Urban development is a critical but often overlooked obstacle to achieving global climate goals. According to extensive research of more than 1,000 cities around the world, unregulated growth of emissions associated with building materials (as cement and steel) in direct contradiction to the Paris climate commitments. If cities want to stay within the 2°C warming limit, they must reduce construction emissions to less than 10 % current level.

Current status: Construction as a silent polluter

While many cities are successfully decarbonizing their energy systems, emissions from material consumption continue to rise.

• Emissions growth: Between 2010 and 2020, emissions from construction in the monitored cities increased by a total of 20 %.
• Share in population: The 1,033 cities analyzed represent 15.2 % of the global population, but in 2020 they were responsible for 1.4 GtCO2e of construction emissions.
• Inequality: Cities in high-income countries produce on average 1 tCO2e per capita per year from construction alone, while the total global limit for all emission sources should be approximately 2.3 tCO2e per person by 2030.

Less populated cities (under 500,000 inhabitants) account for up to 60 % of emissions in the dataset, which underlines the need to involve smaller municipalities in decarbonization, not just world metropolises.

Climate budgets for cities

Researchers introduced the concept carbon budgets, which set the final amount of CO2 that a city can still emit to avoid catastrophic warming. These budgets can be divided in two main ways:

1. Grandfathering (by current share): Cities will receive a share based on their past emissions, but this model is often seen as unfair because it rewards historically the biggest polluters.
2. Equal-per-capita: Every citizen has the same right to emissions, which drastically shortens the time to decarbonize for wealthy cities.

For example, at an equal per capita share, the 20 largest emitters in the US would have to reach net zero as early as 2018. 2034, while under the "grandfathering" model they would have until 2067.

The path to sustainable building

For cities to meet housing demand while meeting climate limits, radical changes in building practices are necessary.

• Change of building type: Traditional single-family homes are incompatible with the 2°C limit in many regions. The solution is to switch to low-rise to medium-rise multi-apartment buildings, which have a significantly lower emission footprint per housing unit.
• Material innovation: Usage wooden structural systems and reducing the average floor area per capita are key strategies.
• Sectoral compromises: Cities can buy more time for the construction sector by accelerating the decarbonization of other areas, such as transport or household energy.

Methodology and data

These findings are based on the model Exiobase 3.8.2, which tracks global money flows and their environmental impacts. The researchers combined economic data on construction investment with data on building material intensity from the RASMI database, creating the first transparent overview of the climate limits of construction for a thousand cities.

Municipalities must begin to actively use zoning and laws to limit emissions from materials before they exhaust their remaining carbon budgets. JRi&CO2AI