A FOREST the size of Edinburgh would need to be planted every year-andmaintainedfor eternity - to offset all the pollution caused by road traffic in Scotland.

That is the conclusion of an expert reportforthegovernmentagency Transport Scotland, which says that planting trees to soak up climate-wrecking vehicle fumes is "not practical".

The report casts serious doubt on the viability of the previous Executive's plan to plant trees on roadside verges to protect the environment. Even if all the availablevergeswerecoveredwith woodlands, the report says, only 17% of a single year's carbon emissions from traffic would be offset.

Thereport,basedonadetailed analysis by the UK Transport Research LaboratoryandconsultantsYoung Associates, points out that it could cost £53 million to offset pollution belched out by Scotland's road vehicles in a year. The whole idea is "questionable", it says.

To offset the one million tonnes of carbon emitted by vehicles on the trunk road network would require 244 square kilometres of trees, an area about the size of Edinburgh. "This will only offset emissions for one year and the trees will need to be maintained for all time," the report observes.

Environmentalists have condemned mass tree-planting as an "unreliable" and "wholly inadequate" way of combating traffic pollution. "It is a dangerous distraction from making real carbon savings," said Stuart Hay, head of policy at Friends of the Earth Scotland.

"Planting more trees along our roads might help to hide the government's failure to tackle growing traffic levels, but will do little, if anything, to offset rising emissions."

Transport Scotland defended tree-planting by pointing out that delivering carbon savings was a central feature of the national transport strategy.

"The Scottish Executive intends to present a carbon balance sheet' for transport which will present the impact of all Scottish transport policies and projects that are expected to have a significant impact on carbon, whether positive or negative," a spokeswoman for the agency said.

"The aim will be to show that the Scottish government, through its own actions,iscontinuallyreducingthe overallenvironmentalimpactof Scottish transport measures."

BUT concerns about carbon offsetting have been reinforced by newwarningsthisweekend from the government's official environmental advisers.

A report by the Sustainable Development Commission in Scotland says that offsetting should not be used as an excuse to continue polluting. It argues that the first priority should be to cut greenhouse gas emissions, with offsetting only being used as a last resort.

Offsetting projects had to be very carefullychosentoensurethey delivered verifiable, additional benefits andavoideddouble-counting,the report went on to say. Tree-planting projects in Scotland were also regarded withsuspicionbecausemanywere publicly-funded schemes that would happenanyway.Organisationsthat really wanted to reduce emissions would "almost certainly" be better to fund projects abroad, the report suggests.

The report praises "carbon neutral" initiatives from Scottish Enterprise and VisitScotland, but sounds a cautionary note:"Carbonneutralityshouldbe presented as a stage in a journey rather than an easy get-out from responsibility for pollution."

Offsetting should not be seen as an "easy fix", according to the Sustainable DevelopmentCommissioner,Hugh Raven. "We need to know that when a Scottish body claims it's carbon neutral, all possible steps have been taken to reduce energy consumption," he said.

"We need to know that the offset made real cuts in carbon added to the atmosphere - that is, it was not a project that would have happened anyway. Only then can the public trust claims of carbon neutrality."

Raven congratulated companies such as Marks & Spencer and Tesco for taking steps to cut their carbon emissions. "However,goodpracticeincarbon reduction is most important as more organisations consider their impact on climate change," he said.

At the beginning of next month, the Sustainable Development Commission is due to publish a major assessment of theScottishExecutive'ssuccessin meetingitsenvironmentaltargets. While progress on waste recycling and renewable energy will be welcomed, the Executive is likely to face criticism over its transport policy.

An Executive spokesperson said: "The new Scottish government has made clear its intention to reduce greenhouse gas emissions by 80% by 2050 and to bring forward a climate change bill."