The Independent Climate Change Commission has warned the government that it should cut all greenhouse emissions by 80 per cent by 2050 to tackle climate change.
In itself, this isn't particularly surprising; scientists have been recommending this for some time. More interesting - and very welcome - is that the commission wants to include aviation and shipping in the target. That means, for once, that 'all greenhouse gas emissions' pretty much means 'all greenhouse gas emissions'.
We've yet to see whether or not the government will accept the target, which would be the most ambitious legally binding commitment that any country's committed to until now. But Brown has mentioned the 80 per cent figure recently, and Ed Miliband - who heads the new energy and climate change department - has promised to "respond to the recommendations swiftly".
If the government accepts the commission's advice, the implications are enormous. To meet this target, the government will need to stop aviation growth and the new coal rush in the UK. If they allow either the third runway at Heathrow or the new coal plant at Kingsnorth to go ahead, they'll be making a mockery of the target before we've even started.
Here's the maths:
Under this target, Britain would be allowed to emit no more than 118 million tones of CO2 per year after 2050.
The Tyndall Centre has found that, if aviation keeps expanding at expected levels, emissions from aviation alone would wipe out our entire 'carbon budget' in 2050. The only way to meet this target is for the government to stop the growth of aviation - by capping flights at current levels and stopping airport expansion.
A new generation of coal-fired power stations without carbon capture and storage would emit around 56 million tonnes of CO2 per year, ie it would account for 48 per cent of our emissions quota in 2050. To meet the target the government needs to stop the new coal rush, beginning with the proposed new plant at Kingsnorth. That's why it's good news that decisions on energy policy are now being taken by the same department that deals with climate change.
The key to meeting this target lies with the energy sector - which needs to make a very quick transition to being low carbon. ("We have to almost totally decarbonise the power sector by 2030, well before 2050," said Lord Turner of Ecchinswell, the committee chairman.)
Ed Miliband needs to fix the UK's very broken renewables strategy. I explored this in depth on Friday but, to summarise, our energy system - from physical infrastructure to regulatory standards - is designed to deliver bulk quantities of energy or fuel across the country and is actively hostile to low-carbon technologies.
If we want a coherent, low carbon energy system, Miliband needs to end the government's love affair with nuclear and coal and take a bolder, more sophisticated approach.
This approach should include giving low carbon projects priority access to the grid (at the moment, fully working wind farms are sometimes turned off because a fossil fuel plant has priority access to the grid). It should include reassessing the role of Ofgem, which often condemns new technologies to failure. And it should include concrete, legally binding measures to reduce demand / increase efficiency - not just voluntary schemes for which no one volunteers.
Over two thousand of you have emailed Ed Miliband to congratulate him on his new post and ask him to take the steps needed. And he seems to be listening. Since you started emailing him, he's changed his email auto-reply to include the pledge that "We will do all we can to... put Britain at the forefront of creating green jobs and play our part in ensuring every country meets the climate change challenge."
Time will tell if he delivers on this. And not much time, as it happens; within months the government is due to make decisions on the proposed third runway at Heathrow and the new coal plant at Kingsnorth. Giving the go ahead to either will mean our chances of reducing emissions by 80 per cent by 2050 just got a whole lot harder.
