From Toronto to Tokyo to Tijiuana, a new, open-source model created by University of Toronto Engineering researchers provides — for the first time — construction-related greenhouse gas emissions budgets for 1,000 cities around the world.
"Many countries have set emissions targets for themselves at the national level," says Professor Shoshanna Saxe, a lead investigator with the University of Toronto's Centre for the Sustainable Built Environment (CSBE) .
"But when it comes to making decisions about what should be built and where should we build it, that typically happens at a more local level."
"Cities around the world are wrestling with this problem, and have been leaders in the climate fight," says PhD student Keagan Rankin, who is co-supervised by Saxe and is the lead author on a paper published in Nature Cities that describes the new model.
"But until now, they've had very little guidance to go on. It's been like trying to run a race without knowing where the starting line or the finish line are."
Saxe, Rankin and Rankin's other co-supervisor Professor Daniel Posen wanted to provide a straightforward way to help cities navigate between the need for new housing and infrastructure and the need to keep global warming below the levels set out in the Paris Agreement .
Collaborators on the project included Professors André Cabrera Serrenho at the University of Cambridge and Christian Bachmann at the University of Waterloo.
Their first-of-its-kind approach uses an extended environmental input/output model, along with statistical regression techniques, to allocate the global carbon budget estimated from climate models to 1,000 different cities around the world.
"Thanks to the work of climate modellers, we have really good estimates for the amount of emissions that will enable us to stay below two degrees of warming at a global level," says Rankin.
"Where it gets complicated is deciding how to divide that up between countries, cities and even specific sectors. As far as we know, our model is the first one with the ability to provide city-level emissions budgets for the construction sector: we can finally see where we're at now, and where we need to be by 2050."
Rankin says that while the model can be used to allocate the emissions budgets in any number of different ways, for the demonstration purposes the team chose two common strategies: equally-per-capita and grandfathered.
"With the first method, you say that each human is allowed a certain amount of emissions to meet their housing and infrastructure needs. This means that bigger cities that need to service more people get bigger budgets," says Rankin.
"By contrast, the grandfathered method accounts for the fact that cities in the developed world are already building at a much higher level of emissions per capita than those in the developing world. It reflects the reality that it will take time to re-imagine our construction habits and learn how to build more with less."
The team has provided a handy dashboard that enables anyone to look at the carbon budget for their city, or to compare and contrast those for several cities at once.
Using this tool, it becomes clear that no matter how emissions are allocated, all cities need to get better at low-carbon construction very quickly.
For example, under the grandfathered method, the city of Toronto has only 21 years left before it exhausts its emission budget for construction — and under the equally-per-capita method, it has only seven years.
"Our model suggests that if we in Toronto want to stay below two degrees of warning, we must reduce emissions associated with construction by between 20% and 40% every year between now and 2050," says Rankin.
"It is possible to do that: in fact we've published other papers that show how various strategies, such as an increased proportion of multi-unit buildings or placing new housing in existing neighbourhoods, can significantly reduce the emissions associated with new construction."
Whether or not cities choose to adopt some or all of these strategies, Saxe says that finally having specific targets to aim at is a critical first step.
She also says the tool's open-source methodology can be used to set emissions budgets for other sectors — such as public transportation — at the city and/or regional scale.
"Hundreds of these cities have already signed carbon neutral pledges. If they're serious about that, they need to have a budget and a plan to meet it," she says.
"This model provides all of them with a plausible upper and lower bound, as well as a template they can use to create their own budgets if they want. Hopefully, that will help us get from simply having a commitment to having a real, actionable plan to achieve our goals."
