Why Carbon Emissions From Transport and Buildings Cannot Be Separated
Rethinking how we address carbon emissions by recognizing the inseparable link between transport and buildings.
Carbon Emissions: Transport and Buildings Are Inseparable
Climate change mitigation has often separated carbon emissions into distinct sectors—those coming from transport and those resulting from buildings. However, growing research and real-world observations show that these categories are deeply entwined, and treating them separately restricts our capacity for transformative climate solutions. This article explores why transport and buildings must be examined together, the consequences of siloed thinking, and how integrated design can unlock significant emissions reductions.
The Problems With Siloed Carbon Thinking
Many carbon accounting and reporting frameworks split emissions into operational categories—such as the energy used to heat, cool, and light buildings, versus fuels used for cars, trucks, and public transit. While simple, this approach overlooks the profound influence that urban form, community design, and infrastructure have on both transport and building energy demand.
- Separated metrics miss synergies: Improvements in public transit and walkable neighborhoods can slash both building and transport emissions.
- Policy fragmentation: Regulations and incentives often target building codes or vehicle technologies, but rarely the spatial links between them.
- Barriers to systemic change: Siloed approaches make it harder to create low-carbon communities through integrated planning.
Urban Form: The Hidden Driver of Carbon
The layout of cities—where buildings are placed, how they relate to transit hubs, roads, pedestrian corridors, and social infrastructure—profoundly shapes carbon emissions. Dense, mixed-use urban neighborhoods foster shorter travel distances, encourage walking, cycling, and enable public transport systems, simultaneously reducing the demand for private vehicles and decreasing building energy consumption per capita.
| Urban Pattern | Transport Emissions | Building Emissions |
|---|---|---|
| Sprawl (low-density) | High (car-dependent, longer trips) | High (larger homes, more heating/cooling) |
| Compact, mixed-use | Low (public transit, walkable) | Low (smaller homes, shared infrastructure) |
The Interplay of Embodied Carbon and Operational Carbon
Carbon emissions from buildings are typically divided into operational carbon (from energy used in running the building) and embodied carbon (from materials, construction, and demolition). Similarly, transport emissions include both vehicle operations and the emissions embodied in construction of roads, rails, and vehicles.
- Embodied carbon in infrastructure: Building new roads and car parks encourages driving and increases both transport emissions and the upfront emissions from concrete, steel, and asphalt.
- Retrofitting vs. new construction: Upgrading existing buildings and transit links often result in far lower overall emissions than demolition and new construction.
- ‘Upfront Carbon Emissions’: Experts advocate renaming ‘embodied carbon’ as ‘upfront carbon emissions’ to emphasize the immediate climate impact of construction activities.
Case Studies: Evidence of Integrated Emissions
Research supports integrated strategies for reducing carbon footprints:
- In European cities, higher density and greater transit options correlate with lower per capita emissions from both buildings and transport.
- Retrofitting building stocks is almost always preferable to demolition and rebuild. For example, refurbishing a detached house emits only eight tons of CO₂e compared to 80 tons for demolition and rebuild; operational savings only offset upfront emissions in 15-20 years.
- Changing exterior wall materials (from standard to hollow blocks with stone cladding) can decrease embodied emissions by 18% in certain projects.
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The Carbon Hierarchy: Avoid, Replace, Reduce, Offset
Building on sustainability frameworks, experts propose a carbon hierarchy for construction, much like the waste hierarchy (reduce, reuse, recycle):
- Avoid: Minimize carbon through regenerative, low-carbon design and planning—prioritize retrofitting and design buildings as carbon sinks.
- Replace: Substitute high-carbon materials and methods with lower-carbon, regenerative solutions (e.g., switching from concrete to mass timber).
- Reduce: Limit emissions by using local materials, improving procurement, and cutting construction-related travel and transport.
- Offset: Where emissions cannot be eliminated, responsibly offset using restorative methods, while recognizing limits (e.g., toxic non-offsettable pollutants).
Mass Timber and Bio-Based Materials: A Path Forward
The shift to mass timber and other bio-based building materials offers significant carbon savings over steel and concrete.
- Durability and safety: Modern engineered timber meets rigorous fire and structural standards.
- Lower weight: Timber buildings typically weigh only 20% of their concrete counterparts, resulting in smaller foundations and lower emissions.
- Sequestration: Timber locks in carbon throughout its life, outweighing the emissions from harvesting, transporting, and manufacturing.
- Other bio-based alternatives: Bamboo, grasses, and agricultural by-products are also emerging as promising options.
Despite some emissions from machinery and adhesives, the net impact is strongly positive when accounting for timber’s carbon storage capacity.
Urban Height, Density, and Embodied Emissions
Urban density can reduce per capita energy use and transport emissions, but there are limits:
- Embodied emissions rise as buildings grow taller, mainly due to heavier foundations and increased use of high-carbon materials like concrete and steel.
- Around 20-50% of concrete in mid-rise and high-rise buildings is below grade for foundations.
- Studies: Emissions climb sharply after 12 stories; for example, a 12-story building uses 40% more embodied energy than an 8-story building.
- Tall buildings typically require more spacing for stability and daylight, sometimes offsetting the land-use efficiency.
Retrofitting: The Climate Imperative
The focus on operational efficiency in new buildings often overlooks the substantial upfront emissions from construction. Retrofitting existing structures, when feasible, frequently delivers greater long-term carbon savings. Foundations, structural elements, and facades have long life cycles—sometimes lasting multiple decades or centuries—making retrofit strategies environmentally sound.
It is crucial, however, to address:
- Structural limitations: Not all older buildings are easily retrofitted, but technical advances are expanding possibilities.
- Cost barriers: Financial incentives and codes can help make retrofits viable for more property owners.
Designing Carbon-Positive Projects: Learning From Nature
A paradigm shift is needed. The best climate solutions ask: How can every act of construction make the world better? To achieve this, thought leaders emphasize three guiding principles for design and construction:
- Take Back: Actively remove carbon from the atmosphere; design buildings as carbon sinks.
- Stop: Eliminate, or drastically reduce, all emissions and toxins released into the atmosphere—especially in the built environment.
- Think Like a Tree: Understand carbon as a natural cycle; improve ecological literacy to design spaces that actively store and cycle carbon, rather than simply minimizing emissions.
Policy Implications and Recommendations
- Integrate transport and building strategies: Urban planning, building codes, and climate regulations must evolve to treat transport and buildings together.
- Promote retrofitting: Policymakers should reward upgrades to existing structures, recognizing their lower total lifecycle emissions.
- Encourage regenerative materials: Support research, development, and adoption of timber, bamboo, and other bio-based materials.
- Foster compact, walkable communities: Incentivize development that reduces both transport and building emissions via thoughtful design.
Frequently Asked Questions (FAQ)
Q: Why shouldn’t we treat transport and building carbon emissions as separate issues?
Emissions from transport and buildings arise from interconnected urban patterns, infrastructure decisions, and policymaking. Treating them separately prevents holistic solutions and misses major opportunities for carbon savings.
Q: What is the difference between operational and embodied carbon?
Operational carbon refers to emissions from using buildings and transport (e.g., heating, lighting, driving vehicles), while embodied carbon accounts for emissions from the extraction, production, and assembly of materials and infrastructure—sometimes called ‘upfront carbon emissions’.
Q: Isn’t it better to just build new, more efficient buildings?
Although newer buildings may have lower operational energy emissions, demolishing and rebuilding introduces massive upfront embodied emissions. Retrofitting existing buildings almost always results in lower total emissions over time.
Q: How can cities be designed to minimize both building and transport emissions?
Compact, mixed-use, and walkable neighborhoods—linked by efficient public transit—reduce the need for private cars, shrink home sizes, and maximize shared infrastructure, leading to lower emissions across both categories.
Q: What role do bio-based materials play in reducing carbon footprints?
Materials like engineered timber sequester carbon, are much lighter than concrete, and can both lower construction-related emissions and lock away climate-changing CO₂ within the building structure.
Key Takeaways for Climate Action
- Carbon emissions from transport and buildings are fundamentally intertwined.
- Integrated urban design and policy can achieve deep emissions cuts impossible with siloed approaches.
- Retrofitting, material innovation, and ‘carbon-positive’ thinking are vital steps toward meaningful climate solutions.
References
- https://openknowledge.worldbank.org/bitstreams/a3474878-71ca-44ad-a584-3c888722673e/download
- https://fairsnape.com/2020/01/07/a-carbon-hierarchy-for-net-zero-carbon-construction/
- https://www.greenbuildingadvisor.com/article/lloyd-alter-on-upfront-carbon-emissions
- https://ecooptimism.com/?tag=treehugger
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