Does this measure people working on a day to day basis in the area, or people living in the area who are working outside the area?

Somewhere between the two figures (where people live and where people work) is the important commuter information.

The range of land uses within the borough is put is relevant to any sensible interpretation of the density figures. This may be included in a measure of urban density.

Perhaps the overall density figure used is the weighted density?
[ http://en.wikipedia.org/wiki/Urban_density ]

Yes: an overall density figure for a city is pretty useless and it is hard to know what geographical unit should be used. if cities were made of ‘housing estates’ then this would be relevant but local government areas do not seem useful. I guess people started calculating densities as an argument for ‘slum clearance’.

Equally carbon per person useage would not be equal either, the figures are only an average.

Monitoring CO2 levels from satellite is a great idea, providing the figures are accurate. Since power stations tend to be outside cities it would not however provide a full picture. Well done Paris for collecting its own figures.
It would be particularly interesting (as a pointer to the future) to have carbon (and density) figures for Gurgoan – my guess being that the carbon emission is high and the density average.
As for how to reduce energy consumption in cities, I think it is fairly easy – making it all the more remarkable that the policies are not pursued with more vigour:
1) buildings must he very well insulated, both for hot and cold climates
2) urban transport should rest on a full integration of public transport with cycling – with cycle storage at train/bus stops. I like trains but a Curtiba-style bus system is much cheaper and much more flexible. I also remember that they started building cycle tracks in Delhi and they were not a success (I think because all types of road user used them).
3) building envelopes should not be passive: they should make a contribution to energy generation or they should be vegetated (using grey water for the purpose in hot dry climates).
If politicians do not follow these policies then I do not think they are serious about making cities more sustainable!

And it turns out Paris is measuring its emissions (the only European city to do so, according to the Earth magazine article); see here: http://co2-megaparis.lsce.ipsl.fr/index.html

The map of Paris you found is interesting, if the figures are right, as it suggests that for all its low-rise buildings the city itself is still denser than much of its higher-rise outskirts. Where most carbon is emitted is another question. The out-going mayor has done much of course to try to reduce the city’s carbon output, but I don’t know how far he has measured shifts in emissions or compared Paris with other cities.

Like you I could not find anything comparable for Delhi but this one: http://spie.org/Images/Graphics/Newsroom/Imported/0910/0910_fig1.jpg at least shows the scale of development over a 12 year period – and a 2013 version would be dramatically different again. I am not sure I fully understand the legend, but think it confirms your view that New Delhi is not very dense, whereas East, West and North Delhi are. It would be fascinating to see if the carbon output here bears much relation to the density…

I share your scepticism about density figures and wonder if they tell us anything worth knowing. Here is a density map of Paris http://www.thetransportpolitic.com/wp-content/uploads/2010/09/Paris-Region-Grand-Huit-Density.jpg I have not found a comparable map of Delhi – but New Delhi must have a pretty low density. Nor do I think density is of much use as a criterion for assessing cities. If Glaeser is interested in carbon usage/hectare then this is what he needs figures for. Then he needs to find out what they correlate with. I will be surprised if density provides the best correlation. I am in full agreement that (1) carbon output differs between cities (2) it should be reduced.

I don’t know the Glaeser book and am surprised that it seems to be arguing Paris is a good model of development, with its low-rise centre ringed by seemingly uncontrolled and often desperately inadequate high-rise blocks, a ‘badlands’ of unemployment, crime and hopelessness. It may be low carbon, but it is arguably a worse place to live than the vibrant and much-loved area around Chandni Chowk…

Debate is FUN.