The lights on some of the world's most iconic buildings and tourist destinations have gone dark as cities in Australia and Japan became the first to mark Earth Hour.
More than 350 landmark buildings across the world - including the Eiffel Tower, the Empire State Building and Big Ben - are set to take part in the 10th annual Earth Hour; a global lights-out event to highlight the dangers of global warming.
Sydney Opera House, normally brightly lit, was among the first to turn off its lights at 8.30pm local time (9.30am GMT), along with Sydney Harbour Bridge and dozens of others across Australia.
In Japan, the Tokyo Tower went dark; while buildings in the Pudong financial district of Shanghai, China, followed suit. The Taipei 101 building in Taipei, Taiwan, also took part.
Landmarks in the UK will join in at 8.30pm.
The theme of this year's Earth Hour is 'Places We Love', celebrating the beaches, forests, reefs, snow-capped mountains and rivers which are at risk from climate change.
Earth Hour Global executive director Siddarth Das said organisers were hoping that this year's event would help propel "new momentum" in the wake of the climate conference in Paris in December.
At the summit, world leaders agreed a number of measures designed to reduce carbon emissions to a net zero by 2100, moving away from fossil fuels and shifting favour to greener energies such as solar and wind power.
Earth Hour began in Sydney in 2007 as an initiative by conservation charity WWF. It went global the following year, and attracts millions of participants every year, organisers say.
A total of 178 countries and territories have agreed to take part in this year's event.