IN THE bush behind their Castle Hill home, twins Freya and Imogen Wadlow have been poisoning privet, the weed that strangles their gum and wattle trees.
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Landmarks across the world go dark as part of Earth Hour, a global campaign to highlight climate change.
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 last night's Earth Hour ceremony.
The twins also picked up the People's Choice Award, which people voted for online over the month leading up to Earth Hour.
Dr Robert Dane, of Castlecrag, won the Futuremakers Award for his cutting-edge idea for solar-powered ferries with wind veins, inspired by his study of beetles' wings.
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 Julia Gillard, John Howard and Prince Charles.
They said it was a thrill to see how big Earth Hour had become, and believed that those who slam 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.
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 buildings joining the Opera House and Sydney Harbour Bridge in the annual blackout were the UN headquarters, Times Square, the Brandenburg Gate, Eiffel Tower and the world's tallest skyscraper, Burj Khalifa in Dubai.
Energy use across the Sydney CBD fell by about 10 per cent during Earth Hour last night, according to analysis from Ausgrid's energy efficiency team. Ausgrid energy efficiency expert Paul Myors said the decrease in electricity use was the equivalent of switching off more than 1.8 million energy-saving light bulbs for an hour.
The Intergovernmental Panel on Climate Change last week released a report that said rising temperatures were triggering an increase in extreme weather, and this trend would intensify. Climate scientists agree there are only a few years left to avoid some of the ''tipping points'' for dangerous climate change. Will Steffen, the director of the climate change institute at the Australian National University, told a conference in London the world may have already wasted its chance to stop the break-up of the West Antarctic and Greenland ice sheets. However, Imogen Wadlow said: ''We're optimistic.''