A team of researchers just traced the supply chain of every building and bridge across 1,000 cities. The resulting carbon data makes our current housing targets look mathematically impossible. For years, we lacked a way to calculate the true environmental cost of urban growth. Global economic models track the flow of money and ecological impacts at the national level. We knew exactly how much carbon a country emitted making steel and concrete. We just couldn't trace those materials down to the specific metropolitan areas consuming them. So a team built a top-down allocation model to find out. They took 20 years of global input-output data and merged it with local economic proxies like construction employment and regional GDP. They effectively generated an itemised receipt for the embodied carbon of every highway, pipeline, and residential tower built in major global cities. Then they calculated strict cumulative carbon budgets. They took the remaining global carbon allowance required to stay below 2°C of warming and divided it up. They allocated shares to specific sectors and then distributed those shares to individual cities based on population and historic emission rates. A city like Montréal gets a hard mathematical limit on how much carbon its construction sector can emit from 2020 onwards. This is where the climate data collides with the housing crisis. Most major cities are projecting massive population growth and authorising immense residential construction programmes to match. Toronto plans to build hundreds of thousands of new units by 2031. The researchers calculated the material intensity of this future housing stock. They pulled data on the concrete, steel, brick, and glass required for different building types. They then ran complex simulations to model the life-cycle emissions of manufacturing those exact materials. When you multiply the required new floor space by the embodied carbon of standard construction materials, the budgets immediately fail. Building enough housing to meet projected population growth using our current supply chains guarantees a massive carbon overshoot. The gap between these two realities forces a brutal choice. We either accept that we will blow past our remaining global carbon budget, or we drastically change how we build. Meeting future housing demands within the required climate limits demands an immediate shift away from high-emission concrete and steel toward timber and radical material efficiency. The maths is completely unforgiving. You can have the necessary housing, or you can use traditional building methods. You can't have both. Link to article: nature.com/articles/s44284-0…