Any car you like, as long as it's green

As posturing politicians and embattled environmentalists meet in Kyoto to help save the planet, it's a good time to look at how we humble drivers can contribute to a greener, more cheerful, world.

Let's get straight to the point: what's the greenest car on sale in Britain? The most fuel-efficient car, according to EU fuel economy figures, is the Seat Ibiza 1.9 TDi, at 58.9 mpg.

But isn't that a diesel, and aren't diesels dirtier than petrol cars? They use less fuel, so create less carbon dioxide, the primary issue discussed in Kyoto. But they pump out more gases linked to respiratory illnesses. Mind you, modern car diesels are miles better than buses and taxis. Nowadays, they're pretty clean.

Fine, but I need something bigger than an Ibiza. And I don't want a car named after a chair. What's the most fuel-efficient car I can buy that can comfortably seat four people and carry some luggage? A Volkswagen Passat TDi (53.2 mpg). The turbodiesel Passat is not just the most fuel- frugal big family car, it's also one of the best. Nowadays, you don't lose any style or driving pleasure by buying economical cars.

I'm still not sure about a diesel though. The stuff is so smelly, and the only time I filled up a diesel car it stained my loafers. Then you're after petrol power. If you want to average over 40 mpg, there's a big list of good, small, petrol cars. You could try the Fiat Cinquecento Sporting (45.3 mpg) - which is a real hoot to drive. The Ford Ka (47.9 mpg) is also fun to drive and to look at, although the back seat is for kids only. And the Fiesta 1.25 (42.2 mpg) is a brilliant little machine, although it has a face that looks more like a carp than a car. If you need more room, the Fiat Punto 60 seats four easily and averages 41.5 mpg.

I've already told you: I need more space. Then buy one of the new Mitsubishi Carisma GDI models (45.6 mpg), which uses a newfangled direct-injection petrol engine, one of the new "Big Ideas" which should see petrol power soldier on for another couple of decades. It combines diesel economy and petrol performance. In a few years, most car makers will offer direct- injection petrol engines. It's the future, short- to medium-term.

And long-term? After all, surely petrol cars will always be dirty? Not necessarily. You'd be amazed how clean a modern car's exhaust is -- although it still pumps out carbon dioxide, the greenhouse gas. But in terms of exhaust toxins, a modern car with catalytic converters and electronic engine management is about 20 times less polluting than a car from the late Seventies. Long term, petrol power will disappear, though. Some electric "hybrid" cars which use both petrol and electric power are about to go on sale in Japan, one from Toyota and one from Honda. They're greener than conventional petrol cars, although they're pricier.

Longer term, we'll almost certainly see hydrogen fuel cells. Most car makers agree it's the answer. Mercedes is probably at the cutting edge, although Bill Clinton recently hinted that the Yanks are at the forefront, as a way of mitigating their appalling environmental record. Mercedes reckons it will have a fuel-cell version of the A-class on sale in just over a decade.

In quick succession, other car makers will probably offer fuel-cell cars too. Potentially, these cars pump out no tailpipe pollution at all. The Kyoto delegates would love them.

How much do cars contribute to global warming, anyway? About 10 per cent of total carbon dioxide output, according to the OECD. But cars are the fasting growing source, owing to their explosion in the Third World. Buses and trains may be greener. But, as in the West, people would much rather use cars.