On March 31, 2007 at exactly 7:30 p.m., Sydney Harbour was cast into darkness.

First the lights of the Harbour Bridge were turned off, followed by those of the famous Opera House, then the office buildings and restaurants along the city's coastline. Nearly a hundred McDonald's restaurants dimmed their golden arches in a sign of environmental solidarity. One by one, businesses, government departments and individuals pulled the plug for Earth Hour – a project aimed at bringing the issue of energy conservation and climate change to the fore.

The idea behind Sydney's hour of darkness began as a joint effort between the World Wildlife Fund (WWF) and the Sydney Morning Herald newspaper. Their goal was two-fold: to launch a year-long campaign to reduce Sydney's greenhouse gas emissions by 5 per cent, and to find a way to bring the fight against climate change into every home.

"It had to be simple," Andy Ridley, WWF co-ordinator of the global Earth Hour movement, said in a telephone interview from Sydney. "We wanted to break out of the classic environmental audience and take it to the mainstream."

They first approached the city with the idea, and then moved on to commercial businesses occupying the equivalent of 30 per cent of Australia's office space.

Eventually, the whole city caught green fever, and ideas for participating in the hour of darkness began pouring in.

That spirit – and idea – has spread. This year, at 8 p.m. on Saturday, March 29, 17 cities in seven countries will take part in the first global Earth Hour. Toronto was the first city outside Australia to join in. "We wanted this to be a street party instead of a street protest," Ridley says of last year's inaugural event.

So, Sydneysiders dusted off their flashlights, stocked up on candles and got in the mood. Couples held wedding receptions in candlelight; restaurateurs fed hungry customers under the moonlight. Thousands of families sat by the harbour watching the lights go out, and, thanks to the reduced light pollution, the stars as they began to reappear. Stadiums dimmed their lights and sports fans watching games at home were reminded to turn off theirs. Politicians, musicians and Hollywood actors helped with extensive media coverage counting down the weeks, days and seconds to the event.

In the minutes before darkness was to fall, Ridley held his breath.

"I couldn't help be nervous," he recalls. "At 7:29 p.m, you just don't know if it's going to work, if it's something people will think is important enough to do."

It was. More than 2 million residents pulled the plug and the city went dark. Energy usage dropped by 10.2 per cent across the business district, more than double what organizers were aiming for, representing a reduction of 25 tonnes of carbon dioxide – the equivalent of taking 48,613 cars off the road for an hour.

"It was amazing," Ridley says. "These are the kinds of events that show how much spirit a city has, and I was impressed by Sydney's."