Louise Batchelor BBC Scotland environment correspondent

Edinburgh's trams are being presented as a clean, green alternative to cars and buses in the capital.

So, do the claims stack up?

The Edinburgh trams will run on electricity

Modern electric trams may not be an obvious source of pollution, but it all depends on how the electricity is made.

Then there is the question of how many motorists will be prepared to get out of their cars.

As you would expect, the company set up to build them says trams will be an attractive option.

Tie Limited - an arms length operation for Edinburgh City Council - says research shows that 20% of peak hour and 50% of weekend tram passengers in other parts of the UK previously travelled by car.

They also believe trams will be good for city centre business. Dublin is held up as the example where they say that, after trams were introduced, there was a rise of between 20% - 35% of visitors to Grafton Street, the city's main shopping thoroughfare. Some retailers reported a 25% increase in trade.

Willie Gallagher, executive chairman of Tie Limited, said: "Trams are proven to enhance the quality of environment in the cities that invest in them.�

Trams are an environmentally friendly way of moving large numbers of people efficiently, cutting congestion and local air pollution Stuart Hay Friends of the Earth

"They will enhance Edinburgh's reputation as a good place to live, work and visit through a range of environmental benefits from air quality to noise reduction to less congestion."�

Commuters to the north and west of the capital will have the option of leaving their cars at the Ingleston park-and-ride near the airport and travelling to the city centre by tram. Tie says it'll be a speedy and frequent service, with trams leaving every five minutes.

As trams run on electricity, there'll be no nasty exhaust fumes along their route. But that doesn't mean there are no consequences - the pollution will simply appear somewhere else.

The trams obviously have to rely on available power supplies and in Scotland that's about a third from coal-fired plant, between a quarter and a half from nuclear and the rest from gas and renewables like hydro and wind energy.

One of the main suppliers will be the Longannet power station in Fife - Scotland's biggest single emitter of the greenhouse gas, carbon dioxide.

The other coal-fired supplier is Cockenzie in East Lothian.

Friends of the Earth Scotland stress that trams are only as green as the fuel they run on.

Favourable comparisons

Head of projects Stuart Hay said: "Trams are an environmentally friendly way of moving large numbers of people efficiently, cutting congestion and local air pollution. �

"Despite their efficiency, they will still have global impacts such as CO2�emissions,�unless they are powered from renewable energy sources - �something Friends of the Earth Scotland wants to see adopted by the Edinburgh scheme."

They're seeking assurances that Edinburgh City Council will buy power from one of the "green" tariffs, meaning that the supplier must guarantee to buy energy from renewable sources to match the trams' consumption of electricity (even though the power will coming through the cables could be from any source.)

The council says they haven't reached that stage in the contracts yet.

But even if trams are not wholly green, in the beauty parade of environmentally-friendly transport they compare favourably to buses.

Trams can transport twice as many people as buses, far more comfortably and efficiently, eventually with zero emissions Robin Harper MSP

Based on other tram systems, eg in Manchester and Croydon, Department for Transport figures show that trams emit 65.0 grams of CO2 per "passenger kilometre" whereas for buses it's 89.1.

The average petrol driven car emits 182.2 grams of CO2 per kilometre.

For the Scottish Greens there's no contest. MSP Robin Harper said: "Trams are particularly effective at encouraging people to switch to public transport and away from the car, and will help reduce pollution, congestion and frustration in the capital.

"Over any given length and breadth of road space, trams can transport twice as many people as buses, far more comfortably and efficiently, eventually with zero emissions once we move to 100% renewables."

Edinburgh's trams will each have space for 250 passengers, compared to 103 on a double-decker bus.

But hang on a minute. The first phase of the Edinburgh tram scheme will take passengers to the airport - and the Greens want to see less flying.

Robin Harper says: "Without the tram, the numbers of car journeys to the airport would increase, and the area of land sterilised and under tarmac for parking spaces would also increase.

"Simply making the airport less accessible isn't the right way to reduce the numbers of flights taken. Ministers need to abandon their plans to expand Scotland's airports and also work to improve train links and other public transport alternatives."

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