Study of 1,000 cities shows new single-family homes incompatible with 2°C target

Massive construction in Dubai — This study shows that construction in high-income cities was consistently responsible for the equivalent of ~ 1 metric ton of CO2 per person per year. For context, the average global citizen must reach 2.3 tons of CO2 equivalent from all emissions sources by 2030 to align with the Paris Agreement. getty

Many cities are viewed as climate leaders, often setting more ambitious targets to reduce emissions than national governments. But in the question of urban decarbonization, something big has been overlooked – or perhaps quietly ignored – the emissions of construction itself.

A new study published in Nature Cities has, for the first time, calculated the consumption emissions and carbon budgets for construction in over 1,000 cities worldwide. The findings are stark: across these cities, construction produces the equivalent of 1 – 3 metric tons of CO2 per person (tCO2e), per year. The authors say that continuing at that rate could consume “most or all emissions allowed by a 2°C climate target in 2030.”

Staying within these limits requires emissions from construction to drop to below 10% of current levels within the next 45 years. And while this might sound impossible – especially with the need for construction predicted to double by the middle of the 21st century – this study shows that it is still achievable.

And the good news is that cities can do it without enacting a total ban on construction, nor waiting for ‘silver bullet’ solutions, or yet-to-be-invented materials. What’s needed instead is a smarter approach to designing, building, and maintaining urban homes and infrastructure.

“The thing that surprised me most was that it is actually possible,” says Shoshanna Saxe, Associate Professor in the University of Toronto’s Department of Civil & Mineral Engineering. “I wasn’t sure that it would be, when we set out to do this work. But we found that even in very rapidly growing cities, we can stay within target using existing technologies, tools and knowledge.”

Concrete pouring on the construction site. — Concrete is everywhere in our cities, but who pays for its emissions footprint? Construction emissions are challenging to measure because they are spread across global supply chains. But Toronto researchers say that cities need to take ownership of this, in order to make construction more sustainable getty

Construction’s Carbon Budgets Are Complex

Unlike transportation, tracking emissions from construction is a significant challenge because there’s no obvious point at which to measure them. Or as Saxe’s colleague and co-author Keagan Hudson Rankin put it, “Construction emissions aren’t like a car – there’s no tailpipe to measure at.” Take concrete as an example. Of its three main components, only the water tends to be sourced within a city’s limits. The aggregate may come from a quarry tens of kilometers away, and the cement will likely have been transported hundreds or even thousands of kilometers before it is put into the mixer. Then, it’s transported to a construction site where it is poured to form structures that will eventually support a building. And that’s just one material at one site. With so much complexity, is it any wonder that most cities haven’t yet considered construction emissions?

“We know that a lot of cities want to be more sustainable, but when it comes to construction, they don't even know where they're starting this race – by that I mean their current emissions,” says Rankin. “They’re clear on where the race has to end, which is by hitting net zero, but they don't know what the path looks like between those two points. So, our goal with this paper was really to give as many cities as possible that information.”

Saxe and Rankin started by collecting relevant data from across 40 countries; a process that required “a lot of long nights in the lab”. They relied on Exiobase, a global economic model that tracks how money flows between sectors (e.g construction and manufacturing) and between countries. It then links those flows to their environmental impacts in the form of CO2 emissions. From this, they could isolate construction investments from everything else, giving them each country's total construction emissions. Transforming this national-level analysis into a city-level one involved collecting real construction investment data (e.g. number of granted building permits) for 50 cities and using that to train a regression model that they could then apply to all 1,033 of the cities in their study area. The final dataset covers cities that, in 2020, were home to a total of 1.2 billion people (15% of the global population).

“This is the very first time that anyone has systematically created construction carbon budgets at a city level for so many cities,” says Rankin.

Even so, they admit, there are significant gaps in their data. Exiobase doesn't have current economic statistics for every region on the planet – e.g., there’s no data from Sub-Saharan Africa, India hasn’t held a census since 2011, and just three cities in China made verifiable data available. This is important says Rankin, “because it’s those cities that are going to be growing the fastest. Resolving that gap will be critical for understanding future global construction.”

Saxe explains, “Exiobase is a big research effort – it isn’t maintained by the UN; it’s run by a group of researchers because they want to make tools available that we can all use to understand the world. They are often working to expand it. And so, for countries not included, I feel fairly confident in saying that Exiobase researchers would love to hear from them.”

Their Headline Findings

The Toronto team found that between 2010 and 2020, construction emissions in these cities grew 20% overall, but the specifics vary. The fastest growth has occurred in middle-sized North American cities and middle-income Asian cities (+32% and + 36% growth, respectively), while construction emissions shrunk in others (e.g. −85% in Chapecó, Brazil)

High-income cities were found to be consistently responsible for the equivalent of ~ 1 metric ton of CO2 per person per year since 2000, solely from construction, with some larger cities reaching 2 – 6 tCO2e. Cities in middle-income countries tended to have lower annual construction emissions (0.3 – 0.4 tCO2e per person). Contrary to previous studies that suggest smaller cities can be excluded from emissions metrics, Saxe and Rankin found that they are actually “responsible for a substantial portion of emissions.”

Importantly, they also found that unlike the energy sector, construction emissions remain tightly linked to investment. When cities build more, they emit more, and in the twenty years their study covered, there's been no significant improvement in emissions per dollar spent.

For context, the average global citizen must reach 2.3 tons of CO2 equivalent from all emissions sources by 2030 to align with the Paris Agreement. This study shows that current construction alone in 19 high-income cities would overshoot this entire limit.

Meeting construction climate limits, write the authors, “requires immediate and unprecedented emissions mitigation.” So, what does that look like in practical terms?

Planned Desert Community In lancaster California, USA — Single-family home neighborhoods far from urban centers are inefficient in terms of resources, and costly in terms of emissions and construction. (Photo by Sam Lafoca/Construction Photography/Avalon/Getty Images) Getty Images

We Already Have The Tools

“You really have three options: you can build less, you can build better, or you can build out of better stuff,” explains Saxe. “Building less is rarely an option because most cities need new housing [to support their growing populations]. Building out of better stuff will take time. Yes, of course we hope that our construction materials will be manufactured in ways that have less environmental impact, but that won’t happen wholesale within the next decade, which is when we need to act.”

So, she continues, “that leaves us with building better, and on that front, we actually have lots of tools already available to us – we don't need to reinvent the wheel.”

“A big one for me is underground construction,” says Rankin. “Changing it might mean a smaller basement or no basement in a family home. Or, we've done studies where we look at two very similar tall buildings, but one will have upwards of 50% more embodied emissions just because it has extra underground parking. But even smaller things like the choice of insulation makes a difference – wool batt has much lower emissions than polystyrene insulation but is thermally just as good.”

Urban design plays a huge role too, says Saxe. “We’ve done a lot of very complicated math and ultimately, it shows that we need to build reasonable size buildings that are well-designed with not too much consumption, not too far apart.”

When asked for a single piece of advice she'd give a mayor serious about reducing construction emissions, her answer was unequivocal: "No more single-family homes." She clarified that doesn't mean people can't live in single-family homes; there are already hundreds of thousands in most cities. "But no new single-family neighborhoods. If you're serious about the climate or affordability or transportation or really any of it, that's a big one." For Toronto, meeting 2032 housing goals requires limiting emissions to 4 – 19 tons CO2 equivalent per housing unit; a range only compact, multi-unit buildings with timber structural systems can currently achieve.

She continues “If you are going to have single-family neighborhoods, there are ways to make them less unsustainable. In Canada we have doubled the size of housing since the 1950s, while approximately halving the size of families, because people have fewer kids today. Even going back to the average size of single-family home from the post-war period would in and of itself dramatically reduce resource use and embodied greenhouse gases.”

Her second recommendation: "Raise the standard of your design and engineering. We spend a lot of money to pour concrete, and in many places very little money on the quality of design. There's a big opportunity there to pay a little bit more upfront and save a ton of money and emissions downstream."

Rankin's advice for city leaders was straightforward: "Have a plan. No plan is perfect, and there's a bunch of subjective choices to make, but at least having a plan sets you off on the right path to start implementing things."

Saxe agrees, "There’s been huge success from having plans. Most wealthy countries around the world have now decoupled their emissions from economic growth. We've seen a huge amount of progress in the last couple years in renewable energy. We've shown ourselves capable of a massive amount of change over the past 15 years, while keeping quality of life pretty much the same. We can do this if we try."

The study shows that cities simply cannot build their way out of climate limits using current construction practices. But it also provides a new and valuable dataset, and a roadmap that enables evidence-based planning. The question is, will cities adopt it quickly enough?

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Urban Construction Boom Threatens Climate Goals