More than 350 of the world's most iconic landmarks will switch off their lights at 8:30pm tonight.
The Eiffel Tower, the Empire State Building and the Sydney Opera House - and millions of homes around the world - are all taking part in Earth Hour.
The campaign, which encourages people to plunge their homes into darkness for one hour, is part of a united global effort to raise awareness of climate change.
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Here at home, all non-essential lights at Áras an Uachtaráin will be switched off to mark the event.
Leader of the Green Party, Eamon Ryan, says it's one small way of highlighting the effects of global warming;
The Telegraph have posted a video of some of the famous landmarks in Asia turning their lights off for the hour:
