (See before-and-after Earth Hour pictures from past years.)
Earth Hour's energy-saving impact is limited: It does very little to reduce the greenhouse gas emissions that contribute to global warming, according to its partner WWF.
Andy Ridley, CEO and co-founder of Earth Hour, said in a statement that the event was created "to unite people and show our desire to protect the planet," as well as "to encourage and empower people to take action beyond the hour itself."