Homes, office blocks and Australian landmarks have gone dark as millions of Australians take part in Earth Hour - a mass symbolic gesture urging action on the environment.
And, in the bushland behind their Castle Hill home, twins Freya and Imogen Wadlow have been poisoning privet, the weed that strangles their beloved gum and wattle trees.
To rally their neighbours, the 16-year-olds did a letterbox drop – on recycled paper of course – and their council lent the girls tree loppers and a gardener to help heal the scrub.
The bushland job was the Wadlow twins' latest in a long line of environmental good works, which netted them the Young Panda Award at tonight's Earth Hour ceremony.
The Castle Hill High School students began their activism aged 10, when, concerned that "kids our age didn't really know about what was going on in the world", they built their own environmental website to explain science in language children would understand.
They now run two websites – a children's site called Planet Patrol and a teen version called i-fink – and their material is now being used in schools across the US, Britain and Canada.
Earth Hour was about "symbolism", Imogen said, and they would use any attention from their prize to "spread the message".
"By doing little things as a mass we can really contribute," Freya said.
The girls are skilled politicians, and have exchanged letters with Prime Minister Julia Gillard, former prime minister John Howard and Prince Charles.
They said it was a thrill to see how big Earth Hour had become, and believed that those who slammed the idea for its gimmickry were missing the point, which was to encourage lots of people to do small things every day to help the environment.
The twins also picked up the People's Choice Award, which people voted for online over the month leading up to Earth Hour.
Other award winners included a small sustainable Brisbane-based tourism business, EC3 Global, which assists travel and tourism businesses to measure and improve their environmental footprint, and Beaconsfield State School in Queensland, where students are working to protect nesting sites for marine turtles.
The Creative Arts Award was presented to Ghost Nets Australia, which holds workshops in indigenous communities across Queensland and the Northern Territory on how to make art out of marine debris that would otherwise threaten marine wildlife.
Earth Hour began in Sydney in 2007 as a joint initiative between the World Wildlife Fund and Fairfax, publisher of The Sun-Herald. It has now spread to 5251 cities and towns in 147 countries and territories, making it the biggest voluntary environment movement in history.
Among the iconic buildings across the world joining the Opera House and Sydney Harbour Bridge in the annual blackout were the United Nations headquarters, the Las Vegas strip, Times Square, the Brandenburg Gate, the Eiffel Tower and the world's tallest skyscraper, Burj Khalifa in Dubai. And the idea has now travelled beyond Earth. The crew at the International Space Station pledged to switch off or dim all non-essential lights.
Adding urgency to tonight's message, the Intergovernmental Panel on Climate Change this week released a gloomy report on the state of the planet. The paper, drawing on research from 220 authors, said that rising temperatures were already provoking an increase in extreme weather, and that this trend would only intensify.
Climate scientists agree there are only a few years left to avoid some of the "tipping points" for dangerous climate change. Will Steffen, director of the climate change institute at the Australian National University, told a global conference in London the world may have already squandered its chance to stop the eventual break-up of the West Antarctic and Greenland ice sheets. However, Imogen Wadlow said: "We're optimistic."
YOUNG PANDA AWARD: Freya and Imogen Wadlow, 16, Castle Hill, NSW.
CREATIVE ARTS AWARD: Ghost Nets Australia.
EDUCATION AWARD: Beaconsfield State School, Queensland.
FUTUREMAKERS AWARD: Dr Robert Dane, Castlecrag, NSW.
SUSTAINABLE SMALL BUSINESS AWARD: EC3 Global, Queensland
ECO-BLOGGER AWARD: Speak Your Mind.
PEOPLE'S CHOICE AWARD: Freya and Imogen Wadlow, 16, Castle Hill, NSW.
- with Jim O'Rourke and AAP