The carbon devil in the detail on urban density

High-rise residential developments have been springing up in all Australia’s major cities. The view that carbon constraint, ecological protection and liveability can only be achieved by remodeling our cities at high-rise densities has taken root among much of Australia’s policy intelligentsia. This view is inconveniently flawed.

Most people agree climate change should be understood through robust scientific evidence. Such a standard should also apply to measures to reduce the climate impact of our cities.

Unfortunately a simple formula equating high-rise urbanism with low carbon or ecological impact finds at best partial confirmation in the scientific literature.

When different building scales are compared on objective environmental criteria the evidence suggests that high-rise apartments are often the worst performers. The building scale with least overall ecological impact – measured in energy, CO₂ and water use per capita – tends to be medium-rise of between three to six storeys, with individual detached dwellings the next best.

Alan Perkins and colleagues' work in Urban Policy and Research, for example shows that on per-capita analysis, attached low-rise dwellings perform best in terms of CO₂, with suburban and high-rise successively worse.

Such patterns arise because any valid assessment must account for all energy use. This includes energy “embodied” in a building during construction, plus energy used in ongoing operations.

While residents of detached houses use high levels of energy to get around, because of their greater car reliance, their overall energy use per capita tends to be lower than high-rise residents. This is because detached dwellings can consume less embodied and operational energy and that use is divided among a higher average occupancy rate (detached dwellings generally house more people than apartments).

The relationships between building type, urban form and CO₂ emissions become even more complex when income – or “lifestyle” – factors are included in the analysis. But it seems that high-rise urbanism exacts a high carbon cost.

The Australian carbon consumption atlas, prepared for the Australian Conservation Foundation by Chris Dey of the University of Sydney and colleagues, provides a striking illustration of this pattern.

Their work shows the highest per capita residential environmental consumption occurs in the higher density inner urban areas of Australian cities. The 37 tonnes of total CO₂ consumed per person each year by downtown Sydney residents is, for example, more than double the 16 tonnes produced by residents of Blacktown. There’s a carbon devil in the detail on density.

Such findings fundamentally confound the simple “high density good, low density bad” assumptions in current debates. High-rise apartments are far less of a solution to our urban environmental challenges than the prevailing consensus suggests. Even if better design could improve high-rise performance, so too could it improve that of other building types.

Some look to New York City as a model for Australian cities. But high-rise Manhattan is a small island set among a continuous interconnected urban mega-region of some 21 million inhabitants, only some of whom live at high densities.