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Sydney Before Earth HourLights from Sydney, Australia's many buildings paint the harbor two days before Earth Hour 2013, which took place on March 23.The movement, which began in Sydney in 2007, is a way for people around the world to show their support for tougher action on global warming by turning off the lights for one hour starting at 8:30 p.m. local time.Going dark is just the start for many organizations and governments around the world. Environmental challenges inspired by Earth Hour include planting 500,000 trees in Uganda to highlight deforestation in the African country to gathering over a hundred thousand signatures on a petition to ban logging in parts of Russia, according to the Earth Hour website.The annual event is held at the end of March to coincide with the spring and autumnal equinoxes in the Northern and Southern Hemispheres.

Photograph by David Gray, Reuters