"Concrete and steel have their place. They're used widely and, in many cases, for good reason; they're cost-effective and so on, but they're extremely carbon intensive. There's an awful lot of energy that goes into their manufacture. In fact, both individually are responsible for about 8% of total CO2 emissions. Now, if they were each of them a country, they'd be the third largest emitter in the world behind the US and China."