High density good or bad?

In comments a couple of posts back, I noted how the writer of the Slate article about environmentalism claimed that the "greenest" way of living was in a high density city like New York; not by being in a city with lots of suburbs.

But today, in the Sydney Morning Herald, someone from an organisation "Save our Suburbs" argues with figures that high density development in Sydney would make more CO2, not less. Here's some of his arguments:

The Australian Conservation Foundation's consumption atlas shows people living in high-density areas have greater greenhouse gas emissions than those living in low-density areas. A study by EnergyAustralia and the NSW Department of Planning shows the energy used by a resident in high-rise is nearly twice that for a resident in a detached house. Think of all the lifts, clothes dryers, air-conditioners and lights in garages and foyers. ...

Research in Melbourne shows people squeezed into newly converted dense areas did not use public transport to any greater extent and there was little or no change in their percentage of car use.

There is not enough difference in the emissions of public versus private transport to counter the increased emissions of high-density living. For each kilometre CityRail carries a passenger, it emits 105 grams of greenhouse gases, while the average car emits 155, and modern fuel-efficient cars such as the Toyota Prius emit just 70.

I am a little suspicious of the slant being put on some figures here. In the second paragraph, for example, he talks of little change in "percentage" of car use, but is that taking into account the much shorter distances that may be being driven when you live close to the city, even if you still use your car to get to work?

Similarly, given that air-conditioning is so popular now, I would expect that one saving in energy use would be that small apartments don't take much energy to heat or cool, and are insulated by the other apartments around them.

On the other hand, suburban gardens and plants must be given some credit for absorbing CO2, I guess.

Maybe (just guessing here) to get full credit as a low CO2 emitting city, you have to reach a threshold level of density where a very large proportion of residents almost never have to rely on a car, such as in New York or Tokyo.

I'm sure someone's looked at this, but I don't have time to find the answer now.