Global landmarks from the Sydney Opera House to Dubai's Burj Khalifa are dimming their lights to raise awareness about energy use and our planet's vanishing biodiversity.
The thirteenth edition of Earth Hour, organised by the green group WWF, will see 24 landmarks each go dark for 60 minutes throughout the day.
"We are the first generation to know we are destroying the world. And we could be the last that can do anything about it," the charity said.
"We have the solutions, we just need our voices to be heard."
Dozens of companies around the world have said they will join in this year's switch-off.
The event comes after some of the most dire warnings yet on the state of Earth's natural habitat and species.
WWF's own "Living Planet" report in October said that 60 percent of all animals with a backbone -- fish, birds, amphibians, reptiles and mammals -- had been wiped out by human activity since 1970.
Another dataset confirmed the depth of an unfolding mass extinction event, only the sixth in the last half-billion years.
Paris's Eiffel Tower, New York's Empire State Building and the Acropolis in Athens will all take part in Earth Hour.
Last year's event was observed in more than 7,000 towns and cities in 187 countries, according to organisers.
On social media, people posted messages about how they marked the event.
