British Chancellor of the Exchequer, Alistair Darling has been trumpeting a “34% cut in UK emissions by 2020″ in his budget speech:

Today, I am presenting the world’s first ever carbon budget, which commits Britain to cut carbon emission by 34 per cent by 2020.

These budgets give industry the certainty needed to developed and use low carbon technology – cutting emissions, creating new businesses and jobs.

They are a landmark step, which point the way to the vital decisions which must be made at the Copenhagen Climate Change Summit later this year.

But this cut is not quite what it seems. The baseline year isn’t 2009 or 2010, but 1990. UK emissions had already fallen 19.5% below 1990 levels in 2008 and will be falling fast in 2009 due to the recession. So the cut over 2010-2020 will be considerably less than 15%

If there’s a deal at Copenhagen, the UK will almost certainly have to do more. According to the background briefing, the UK aims to help the EU achieve a 20% cut by 2020 (again, against a 1990 benchmark). But Europe has said it will accept a 30% cut if others reciprocate – so the UK’s rather unambitious carbon budget may soon need to be tightened…

Update: The media seem quite happy to propogate the figure without explaining that it doesn’t mean what it seems to mean.

BBC:  Britain commits to cut carbon emissions by 34% by 2020

Guardian: Carbon budget commits UK to reduce emissions by 35% by 2020.

Telegraph: Chancellor presenting the world’s first ever carbon budget, committing Britain to cut carbon emissions by 34pc by 2020.

Times: Commits to cutting carbon emissions by 34% by 2020

Independent: Chancellor, presenting the world’s first carbon budget, committed Britain to cut carbon emissions by 34 per cent by 2020.