More than 7,000 cities across 172 countries are expected to turn off the lights this evening in the biggest Earth Hour event ever.
At 8:30pm GMT the Houses of Parliament will go dark, as will the Eiffel Tower in Paris and the UN building in the New York, as part of a global symbolic demand for action on climate change.
Thousands of people around the world will, for just that hour, also opt for candlelight.
The WWF-led demonstration, which started in Sydney nine years ago, is particularly potent this time round, with the pivotal Paris climate change conference due to be held at the year's end.
It's there that the definitive global climate change deal may be struck.
UN Secretary General Ban Ki-moon, a staunch advocate for climate change action, said in a video address: "Climate change is a people problem. People cause climate change and people suffer from climate change.
"People can also solve climate change. This December in Paris, the United Nations is brining nations together to agree a new, universal and meaningful climate agreement."
Ban said: "By turning out the lights we also highlight that more than a billion people lack access to electricity. Their future wellbeing requires access to clean, affordable energy."
Ed Davey, the secretary of state for energy and climate change, said it's "time to recognise climate change will touch just about everything we do, and everything we care about".
He said: "Earth Hour is an excellent opportunity for millions of people across the world to take one simple step to show they're serious about backing action on climate change."Reuse content