It was used to build the Great Wall of China and Spain’s mediaeval Alhambra Palace — and now earth is back in vogue as a building material. Climate change has spurred renewed interest in the ancient technique which sees polluting concrete swapped where possible for earth. For centuries, mud and clay were an abundantly available way to put a roof over one’s head, but earth’s environmental credentials are behind its modern-day resurgence. “A kilo of cement emits a kilo of CO2. Whereas a kilo of earth emits none,” Xavier Chateau of the Navier Laboratory at the French National Centre for Scientific Research said. “If we could reduce by 25 percent the volume of cement consumed globally it would be equivalent to negating the impact on the climate of all air transport,” he estimated. Known as rammed earth construction, the practice dates at least as far back as the Neolithic era. It involves compacting certain soils into a mold, of sorts, to make building blocks or build up whole walls, layer by layer. More than two billion people across some 150 countries live in buildings made of earth, according to a 2006 guide on earth building by French authors Hugo Houben and Hubert Guillaud. Advocates say it can help reduce reliance on concrete, which accounts for about eight percent of global CO2 emissions. Earth also has a high thermal capacity by self-regulating its humidity, is fireproof, non-toxic and can be completely recycled. But it has downsides too, not least the cost, given the need to find builders qualified in ancient techniques.