Yesterday, the first of a new fleet of electric taxis rolled out onto Hong Kong's crowded streets.
Encouraged by the prospect of generous subsidies from the government's HK$300 million "green transport fund", the Hong Kong Taxi & Public Light Bus Association is leasing 45 battery-powered "e6" vehicles from Shenzhen-based manufacturer BYD.
In time, BYD hopes to replace at least a quarter of Hong Kong's 18,138 liquid-petroleum-gas-fuelled taxis with electric models.
According to the Reuters news service, BYD founder Wang Chuanfu said yesterday that if the city's entire taxi and bus fleets were replaced with electric vehicles, "it will reduce Hong Kong's emissions by 50 per cent".
You have to wonder what he's been smoking.
BYD boasts its e6 cars have zero emissions.
This is nonsense.
For one thing, it takes a lot of energy to manufacture battery-powered cars. BYD's e6 weighs 60 per cent more than a Toyota Crown, the model which makes up the vast majority of Hong Kong's existing taxi fleet. That means it takes a lot more energy to build.
And more energy means greater emissions.
According to a research paper published in last October's edition of the Journal of Industrial Ecology, manufacturing a battery-powered vehicle like the e6 pumps out twice as much greenhouse gas as building a conventional car.
And even when electric cars reach the road, they are not emission-free. Their batteries have to be recharged, which means plugging them into the mains power supply.
That of course means electric cars are only as green to run as the electricity that charges their batteries.
So if Hong Kong were to generate all its electricity with wind turbines or solar panels, then BYD could legitimately claim its taxis were green.
But Hong Kong's power mix is much dirtier. The government likes to claim the city generates 23 per cent of its electricity from natural gas, 23 per cent from nuclear power and 54 per cent from coal.
But if you scour through the Hong Kong Energy Statistics 2012 Annual Report published two weeks ago by the Census and Statistics Department, you get a different picture.
As the first chart shows, last year Hong Kong generated only 16 per cent of its electricity from relatively clean natural gas. The lion's share - 61 per cent - came from coal.
Working from vehicle emission figures compiled by WWF-Deutschland, we can estimate that a battery-powered car recharged from Hong Kong's electricity supply would be responsible for about 140 grams of carbon-dioxide emissions for every kilometre driven.
As the second chart shows, that is roughly the same as a petrol-driven car would pump out, and about 8 per cent more than a car with a diesel-hybrid engine.
When the additional emissions involved in manufacturing battery-powered vehicles are factored in, the case for electric cars looks even more shaky.
According to the Journal of Industrial Ecology paper, across their whole life-cycle, battery-powered cars recharged with electricity generated from natural gas emit the same amount of greenhouse gases as diesel-powered cars.
If the electricity is generated largely from coal, as in Hong Kong, battery-powered cars are responsible for significantly more emissions than conventionally powered vehicles like Hong Kong's existing LPG-fuelled taxis.
That doesn't mean electric vehicles are entirely pointless. They do shift emissions from the streets to power station smokestacks, which may have health benefits.
Just don't kid yourself that these taxis are green. In environmental terms, they are bright red.