By Simon Gompertz
A sheep farm in the Brecon Beacons in mid-Wales is providing a glimpse of a post-carbon future, one in which green electricity is generated locally.
And the farmer involved, Howell Williams, has found that low emissions can mean high profits. He is making £900 ($1,500) a month from going green.
Howell has worked on the damp, golden hills above the village of Abercraf for 30 years. One day, tramping back down along one of the streams that tip down from the hilltop, he had an eye-opening moment.
"I thought: 'What terrific power!'" he remembers. "If only we could harness this."
Howell Williams is making power for 20 households from one small stream
Howell Williams spent £25,000 ($41,000) on a water turbine, housed in a small shed over the stream, just next to his farm. A £2,500 grant from the Brecon Beacons National Park helped him reduce the cost.
He showed me the equipment after a rainy few days and it was generating at its maximum rate, a constant 11kW, all of it sold to the local electricity company.
"I think I am generating enough electricity for 20 houses," Mr Howell says.
World leaders are under pressure to agree a climate deal in the Copenhagen talks, but the only pressure Howell Williams feels is from the water gushing 90 metres down the hillside.
He is thinking about installing another turbine. On a recent Sunday, 20 local farmers came to see his mini-hydro station and 14 of them said they could see the potential for similar installations on their own farms.
Welsh hill farmers have always struggled to scrape a living. Water power offers the prospect of a steady income to supplement the up and down earnings from selling lamb.
A few miles away, in the picturesque Dyffryn Crawnon valley, plans are being hatched for a bigger water turbine on another steep hillside dotted with woods, sheep and and stone farm buildings.
The difference is that this is to be a community project. The profits, expected to be £12,000 a year, will be spent on energy-saving schemes for residents and more water turbines.
Projects like this are tiny compared with the overall demand for power, but the plans are just a start.
"It's not going to generate all of our electricity needs but it's going to produce a lot of it," explains Chris Blake, who runs a local initiative called The Green Valleys, "And if we can get 40 or 50 schemes in this area then it really does start to make a big difference."
He points out that the water is only "borrowed": it's extracted upstream and returned to the river below the turbine. And the turbine house will be hardly visible, half-buried and covered in a turf roof.
The 16kW Dyffryn Crawnon turbine will generate almost as much power as the inhabitants of the valley currently use.
"This valley probably has scope for four or five such schemes which would make it a carbon negative valley, this within 5 years. It's something fantastic to show to Copenhagen."
Howell Williams took me to round up sheep and lambs and move them to some fresh, glistening pasture, in order to show off the spring above Abercraf which feeds his stream.
Howell Williams' turbine leaves the landscape unscathed
Water is diverted at this point to shoot down the hillside in a buried plastic pipe, gathering speed and force along the way.
He restricts grazing and the size of his flock, to try to control its impact on the landscape. But the water turbine has transformed his environmental credentials and he is beginning to look at the sustainability of the farm as a whole.
Farming has had to face difficult questions in the debate over global warming. There are the tractors, the manufactured feed, the methane given off by livestock: all combine to create a carbon footprint which has to be justified.
"You're producing carbon-negative lamb," I suggest.
"Yes," Howell agrees, "I should imagine 90% anyway."
"I think it's a good route to take with livestock. We should make use of any natural resources we've got - wood, water, the air you use. It can all be brought together in one package."
Not many people are lucky enough to have a steep slope and a fast-running stream on hand.
But if objectives set in Copenhagen are to be achieved, everyone will have to look around them, like Howell Williams, and find whatever opportunities they can to reduce their carbon emissions.
Simon Gompertz's television report can be viewed
on the Working Lunch website.