Re: Won't someone think of the children . . . (no, really)
I've thought this for the long time.
The real "Inconvenient Truth" is not that there might be anthropogenic global warming, but that there ARE, RIGHT NOW, real, quantifiable health issues with fossils.
Aside from the economic idiocy of signing over our energy security to Saudi oil and Russian gas, there are clear and proven health risks associated with a whole raft of avoidable modern industrial processes.
Why is California so hot on emissions? Well because their topology traps pollution over the city, same for Mexico City. New Delhi is the first place I've seen a sunset where the sun didn't actually go below the horizon - just got closer and closer to the ground until the smog blocked it out.
But that crap is still floating around you whether you're in London or Delhi. It's just at a low enough density that you're not constantly aware of it. Funnily enough, we don't use coal train on the Underground any more, because it's obviously bad. Older buses are little better, but it's outside, and we're not forced to address the poison pouring out the back.
A lot of that comes down to particulate matter (certainly when you're discussing asthma and endemic incidence of respiratory complaints, etc), but gases play their part as well.
TfL runs a fleet of 8000 buses, mostly diesel. Converting them to electric or hydrogen would have a direct and measurable impact on air quality in London, especially if the London Taxi Company joined in as well (there's 21,000 black cabs running round London, of which 5 are prototypical fuel cell models).
Yes, okay, the electric comes from fossils, but from an industrial power station boasting far better PM filtering, de-sulpherization, etc than can be squeezed into a car's exhaust system (even assuming the car is maintained properly), and when (if) the politicos pull their finger out and build some nukes, then the infrastructure will accept their power just as readily as it accepted coal-derived electric.
No doubt someone will point out there are 2.5million cars in London, and sorting 30,000 vehicles is a drop in the ocean, but of course most cars are driven to/from work and are off for >20hours of of the day/night. Bus and taxi fleets run almost 24/7, with vehicles handed from one driver to the next to maximise utilisation and ROI. Cutting their emissions has a far greater impact than those of commuter cars.
Taxis are probably better suited to Hydrogen unless they're depot based. Buses could use either - being based at depots means a sane design would allow the batteries to be unloaded off a roll-out sled in 5 minutes and replaced with a charged unit allowing turn-arounds no slower than diesel units.
Tesco understood this, recognising that Modecs, even with their limited range, were perfectly suited to in-town deliveries, where you spend half your time idling at traffic lights anyway, only cover a few miles a day (and they don't want to deliver at night because customers are asleep, so you can charge overnight).
Forget global warming, and think about local level air quality and environmental pollution.
It's funny that Al Gore completely missed the point with "An Inconvenient Truth" - most of what he advocates is absolutely necessary, but he could have justified it all with proven case studies and verifiable fact, rather than campaigning on the back of a contentious and debated phenomena.
I've used London there because TfL numbers were easy to get. Scale that out to a location that suffers major smog and pollution issues and the benefits become clear.