Today, more than half of the world's population lives in cities with 3 million people moving there every week. To absorb these new arrivals, the world is witnessing the emergence of new real estate infrastructures as large as New York every 34 days! No wonder then that buildings and housing are responsible for more than 70% of global greenhouse gas emissions, a figure that is constantly increasing. At first glance, this figure is frightening. But if you think twice, the concentration of these nuisances in well-defined areas is a great opportunity to act more effectively: if all the problems are concentrated there, so are solutions!
The first question is to seek to build in a more ecological way. Concrete is the most used resource in the world after water. We produce 30-35 billion tonnes of concrete per year, which represents 8-10% of global CO2 emissions, a huge amount for just one material. But solutions do exist: most of the CO2 emissions during the production of concrete come from the decomposition of limestone, but this chemical reaction can be optimized. Cement manufacturers are now able to reduce their emissions by 50% and are moving towards a cement that captures more CO2 than it emits. There are also techniques to use 20% less concrete by using a mesh system that makes them just as stable.
Beyond optimization, alternatives emerge. Wood chips treated as lightweight aggregate begin to replace sand and stone in concrete; building materials are made from wood; cement incorporates demolition waste previously treated to replace the raw material.
These are the avenues to be exploited in regions with strong demographic growth, to design carbon neutral projects from their inception, while reducing the energy bill of residents. The initial investment is roughly 10% higher than badly insulated buildings but it pays for itself in less than 10 years on energy savings. The solution will come here from the financial world which will agree to what is called “upfront investment”.
But it is clear that the cities of the world respond to very different problems: while, for example, the cities of Africa and South-East Asia emit annually about 1.5 tonnes of CO2 per capita, those of Europe are at 5 tonnes and those of the United States and Australia at 15 tonnes. Therefore the answers to be given are different. For the latter, it will require modernising all existing infrastructure. This is a key sector because it involves renovation almost everything: thermal insulation, optimization of heating and cooling, management of energy consumption, lighting efficiency, etc. As the renovation of existing buildings has become profitable thanks to the energy savings obtained, the question is no longer technical, but legislative. How to allow an owner who invests in a property to value his investment by repaying it with part of the savings made by the tenants? Everyone would benefit.
The regulations, for their part, should they ban excessive heating and air conditioning which is responsible for an incredible waste of energy? In the general interest, certainly, when we know that an ambient temperature of 25 ° C requires 40% more energy than 20 ° C. But resistance is likely to be strong.
It also goes without saying that these solutions must be accompanied by public policies that support their deployment. Globally, a large part of the urban housing stock is in the hands of developers. On the scale of a financial operation, the renewable gains do not weigh much. In order to initiate the transition, we could, for example, put in place a public policy authorizing an additional height to a building provided that it is topped by a renewable structure (photovoltaic panels, urban wind power, solar thermal, etc.) . This is how the Louvre Post Office in Paris was redeveloped into a hotel, integrating a renewable infrastructure on the top floor.
If the cities of the United States and Australia far outperform the rest of the world when it comes to CO2 emissions, it is also because they are built around cars. Thus, the transport revolution appears essential to fight against the climate crisis and air pollution. The development of public transport is obvious, provided of course that they are electrified. Individual mobility, for its part, must also become electric, and serve to store, in the batteries of parked vehicles, the intermittent renewable energy needed by the community.
When you see all the solutions that already exist today, the status quo backed by lack of imagination is a nightmare. It reminds me of the beginnings of the Solar Impulse project, when aircraft manufacturers told me that the sun would never give enough energy. However, we have succeeded in building an ultralight aircraft, in using alternative materials and in inventing new manufacturing techniques. We have completely changed the paradigm! I am optimistic that we will also achieve this in the areas of construction, housing, energy and mobility to finally make cities sustainable.