With “ambitious and immediate” mitigation efforts, there is still time for cities to achieve net-zero consumption-based emissions, the Intergovernmental Panel on Climate Change (IPCC) in its latest report. But strategies for hitting the target will not be one-size-fits all, and will prove sufficient only if communities can address supply chain emissions that extend beyond their administrative boundaries.
To successfully slash emissions, cities need to do three things, the report says: slash energy and material use at all points along their production and consumption chains; electrify everything using green energy; and increase urban carbon uptake and storage by all means possible.
Reporting with high confidence that “the urban share of combined global carbon dioxide and methane emissions is substantial and continues to increase,” the IPCC pegs total urban emissions based on the consumption of goods and services at roughly 28 gigatonnes of carbon dioxide equivalents (CO2e) in 2020, representing a whopping 67 to 72% of global emissions.
And thanks to growing urban populations—with associated infrastructure and service demands—these emissions are projected to rise to 34 Gt by 2050 with moderate mitigation efforts by mid-century, or 65 Gt with none.
With “aggressive but not immediate” urban mitigation policies aiming to limit global heating to 2°C, urban emissions will stand at 17 GtCO2-eq in 2050.
But with “aggressive and immediate” policies aimed at keeping global heating below 1.5°C by the end of the century, “urban GHG emissions could approach net-zero and reach a maximum of 3 GtCO2-eq in 2050.”
To meet this final scenario, city policies must bring systemic reductions and changes in both the energy and materials used to build them and keep them ticking.
The risk today is that “urban land areas could triple between 2015 and 2050, with significant implications for future carbon lock-in,” writes the IPCC. “Given past trends, the expansion of urban areas is expected to take place on agricultural lands and forests, with implications for the loss of carbon stocks.”
Noting that “the construction of new and upgrading of existing urban infrastructure using conventional practices and technologies can result in significant increase in CO2 emissions, ranging from 8.5 GtCO2 to 14 GtCO2 annually up to 2030,” the IPCC adds that doubling down on the status quo will also leave cities demanding “more than double annual resource requirements for raw materials to about 90 billion tonnes per year by 2050, up from 40 billion tonnes in 2010.”
By contrast, achieving “compact and resource-efficient urban growth” by co-locating home and work, changing zoning to allow mixed land uses, and making all development transit-oriented and pedestrian- and cyclist-friendly “could reduce urban energy use between 23 to 26% by 2050, compared to the business-as-usual scenario.”
Given that cities consist very significantly of buildings, getting a handle on energy and material use in construction will be a critical part of “resource-efficient urban growth.”
Surveying the period between 1990 and 2019, the IPCC confirms that the usual building habits just aren’t cutting it, with inefficient use of space and carbon-intensive materials (as well as an increase in the use of emissions-intensive electricity and population growth) increasing global emissions from non-residential and residential buildings by about 55% and 50%, respectively, from 1990 levels.
Urging mitigation “interventions” starting at the design phase, the authors recommend a deliberate push to embed “multi-functionality” into building designs so that they can be easily repurposed as the needs of their users evolve.
The construction phase should include low-emission materials, a highly efficient building envelope, and the integration of renewable energy solutions like small wind turbines, solar panels, and biomass boilers.
And to cut emissions from the point when buildings begin to house people, the IPCC urges highly efficient appliances and optimal use of spaces. Finally, at the end of their lives, buildings must be recycled, and their components reused as much as possible.
Policies that aggressively curb both material and energy use in buildings stand to cut emissions 61%, or 8.2 Gt, by 2050, with energy efficiency policies contributing the vast majority of the potential at 42%. Onsite renewable energy could cut a further 9% off building emissions.
The report notes that “the largest share of the mitigation potential of new buildings is available in developing countries while in developed countries the highest mitigation potential is within the retrofit of existing buildings.” It adds that the next eight years will be “critical for accelerating the learning of know-how, building the technical and institutional capacity, setting the appropriate governance structures, ensuring the flow of finance, and in developing the skills needed to fully capture the mitigation potential of buildings.”
Cities aiming to slash their emissions will also want to pursue “electrification in tandem with a switch to low-emissions energy sources,” the IPCC finds, with potential to save 6.9 gigatonnes by 2030 and 15.3 by 2050 in heating, transport, and cooking.
Finally, cities will want to radically enhance their local carbon uptake and storage through green roofs and spaces and bio-based building materials.
“Globally, urban trees store approximately 7.4 billion tonnes of carbon, and sequester approximately 217 million tonnes of carbon annually,” the IPCC states, emphasizing the value of urban green spaces.
The benefits of green and blue infrastructure go beyond the climate perks of carbon storage, the report adds, providing multiple boons to public health and well-being, from reducing heat stress and stormwater runoff, to improving air quality and mental health. Many of these measures also build urban climate resilience, the authors add.
But urban mitigation pathways will not be a one-size-fits-all proposition. “The degree to which a city is established will significantly determine its mitigation strategy,” the authors note. That means older cities “can achieve large GHG emissions savings by improving, repurposing or retrofitting the building stock, targeted infilling, and supporting non-motorized (eg. walking, bicycling) and public transport.”
For rapidly growing cities, “co-locating jobs and housing to achieve compact urban form,” and “leapfrogging or transitioning to low-emissions technologies,” will be critical to keeping future emissions low.
Finally, the “significant infrastructure development needs” required to achieve a high quality of life in new and emerging cities will be best met “through energy-efficient infrastructures and services, and people-centred urban design,” which also preserve and protect “existing green and blue assets.”
The 21st century will be “the urban century,” the IPCC writes, and “given the regional and global reach of urban consumption patterns and supply chains, the full potential for reducing consumption-based urban emissions to net-zero GHG can be met only when emissions beyond a cities’ administrative boundaries are also addressed.”
So effective emissions cuts will depend on “cooperation and coordination with national and sub-national governments, industry, and civil society, and whether cities have adequate capacity to plan and implement mitigation strategies.”