How should a fair-minded environmentalist make a decision about HS2? How, indeed, should such a person, or even a group of people claiming to be "the greenest government ever", make decisions about huge infrastructure projects for which great environmental benefits are claimed?
Last week, the route of the second phase of the proposed high-speed railway from London to the north was announced, and today The Independent on Sunday publishes an investigation of its effect on nature reserves, wetlands and irreplaceable ancient woodlands. How can the fate of the purple hairstreak butterfly and the Bechstein bat, and their habitats, be weighed against the marginal mitigation of climate change? And how then can the supposed gains for the economy, and in particular for closing the north-south divide, be measured in the equation?
We have been here before. This newspaper supported the Severn barrage scheme before the election, but we accepted reluctantly that the electricity it would generate would be much more expensive than the main green alternatives. We accepted that, if Chris Huhne, the former energy secretary who abandoned the scheme in October 2010, could not make it work, no one else was likely to.
Similar arguments apply to our conditional support for renewing nuclear power stations. If the operators can make them pay without subsidy, let them go ahead. Yet the timetable is forever slipping, and the big difference in green energy is being made by wind turbines.
The HS2 plan runs on sentiment. High-speed rail is the sort of shiny project that any go-ahead government ought to be proposing. Cars and planes bad; trains good, and green. The domination of the economy by London and the south-east is The Problem and high-speed trains are The Solution.
There are far too many assumptions behind the project that need to be challenged. Even if the high-speed line were extended to Scotland this century, would it really suppress demand for domestic air travel? It would make no difference to motorway traffic volumes; it might simply allow a little extra traffic to be carried by train once the roads are saturated. That might add a tick or two to the Gross National Product – but it would make no difference to climate change. Is that worth the huge cost? We are sceptical. Action on climate change, as the IoS has consistently argued, means putting a price on carbon and it needs to be global.
What, then, of the argument that HS2 would "rebalance" the British economy away from London and the south-east? There is one problem with the case for cutting journey times from Scotland and northern England to the continent: the route passes through London. Look at the HS2 route, and you see that it simply adds a spoke to the existing rail network – starting from London. Although HS2 might extend the London commuter zone and business penumbra along its route, it would increase its gravitational pull on the rest of the economy.
Yes, parts of the existing rail network are running close to capacity. But the truly green and balanced policy might be to increase its capacity, while building fast east-west rail connections across the north of England and central Scotland, and to invest in green technology to cut carbon use by road traffic.
As soon as the green credentials for HS2 are examined, they are found wanting. Nor has the case yet been made to justify the loss of 24 Sites of Special Scientific Interest.