The 13th edition of Earth Hour, organised by the green group WWF, is set to see millions of people across 180 countries turn off their lights at 8:30 pm local time to highlight energy use and the need for conservation.
"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.
Dozens of companies around the world have said they will join in this year's switch-off.
Lights out at Parliament House for Earth Hour conservation campaign
The event comes after some of the most dire warnings yet on the state of Earth's natural habitat and species.
Another data-set 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, Dubai's Burj Khalifa and the Acropolis in Athens are among the 24 global landmarks that will take part in Earth Hour.
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