Some remain leery of these reflectance requirements, particularly in northern cities, where roofs that absorb less sunlight in the winter can drive up heating bills. Chicago-based architect Tom Hutchinson notes that what is suitable in one place may be detrimental in another. “Roofing needs to be a decision based on a lot of things like location, sun, wind, rain, microclimates – there’s no one solution to fit all of those,” Hutchinson says. “Buildings are made up of systems, so you can’t prescribe just one component of the system like a white roof.”

For example, roof reflectance doesn’t address insulation, which can reduce energy bills by slowing the flow of heat into or out of a building. “Insulation is very effective; with enough insulation, roof reflectance doesn’t matter,” Hutchinson says.

Rosenfeld agrees that insulation can be a good way to make buildings more energy efficient, but he says that it has none of ancillary benefits of reflective roofs. “It’s surely a lot cheaper to make a city’s roofs white when they need to be replaced anyway than to bring all their insulation up to code,” Rosenfeld says. Besides, as he points out, “Insulation can’t make the air cooler during a heat wave.”

Rosenfeld also believes in the global benefits of brighter cities, which can make up for a portion of the brightness that is being lost as glaciers and sea ice melt. The ice that has traditionally occupied the poles is bright white, which allows it to regulate the planet’s temperature by reflecting sunlight. As global temperatures rise and this ice melts, it gives way to dark water that absorbs sunlight. While making cities brighter won’t stem sea-level rise, it might slow rising temperatures.

And the movement toward cool roofs is picking up steam around the world. Brazil, China, India, Japan, Mexico, and South Africa have all made commitments to prioritize reflective roofing.

Even where cool roofs have caught on, however, it could take 20 or more years for all structures to transition. Only then will the large-scale climate impacts be known, and it is likely that they are only a small part of the solution to climate change.

But Rosenfeld doesn’t let this slow him down. As long as there are still black roofs and hot cities, his eraser has more work to do. And as the movement he has inspired pushes forward, his handiwork will be ever more visible in the brighter, cooler metropolises of the future.