The idea that living in some semblance of harmony with nature in a leafy suburb is a greener way to exist than cramped in a tenement downtown is under threat. Indeed, a recent study suggests that, when the relative carbon footprints of residents are compared, the opposite is true.
Harvard University professor Edward Glaeser and partner Mathew Kahn of UCLA claim that folk living in dense city-scapes emit about 7 tonnes less CO2 per household per year than suburbanites. Primarily, they say, this is because everything you need is a handy non-driven distance in the centre of a city, and apartment buildings that share walls and floors need less energy for heating or cooling.
Which at first glance makes sense: compacting human built environments makes them simpler to service, and saves valuable land from sprawl. But look a little deeper, and you'd have to say the jury is still out on which lifestyle trumps which in the carbon stakes - or even whether it matters.
Certainly Glaeser and Kahn - professors of economics, by the way, not environmental scientists as such - rather spoil the aim of their report by bad-mouthing environmentalists and using what is a limited data-set to extrapolate very broad and sweeping conclusions. This alone jaundices appreciation of their argument.
But what's important in trying to get a handle on whether the argument is valid are not the factors they've included in reaching their conclusions, but the ones they've left out.
Glaeser and Kahn base their premise on personal transport (including public transport) and energy use. On this basis, they refer to Manhattan as "one of the greenest places in America", and claim New York has the widest gap in relative carbon footprint between the central city and its suburbs, equivalent to 4462 lbs of CO2 per capita in transport-related emissions alone.
The data sets (like this one) and the methodology utilised to interpret them appear perfectly reasonable, but there are a range of factors which remain unexamined.
They do not attempt to quantify the relative (carbon) costs of infrastructure in or to any given location, including for example high-voltage transmission lines (and leakage therefrom), the extent of major roads/motorways and their efficiency, and rubbish water and sewage systems (especially in terms of distance to and from source); or the cost of transport for goods and services, or the cost of goods so transported.
It should also be noted that private public transport - taxis and hire cars/buses, for example - are not included in the data sets; bet that has an impact in a city like New York!
Nor do they adequately attempt to evaluate the effects of the cost of housing in generally higher-priced inner cities compared to outlying suburbs in terms of a dollar's inherent carbon impact. Nor, for that matter, the impacts of telecommunications and computer usage, now recognised as a major emissions source and undoubtedly far more intense in city centres.
But perhaps the primary flaw is that the study ignores completely the ecological impacts: the difference between a tree-lined garden and a paved lot as your backyard. Or having a carbon-sink forest reserve or park down the road, or a local farmers market, or being able to grow your own vegetables for that matter.
In other words, that suburban living inherently contains a number of offsets that, overall, must substantially modify the carbon emissions a suburban family produces - and which inner cities in essence, per capita, completely lack.
For now, many of these impacts have been poorly studied, if at all, and certainly my own search of the net revealed a lack of any robust data with which to forcefully rebut Glaeser and Kahn's conclusions. But the fact is these and other impacts which have not been included in this study make Glaeser's sweeping statements seem just a tad premature.
There are a couple of additional aspects of the study which provide food for thought. One, which seems rather stating the obvious, is that cities in moderate temperate climates emit far less carbon than cities in more extreme climes which need extensive heating or cooling. This is why - using their narrow criteria - five Californian coastal cities ranked best of all the 66 metropolitan areas examined.
Of most interest though, and worthy of further scrutiny by politicians and planners alike, is the correlation drawn between "least emitting" urban areas and relative restrictions on construction/planning. The Glaeser/Kahn research claims to demonstrate a clear relationship between central cities where growth is curtailed on environmental grounds, even though those cities, in this study, rank best as low-emitters.
Glaeser's comments on this aspect are disparaging. "When environmentalists resist new construction in their dense but environmentally friendly cities," he writes, "they inadvertently ensure that it will take place somewhere else - somewhere with higher carbon emissions.
"So California environmentalists have things exactly backward. If climate change is our major environmental challenge, the state should actively encourage new construction, rather than push it toward other areas," he concludes.
He may well be right. But until there is much more substantive and comprehensive analysis, I'd file this under "one to watch".