It's been called the "last mile" of decarbonization.

As companies and countries worldwide map out how they will hit net-zero emissions by 2050, some elements of the vast shift are relatively straightforward: Cars will go electric; power grids will adopt clean energy.

But when it comes to buildings, engineers and policymakers alike hit a hurdle: Even a house covered with solar panels is likely to contain concrete and steel—some of the most intractable sectors when it comes to emissions. To make truly low-carbon buildings, researchers say we must embrace breakthrough technology, from hydrogen to carbon capture, and explore new ways of designing concrete, industrial products, and even houses themselves.

The stakes are high. Between the energy they consume and their construction, buildings are responsible for nearly 40% of the world's emissions, according to the International Energy Agency. To truly produce a zero carbon house, office, or shop, every industry involved in its construction and maintenance must be decarbonized first, says Dabo Guan, a professor of climate change economics at University College London's Bartlett School of Construction and Project Management.

When buildings are constructed, "they trigger the whole economic supply chain," says Guan. "And the emissions of the supply chain are very big."

“Like making a cake”

When it comes to concrete, "the only thing we use more as humans is water," says Jeremy Gregory, executive director of MIT's Concrete Sustainability Hub.

At the heart of concrete is cement: the key binding agent that turns sand and water into one of the world's most ubiquitous materials. In 2019, the world produced roughly 4.1 billion tons of cement, according to the IEA. It's also extremely hard to decarbonize. Cement itself must be formed at extremely high temperatures and is the product of a chemical process that naturally produces carbon dioxide. Collectively, it is responsible for up to 8% of global emissions, says Gregory.

Because it's extremely difficult to use renewable energy to produce the energy intensity needed for ultrahigh temperatures, truly low-carbon cement will likely rely on carbon capture, storage, and utilization, which prevents CO2 from being released into the atmosphere, either by injecting it into the ground or—potentially—into the concrete itself.

There is also another approach that could help, says Gregory: diluting, or even replacing, the cement in concrete. These options already exist: The ancient Romans used volcanic ash as a binding agent to make concrete. But it's possible to use a large number of waste products, including fly ash—a by-product from coal plants. Some blends can reduce the carbon intensity by as much as 70% compared to conventional cement and will produce a product that's just as good.

It's "sort of like making a cake," says Gregory. "You can use whole wheat flour. It'll still look like a cake. It'll just taste a little bit different."

Reduce, reuse, recycle

Steel struggles with some of the same problems as concrete. Mainly, it must be produced at high temperatures, and, to a lesser degree, some CO2 also results from the process. Steel has one advantage—it can more easily be recycled—but that, too, has challenges. There is not enough to meet demand, and reprocessing requires energy, says Richard Curry, a program manager at Sustain, the Future Steel Manufacturing Research Hub based at the University of Swansea in Wales.

Logistically, recycling can be challenging and degrade the quality of the metal. As with concrete, the most feasible solutions are carbon capture, utilization, and storage—even if those are not yet commercially mainstream.

Embracing better design—from buildings to infrastructure to, yes, electric cars—to make them easier to disassemble so that their parts can be accessed and recycled could help, says Cameron Pleydell-Pearce, Sustain's deputy director.

Another option, he says, is reusing.

"One of the things that we're looking at in a very great level of detail is the degree to which we can understand which product and trace which product is coming out of a steel mill at a particular point, and then what happens to it as it goes through its life cycle," he says.

Unlike even recycling, that would offer a major advantage: It comes with almost no CO2 emissions at all.

Warm in winter, cool in summer

When it comes to design, there's another potential solution staring us in the face: drawing inspiration from what our buildings used to look like.

A traditional house in New England, for example, would have had south-facing windows, maximizing the sunshine and minimizing the darkness in winter, says Anna Dyson, the founding director of Yale University's Center for Ecosystems in Architecture.

Houses all over the world have traditionally been designed and built to best work with the climate, she adds, but "over the course of the 20th century, as buildings became more and more reliant on cheap fossil fuels, then it wasn't so required to be really, really careful about orientation and working with climate."

Also, to manage the indoor temperatures, houses were built in shapes and sizes that suited their climates. In humid locations, home designs included ample ventilation and steep roofs to enhance air flow. In arid climates with hot days and cold nights, houses were roomy and light-colored to reflect heat. Those principles, along with making use of biodegradable materials, from timber to straw to coconut husks and bamboo, are ideas that some architects like Dyson are now looking back to.

Of course there are no silver bullets. Houses still need energy for lights and heating, preferably clean energy, Dyson points out. And now we face the prospect of not just making houses that are suited to the next 100 years, but also finding ways to retrofit the ones that have already lasted a century.

"We've got a long way to go," says Dyson. "But we've got a lot that we can do with design."