Locally optimized urban form reduces carbon dioxide emissions

While atmospheric CO₂ levels are higher than ever before, an estimated 2.5 billion additional people are expected to integrate into urban areas worldwide by 2050. Given the link between atmospheric CO₂ levels and energy consumption and their contribution to climate change, interest in energy-conserving urban planning and design measures is growing.

Unfortunately, scientific research in energy-optimized urban forms is lacking, and most existing studies analyze only macro-scale urban form, lack standardization and only assess a limited number of urban form factors. To address this issue, a group of scientists from Hiroshima University, Shiraz University and Northern Arizona University designed a research study to investigate the effects of urban form on carbon emissions using more detailed assessment methods in three different U.S. cities.

The team published their study in the November 2024 issue of the Journal of Environmental Management.

"The relationship between urban form and CO₂ emissions is well recognized. However, most studies so far have been limited to examining the urban form at the macro level and there is a lack of granular understanding of this dynamic relationship. [In contrast,] this study employs the Local Climate Zones (LCZ) framework to investigate the relationship between urban form and CO₂ emissions at the micro level," said Ayyoob Sharifi, professor at the IDEC Institute at Hiroshima University and an author of the research paper.

The LCZ framework is a way of universally classifying urban forms, ten of which are built (such as low-rise buildings, high-rise buildings and heavy industry) and seven that are natural. This framework has been successfully used to better assess urban heat island effects. For this study, the research team applied this framework to classify the urban form of Baltimore, Maryland; Indianapolis, Indiana; and Los Angeles, California using remote sensing methods, such as satellite imagery data.

The three cities chosen for the study were selected because of their differences in both climate and population density. Los Angeles is classified as a warm, dry climate with few temperature variations; Indianapolis is considered a cold, humid climate; and Baltimore is a mixed, humid climate. Baltimore has the highest population density of the three cities, while Los Angeles and Indianapolis have similar population densities. In comparison, Los Angeles is spread over a much larger area than Indianapolis.

The contrasting variables between the three cities were intentionally chosen to determine whether or not differences in LCZ could account for differences in CO₂ emissions over the course of an entire year or individual seasons, depending on the per capita density and climate. CO₂ emissions were calculated using Hestia Project data, which calculates the amount of fossil-fuel CO₂ released in individual cities at street- and building-level scales each hour.

While the study provided new insights into the effect of LCZ on CO₂ emissions, few generalities could be made between cities. "The results demonstrate that the relationship between urban form and CO₂ emissions is complex and dynamic. Patterns could be different depending on the context and factors such as climatic conditions and size and function of the city. Patterns observed in a specific city cannot necessarily be generalized to other cities. This means that one-size-fits-all approaches cannot be applied to determine optimized urban forms," said Sharifi.

Importantly, the study revealed that urban open and green spaces are paramount. "[W]ithout smart and adequate provision of open and green spaces, compact urban development will have limited capacity to mitigate urban CO₂ emissions," said Sharifi.

The research team plans to continue their investigation of the link between LCZ and urban CO₂ emissions to mitigate the effects of urban fossil fuel usage. The authors suggest that further refinement of LCZ resolution in urban areas and studies of individual contributors to urban fossil fuel consumption, such as transportation, residential and commercial sectors, for example, may help scientists tease out patterns between LCZ and CO₂ emissions.

"We aim to conduct more research involving data from a larger number of cities from different parts of the world to [better] understand… the association between LCZ type and CO₂ emissions under different climatic and socioeconomic conditions," said Sharifi.