Well, OK probably the weather - but trains run it a close second.
This past week something quite significant happened: the government scrapped plans to electrify the tracks in areas across Wales, the West Midlands and further up the West Coast. Cue outrage from the greenies.
Why significant? Because electricification is the only known strategy for getting anywhere near the UK's decarbonisation "legally binding targets" of 57% by 2030, 80% by 2050. Electrification of transport and (even more dramatically), of space-heating, see below. Most eyes - and subsidies - have been on decarbonisation of power generation thus far, because it's the easy target. But transport (= oil) is about a third of our energy consumption and space-heating (= gas) another third. So, the theory goes, when you've figured out how to de-carb power generation, you shift as much as possible of transport and heating onto the leccy. Simples.
Accepting that 2050 is a long time in the future, you'd still expect government to be plodding methodically in that direction if it had these targets as a serious policy priority. Around these parts we all know, don't we, that GDP trumps GHG - so we're less surprised. But hard evidence of this important political principle is always interesting to come by.
Notwithstanding what a great topic trains is (cue our good friend Kev ...), space-heating is a really juicy one for fans of irrational policy-making. Above is a graphic from the really quite diligent Energy Technologies Institute which crunches a lot of numbers on these matters. The dark bands at the bottom is energy capacity used in the form of electricity in this country through a particular year (it's 2010, for reference). A bit more in winter months than in summer. And proving very difficult to decarbonise.
Now look at the red plot - that's energy capacity required for space-heating, largely gas-fuelled. What kind of expansion in the grid will we require to switch from gas to electricity? How will it cope with those winter peaks? How much will need to be spent on replacing 20 million-odd old boilers? Paid for by whom? etc etc etc.
In the immortal words of ISIRTA: "I'm sorry, it can't be done!" And I think the government knows it.