If you were going to design a cheap way to trap a lot of heat in cities, a standard asphalt pavement would be a pretty good choice. The mixture of black rocks and gooey black stuff holding it together is an excellent invention if you want to absorb as much sunlight as possible, and re-radiate that energy as heat.
Except, most cities don’t want to absorb as much sunlight as possible. They want to cool down, not get steadily hotter.
To show that there are alternatives to hot asphalt, Lawrence Berkeley Lab, in California, currently has a showcase of "cool pavements" that are designed to reflect between 30% to 50% of the energy, compared to about 5% for conventional surfaces. On some days, according to Benjamin Mandel, a researcher with the Heat Island Group, the new coatings are 40 degrees Fahrenheit cooler than traditional counterparts.
The coatings, which are made by several private companies, come in various shades and colors, and are currently displayed in Berkeley’s (very dark colored) parking lot. Mandel says they’ve tested eight so far. "Our showcase is intended primarily for demonstration purposes, as we show the public and local governments some cool pavement options that are currently on the market," he says.
Mandel says it’s not possible to say which of the eight is best, as they have varied uses. "Some likely perform better under vehicle traffic on city roads. Others may maintain solar reflectance better over time."
But, with more than a third of cities taken up with pavement, using cooler coatings could have a big impact on reducing air temperatures and improving air quality. Studies have shown that lighter surfaces, combined with more vegetation, could impede the formation of smog, and reduce energy costs—for example, from air conditioning.
Mandel says the coatings are likely to be a little more expensive up-front. But this could be offset by savings from longer-lasting pavements, and better preservation of the "system" underneath. If it’s a little easier to walk around in the summer, it sounds good to us.