The construction of tall buildings causes large emissions of carbon dioxide. This is called embodied carbon. In “Tipping the scales”, Roxane McMeeken reports

“A project’s embodied carbon also depends on the type of building it is, adds Sean Lockie … ‘The higher you build, the more steel and concrete you need, and the more foundations you have,’.

Based on calculations that exclude maintenance and regulated and unregulated loads, Lockie says that high-rise buildings produce an average of 1,300kg of CO2e/m2 , while residential buildings come in at 500kg CO2e/m2.”

If tall buildings cannot be built, because of embodied carbon, can human settlements be dense enough to be called cities? The Government now stipulates a target range for new building densities up to 50 dwellings per hectare. The average household size in the UK is 2.4 people so that is 120 people per hectare.

Portsmouth is nowhere near a green city yet but it is one of the densest cities in the UK at 52 people per hectare. It still has considerable open space: Milton Common, Port Solent and Alexander Park & etc. Most of Portsmouth’s dwellings are two story houses. Portsmouth is a dense but low rise city.

(For those that are worried about “this crowded island” the rest of the South East Region has a density of 4.5 people per hectare and to the north of Portsmouth in the Upper Weald there are extensive areas of farm land hundreds of times the area of Portsmouth.)

Almost by definition, a green city has few cars and less space is required for roads. It also has few tall buildings unless embodied carbon is discounted. Portsmouth is not my model city (See The Green Settlement Handbook) but it shows that city densities can be attained without tall buildings.

Over to you Architects. Stop designing tall buildings until you can cut their embodied carbon and stop pretending you are designing the green city of the future.