The social cost of carbon emissions is $85 per tonne. This equates to £72 per person flying to New York or 9p per litre of petrol. This doesn't seem an awful lot. If the purpose of green taxes is to correct externalities, it would seem that petrol tax has overshot quite badly. Yes, I know it covers other things, and that there are non-carbon externalities, but nonetheless it suggests that perhaps motoring has acquired a totemic status among carbon-emitting behaviours. If we have any economists reading perhaps they can explain whether a social cost of $85 means a tax of $85 is correct, or whether and why the two figures should differ.

The other good headline figures to come out of the report are that at a cost of 1% of GDP (which is a huge cost), we can save 20% of GDP in economic decline and equivalent (somehow) loss of health and well being. This is a great expense and a great bargain.

The Lib Dems Green Tax Switch policy, which is perhaps in the process of being copied by the other parties meets the first of Stern's policy challenges: that of setting a price for carbon.

The second challenge is to support the urgent development of low-carbon and high efficiency technologies. I suspect that when the government publishes its proposals, this section will focus on nuclear power. That is, of course, a debate all of its own. But in any case there should be serious money going into offshore wind, tidal lagoons, CHP, transport solutions and so forth.

The third challenge is tackling the barriers to take up of efficient and low carbon technologies. Better product labelling, standards for energy efficiency and that sort of thing.

Stern also mentions the importance of adapting to the consequences of global warming. This is important. It is not a question of stopping warming altogether, but stopping it from being too severe, and adapting to the changes that do happen.

The point is well made that uncertainty over the figures is a reason for more action rather than less. We should act today on the best estimates we have today, and we can always move policy incrementally in response to new evidence.

I think the biggest obstacle we face to progress is the problem of freeloading, and the perception of the problem of freeloading. I will say two things against the problem of the perception.

Firstly, the costs of many of the measures discussed are very low or zero. Green taxes raise revenue and allow low carbon sectors to be taxed less and prosper. Having the carbon pricing infrastructure in place earlier and implementing it, even at a lower than desirable level can mean gaining a lead in low carbon technologies and practises that will pay dividends in future.

Secondly, it is a mistake to think there is a binary divide between tree-huggers and freeloaders. There is a continuum. China, India, the USA, they are all doing more than nothing. And the UK is doing rather less than some other countries. Yes, we want to say to the international community "come on chaps, we can do this" but to say it effectively we have to show some enthusiasm to do our part. Let's not sit on our arses complaining that somebody else is even lazier.