Spatiotemporal characteristics and mechanisms of embodied carbon emissions in the construction industry: Evidence from China's municipalities directly under the central government

Cities are more than just collections of buildings; they are vast, breathing engines of consumption that pull resources from deep within the earth and from distant factories. When a city grows, it does not just emit smoke from its own chimneys. It triggers a chain reaction of production elsewhere: the steel for its bridges, the concrete for its foundations, and the electricity to power its lights all require energy that releases carbon dioxide. This hidden carbon, embedded in the materials and energy used to build and maintain a city, is known as embodied carbon. While we often focus on the smoke rising from a factory or the exhaust from a car, the carbon footprint of the construction industry is far more complex. It is a web of connections where a decision to build a new skyscraper in one place can cause emissions to rise in a completely different province, hundreds of miles away. Understanding this invisible web is critical as nations strive to meet climate goals, because you cannot fix a problem you cannot see, and you cannot reduce emissions if you do not know where they are actually coming from.

A team of researchers from Beijing and Guangzhou has peeled back the layers of this complexity by looking closely at China's four major municipalities: Beijing, Tianjin, Shanghai, and Chongqing. These four cities are economic powerhouses, acting as central hubs that drive development across the country. The researchers wanted to understand not just how much carbon these cities produced, but how their demand for construction materials pulled carbon emissions from other parts of China. They built a detailed map of economic connections, tracing how the money spent on building in these cities flowed to industries in other provinces, effectively pulling the carbon emissions associated with that production out of the city and into the surrounding regions. By analyzing data from 2010, 2015, and 2019, they tracked how these flows changed over a decade of rapid urbanization.

The study revealed a striking pattern in how these cities consume resources. The primary driver of carbon emissions in all four cities was not daily household use, but the massive investment in fixed assets—essentially the construction of new infrastructure, roads, and buildings. This type of spending accounted for more than half of the carbon footprint in each city. When the researchers looked at which industries were most affected by this spending, they found that the construction sector itself was the main culprit, but it was heavily dependent on a few key upstream industries. The demand for construction materials in these four cities was directly responsible for a significant portion of the carbon emissions generated by the production of electricity and hot water, the smelting of metals, and the manufacturing of non-metallic minerals like cement and glass. In fact, the construction industry acted as a powerful magnet, pulling emissions from these heavy industries in neighboring provinces to satisfy the needs of the city.

Over the ten-year period, the story of where these emissions occurred changed dramatically. In the early years, from 2010 to 2015, the construction boom in these four cities largely pulled emissions from their immediate neighbors. For Beijing and Tianjin, this meant neighboring provinces like Hebei and Inner Mongolia saw a surge in emissions as they supplied the necessary materials. However, as time went on, the pattern shifted. By 2019, the carbon footprint of these cities had begun to stretch much further. The demand for construction materials started pulling emissions from the central and southern coastal regions, such as Guangdong, Henan, and Shandong. This suggests that as the local supply chains in the immediate vicinity were exhausted or restructured, the cities began to reach further out, effectively exporting their carbon burden to more distant parts of the country. While the cities themselves managed to reduce the amount of carbon generated within their own borders, the total amount of carbon they caused to be produced elsewhere continued to rise.

The researchers also broke down the reasons behind these changes to understand what was driving the numbers up or down. They found that improvements in technology and efficiency played a major role in reducing emissions during the first half of the decade. The cities became better at using energy, and the structure of their industries shifted in ways that lowered the carbon intensity of production. However, this progress was not steady. In the later years, from 2015 to 2019, the efficiency gains began to stall, and in some cases, the carbon intensity actually increased. The sheer scale of population growth and the rising demand for housing and infrastructure continued to push emissions higher, overpowering the gains made through better technology. The study suggests that while the cities have made strides in becoming more efficient, the fundamental drive to build more and bigger has kept the total carbon footprint high.

The implications of these findings are clear for anyone trying to manage climate change in a rapidly developing world. The study shows that simply looking at emissions within a city's borders gives a false sense of security. If a city reduces its local emissions by moving its construction projects to other provinces, it is not actually solving the problem; it is just moving it. The researchers argue that to truly reduce the carbon footprint of the construction industry, cities must look at the entire supply chain. This means not only building more efficiently but also changing what is being built and how it is sourced. They suggest that reducing the reliance on carbon-intensive materials like steel and cement, and shifting toward cleaner energy sources for the industries that produce them, is essential. Without addressing the deep connections between a city's construction needs and the industries that supply them, the goal of reaching a carbon-neutral future will remain out of reach, no matter how clean the city itself appears to be.