Yesterday was Earth Hour 2011–did you take part? Or watch the lights turn out in your city? Earth Hour was started by World Wildlife Fund, or WWF, in 2007, and now it takes place every year on the last Saturday in March. For one hour across the globe, homes and businesses switch off their lights and electrical appliances to send a message about climate change.

The participation has grown year after year: in 2010, 126 countries took part, compared to 96 the year before. It's always an impressive site to see major cities like Sydney, New York, Paris and Berlin power down for an hour–the Russia Today video above gives you an idea of what it looks like. But every household and every person who takes part is also making a major contribution.

Do you have pictures from Earth Hour 2011? Post them on our GLOBAL IDEAS Facebook site!