So far, just about everyone seems to love the self-styled "world's cheapest car", the Tata Nano.

Writing on these pages, Indian motoring journalist Hormazd Sorabjee writes that "It thrilled me with its 'proper car' feel"; while for Adil Jal Darukhanawala of zigwheels.com, "The Nano has the makings of a mega winner."

And what's not to love? A five-seater car that does about 20 km per litre (that's 56 MPG in old money) and costs $2,000 - come on! - and it's not the end of the line, with Bajaj, the company that principally populates South and Southeast Asia's roads with auto-rickshaws, planning to launch its own tiny car (the Pico?) within two years.

Just about the only people sounding a cautionary note on the tiny Nano's giant appeal are environmental groups, notably the Delhi-based Centre for Science and Environment (CSE).

They judge it inappropriate for Indian cities, choked by traffic, where jams mean a journey across town can already be measured in hours.

"Cars may drive growth and aspirations, but they can never meet the commuting needs of urban India. Cars choke cities, harm public health and guzzle more oil."

CSE's simple prescription is more investment in mass transit schemes.

Although one can see the logic of their argument, it's hard to imagine it prevailing.

Many Indian cities already have swarming bus networks and suburban rail networks. They're slowly being supplemented by true mass transit rail systems - up and running in Calcutta and Delhi, under construction in Mumbai and Bangalore.

That's the good news from the CSE's point of view. Here's some of the bad:

Calcutta's system contains just one line, Delhi's three

during the 30-odd years since a legal framework for the Delhi metro was established, the number of vehicles in the city has risen 10-fold

according to the Delhi metro company, only about 2% of journeys in the city use rail

Delhi, and India, are hardly unique. Bangkok has acquired three mass transit lines within the last decade, which claim collectively to carry 400,000 people per day.

Sounds impressive; but it's less than the population increase in the city since they were built.

I've found these trains a pleasant ride on my all too infrequent visits, by the way. Travel is quite fast; and Delhi's escalators even incorporate special devices to prevent indelicate snagging and ripping of decorative rush-hour saris.

You'd ride them by choice, I think, if you could; but even as their riderships expand, the number of people riding the roads swells further and faster.

Against this backdrop, the CSE's plea for society to put the Nano aside and rely on greater investment in public transport looks more than a little forlorn.

Much of modern Asia is mimicking in a few decades the development that Europe went through in many. When you can upgrade from a bicycle to a moped, you do; and with the advent of cars priced as cheaply as the Nano, the next upgrade - from two wheels to four - is likely to become just as routine.

It's here, in my view, that we find the environmental significance of the Nano.

Regularly these days we hear appeals from politicians, climate scientists and environmental campaigners for societies to curb their greenhouse gas emissions. It's urgent, they say, and - this is the key message - it can be done, if everyone and every country does his/her/its bit.

In the context of a climate change set-piece such as the Copenhagen conference that took place a couple of weeks ago, such words acquire their own logical underpinnings.

When you later emerge blinking into the street, taste the hydrocarbon-laden air and perhaps even hail a taxi yourself, you wonder how firm those underpinnings really are; whether it really can be done.

Perhaps the lexicon of personal transport archetypes needs to acquire a new entry.

Rather than "the man on the Clapham omnibus", a previous generation's fictional commonsense arbiter, or "Mondeo man", the 30-something would-be-upwardly-mobile denizen of an anonymous dormitory town, we now need to introduce "Nano-man" - the Indian (or perhaps not just Indian) patriarch who now finds he can afford to transport his family by car rather than perched on the overcrowded seat of a moped, and - of course - why would you not? - chooses to do so.

(Apologies for the UK-centric examples there - I hope you can fill in your own regional equivalents.)

Western climate campaigners may worry about Nano-man, but if they reach for a stick to beat him with, a pepper-spray of "inequity" will be their desserts.

Car ownership in India, though growing fast, still amounts to only about 10 per 1,000 people - in the West, it's typically 50 times that. Neither western campaigners nor western politicians can make a cogent reason from that as to why Delhi's denizens must remain Nano-free.

And they will find little comfort behind the Nano's chief green claim. At 101 grams per kilometre (g/km), its carbon dioxide emissions are only a fraction down on the most frugal versions of existing small cars such as the Renault Clio (117g/km) or Nissan Micra (120g/km), even though their engines are significantly larger.

I suspect that what Tata sees as an affordable car, history will judge an icon of "small-but-clever-is beautiful" design. I expect that if I take my grandchildren to a motoring museum in a couple of decades' time, the Nano will stand in a line-up of revolutionary small cars alongside the Mini, the Beetle, the Model-T Ford and - er - the Trabant.

Tata's Nano is rumoured to be a great ride for the price. But it should scare the hell out of anyone assuming that the world has an easy trip to a low-carbon future.

