Finally, Declan looked at the BMW Hydrogen 7 - its only emission is water vapour but the technology for this type of green car is still being developed and currently, there are no models on sale.

BBC: NEWS | Programmes | Breakfast | Low carbon family: "Green" cars

Their only emission is water.

CNN: Bus showcases fuel cell technology

On Wednesday he announced his vision to see the world's first "Ultra Low Emission Zone", meaning by 2020 only zero or low-emission cars would be allowed into central London.

BBC: Has London's congestion charge worked?

Of course, nuclear power is not the only carbon emission-free option.

ECONOMIST: Nuclear power

He said the introduction of the Congestion Charge in 2003 and the Low Emission Zone (LEZ) in 2008 had only made a "tiny difference" to air quality in the capital.

BBC: News, London

Its cutting-edge technology ensures that the B-Class Electric Drive is not only sustainable (because it offers local emission-free driving) and agile (because of its high torque and acceleration), but also fully networked.

ENGADGET: Mercedes-Benz introduces B-Class Electric Drive, we go eyes-on

Andrews worries that the proposal will get watered down, as did the next phase of the low-emission zone to cut nitrogen dioxide emissions (it will only apply to city buses).

FORBES: London To Be An 'Ultra-Low Emission Zone' By 2020

Only electric vehicles are capable of meeting a "zero-emission" standard set by California, New York and Massachusetts.

CNN: Natural gas vehicles on a roll in U.S.

Newly programmed engine electronics coordinate the perfect interaction of all new components and ensures not only maximum power yield but also exemplary low emissions that meet the latest emission limits.

ENGADGET: Brabus iBusiness luxury sedan puts an iPad at every seat, a Magic Mouse at every hide-covered armrest