The Las Vegas strip (below) and other global icons went dark on Saturday for Earth Hour.

McDonald's powered down in Chicago. Twitter was alight with Earth Hour tweets.

The annual event, launched in 2007 by the World Wildlife Fund, aims to encourage people to cut energy use and curb greenhouse gas emissions from fossil fuels.

World emissions have risen by about 70 percent since the 1970s. China has recently overtaken the United States as the top emitter, ahead of the European Union, Russia and India.

 What did you do, if anything, to switch off?

(Photos: Reuters\Mario Anzuoni, McDonald's)

Perhaps 50 million people took part in a global Earth Hour campaign to turn out the lights for an hour at 8 p.m. on Saturday to put attention on global warming, organisers said. Did you?

    In Australia, one survey showed that more than half the adults turned off the lights, they said. Bangkok saved 73.3 megawatts, or the equivalent of switching off 2 million fluorescent lights, and organisers said electricity use dropped 8.7 percent in Toronto, Canada.

    You don’t have to be a tree-hugging socialist to see that it makes sense to turn off unnecessary lights and electrical appliances, although it obviously only makes sense if you do so all the time and not as a gimmick one Saturday night a year.

     You can choose from many reasons - you may be worried about climate change, you may want to end a national addiction to oil with prices at $100 a barrel, or curb a dependence on foreign energy supplies.

    Still, I wonder how you estimate how much electricity was “saved” on Saturday. Electricity use typically declines as the evening goes on and people go to bed, starting with kids around 8 p.m., so it may be easy to overstate ”cuts” at 8 p.m. 

    I had a look at the power consumption figures from the Nord Pool exchange for Denmark, a country heavily involved in Earth Hour to try to find out: electricity use did seem to fall faster than normal.

     The Tivoli funfair, the royal palace and the opera house all turned off the lights at 8 p.m. for an hour — there were so many lights out that you could see stars shining from the centre of Copenhagen. The Danish capital will host of a U.N. conference at the end of 2009 meant to agree a new global climate treaty to succeed the Kyoto Protocol, so many people got involved.

    The country’s electricity consumption fell 6.5 percent in the hour from 8 p.m. compared to use the previous hour, more than a decline of 4.7 percent the same hour a week earlier. And it then fell 5.5 percent in the hour from 9 p.m., faster than 5.0 percent on March 22. Less electricity was consumed from 8-9 p.m. than on any other Saturday night this month.

    OK, so there might have been other factors like the temperature steering power use over an evening but it surely indicates that every little bit does count?