But zoom out to city level and it’s a different story. The United Nations predicts that the world’s urban population will rise from 3.6 billion to 6.3 billion between now and 2050. To accommodate these new residents, the cities of the future will have to be much denser – and that means building higher, says David Cooper, chief executive of WSP in the USA. “High-rise buildings enable many people to live, work, spend their leisure time or access public services in a relatively small area and that drives more efficient mass transportation. From an urban planning point of view, towers are very sustainable.”
High-rise developments that combine offices, homes and other uses enable more efficient use of resources, he adds. “Residential buildings typically have different usage patterns to offices, which presents opportunities to share energy and equipment. Technologies such as cogeneration become much more efficient when, for example, waste heat from office cooling can be used for hot water for apartments. We need to start creating vertical communities instead of sprawling horizontal ones.”
Low-density development also consumes greater energy in less obvious ways. Compare New York and Los Angeles, says Honnorat. “In Manhattan, most people don’t even own cars, whereas LA has lots of low-rise, low-energy buildings that can only be reached by car, and require extensive energy and water infrastructure. On a building-by-building basis, it’s a no-brainer that towers use more energy, but when you look at the bigger sustainability agenda it’s far less clear.”
Energy is just one aspect of sustainability, and one which will become less significant as technology improves. Honnorat argues that land is a much scarcer resource and one that is genuinely finite, subject to a range of competing demands from a growing population that needs housing, food and access to green space. Meanwhile, humans are surrounded by free solar, wind and tidal power – if only we could harness enough of it. “Consider a future where instead of being 5% or 10% efficient, photovoltaic cells are 30% efficient,” he says. “All of a sudden, the solar-capturing ability of a high-rise building comes into its own. If we manage to make that quantum leap in technology, it’s a very different equation."