A common refrain from supporters of HS2 is the amount of traffic it will remove from motorways and the ensuing reduction in carbon emissions. Reducing the amount of emissions from transport is a worthy ambition, but the idea that HS2 is an effective or efficient contribution to fighting climate change is very hard to sustain. There is one key reason for this, the emissions from HS2 are largely frontloaded to its construction. This is a massive problem for attempts to make an environmental case for HS2, and I will show you why.