Lights will be switched off tonight at 8 p.m. in 24 global cities, including Atlanta, Bangkok, Bogota, Chicago, Dublin, Montreal, Manilla, San Francisco, Sydney, Tel Aviv and more. This year's "Earth Hour" is an expansion of the event which took place last year in Sydney, Australia on March 31. A somewhat similar "Lights Out" was held in San Francisco in October last year, but this year it has been moved to coincide with Earth Hour tonight.

Businesses, communities and individuals are encouraged to turn off the lights, as well as devices and machinery which use electricity or fossil fuels. Tonight’s Earth Hour is an initiative of the World Wildlife Fund International.

Google is also participating by turning its webpage black. The search engine switches its entire homepage color scheme and posts a simple message below its search box: "We've turned the lights out. Now it's your turn."

"It is about simple changes that will collectively make a difference – from businesses turning off their lights when their offices are empty, to households turning off appliances rather than leaving them on standby," claims the Earth Hour website.