D

A shopkeeper's son breaks a window, causing a crowd to gather. They tell the shopkeeper not to be angry: actually, the broken window is a reason to celebrate, since it will create work for the glazier (装玻璃的工人). In the story, written by a 19th-century economist, the crowd envisions the work involved in repairing the window, but not that involved in everything else on which the shopkeeper could have spent his money - unseen possibilities that would have brought him greater happiness.

If that window were to be broken these days, people might have a different reaction, especially if they were NIMBYs (Not In My Back Yard) who oppose any local construction that affects their quality of life. Their concern might be with the "embodied carbon". The production of a piece of glass would carry a sizeable carbon cost. Similarly, the bricks and concrete in a building are relics of past emissions. They are, the logic goes, embodied carbon.

Conserving what already exists, rather than adding to the building stock, will avoid increasing these embodied emissions - or so NIMBYs often suggest. At its worst, this idea is based on a warped logic. Greenhouse gases released by the construction of an existing building will heat the planet whether the building is repaired or knocked down. The emissions have been taken out of the world's "carbon budget", so treating them as anew debit means double counting. The right question to ask is whether it is worth using the remaining carbon budget to repair a building or it is better to knock it down.

Choosing between these possibilities requires thinking about the unseen. It used to be said that construction emitted two types of emissions. Besides the embodied sort, there were operational ones from cooling, heating and providing electricity to residents. Around the world, buildings account for 39% of annual emissions, according to the World Green Building Council, of which 28% come from operational carbon.

These two types of emissions might be enough for the architects designing an individual building. But when it comes to broader questions, economists ought also to consider how the placement of buildings affects the manner in which people work, shop and travel. Density (密度) lowers the per-person cost of public transport, and this reduces car use. Research by Green Alliance, a pressure group, suggests that in Britain a policy of "demolish (拆除) and densify" - replacing semi-detached housing near public transport with blocks of flats - would save substantial emissions. Without such demolition, potential residents would typically have to move to the suburbs instead, saving money on rent but consuming more energy.

Targeted subsidies (补贴), especially for research and development into construction materials, could speed up the pace at which the built environment decarbonises. What will never work, however, is allowing the loudest voices to decide how to use land and ignoring the carbon emissions of their would-be neighbours once they are out of sight.