A federal judge later barred the city from clearing homeless encampments due to extreme heat conditions, an order that is currently set to expire on Sept. 1.
Residents who live outside experience an intense version of the extreme heat that affects all of Sacramento. In 2021, a report done by nonprofit news organization Climate Central ranked Sacramento as the 11th worst U.S. city for the urban heat island effect, a phenomenon where paved surfaces and dark-colored roofs trap the sun’s heat. Sacramento’s temperatures averaged about 7 degrees hotter than less developed surrounding areas. Last summer, a 116-degree-day set a new temperature-high record for the city.
Some shade structures and misters are set up at Miller Park, according to city officials, to combat the rising temperatures. But what if there was a way the city of Sacramento could quickly cool down the ground — and the surrounding area — with just a bucket of paint and a few paint rollers?
Could Extremely White Paint Help With Sacramento’s Extremely Hot Summers?
Enter, white paint. Regular white paint is already good at reflecting the sun, and has been used to cool buildings in Greece and even rooftops in New York City for decades. But in 2021, researchers at Purdue University released a reflective paint that’s even better — like, Guinness Book of World Records better.
The ultra-white coating can reflect just over 98 percent of sunlight, making the surface’s temperature cooler than its surroundings. In comparison, commercial white paints currently on the market generally reflect about 80-90 percent of sunlight and are often still warm to the touch.
“It’s cool to see that people are interested in this,” said Emily Barber, a doctoral candidate at Purdue University working on the paint. “We’re looking forward to being able to get this product to people and hopefully making an impact and knowing what that impact is — which is my job.”
In terms of effect on temperatures, the paint, when tested on sunny days, kept outdoor surfaces 8 degrees cooler than surrounding temperatures, and at night, surfaces were up to 19 degrees cooler.
Light-reflecting white paints currently on the market do their job by reflecting visible light and near-infrared wavelengths, which together make up a large portion of the sun’s rays. But Xiulin Ruan’s Purdue lab focused on the light particles that were slipping through the cracks: ultraviolet.
Barium sulfate, a compound that is used for sunscreen, oil paints and medical radiology is known to reflect UV rays. So, the team switched out the titanium dioxide often used as a base for commercial paints, as well as varying the particle sizes in the paint to reflect a wider range of wavelengths.
They’re not the first group to try something like this, according to Barber. But one of the reasons that their lab has gotten national attention is because they’re trying to match the commercial process for paint manufacturing as much as possible — Barber said their goal is to “bring this really exciting technology from the lab to the world.”
We Can’t Paint Over All Our Problems
Moving the ultra-white paint from the lab to the world takes away a certain amount of control from scientists — and it can introduce new challenges or complications in the application of the material.
Purdue’s paint likely won’t be released commercially for at least a year, and one of the reasons is that they’re still in the process of making the paint more durable and better at resisting dirt. Part of why the paint doesn’t reflect even more sunlight is because a higher concentration of barium sulfate particles could lead to the paint cracking and peeling more easily.
Research done in 2016 by UC Davis researchers, in collaboration with researchers in China, on the durability and “life cycle” of reflective coatings for pavement — though not specifically Purdue’s ultra-white paint — found that to be effective, reflective coatings may have to be reapplied anywhere from every few months to every few years. However, this may be different for the final paint composition that Purdue releases to markets.
Even if the paint sticks, some studies have found reflective materials to have unintended side effects. Research in Los Angeles in 2019 found that city roads covered in various reflective coatings did their job as advertised in cooling the ground. But by reflecting the sunlight upward, rather than absorbing it as regular pavement does, the pedestrians walking on the streets took in some of the heat themselves, making them feel warmer.
And as far as the cooling paint’s effect on the lives of homeless people in Sacramento who are living in tents and are concerned about hot pavement, Barber said there are even more factors to take into account.
“The paint works due to its large reflectivity,” Barber said. “Because of that, it can only make a huge difference when there is something to reflect — say, sunlight. If someone places a tent on (painted asphalt), they will neglect the ability of that particular area to cool down below surrounding temperatures, since the surface is no longer receiving light to reflect.”
Still, she noted that the force of heat transfer would likely still make the ground underneath tents feel cooler because of the cooler surrounding ground.
“I wish there was a clear answer,” Barber said. “It varies case-by-case, but it all depends on the science that makes the paint possible.”
A lot of the answers researchers have right now to the question of whether the paint would be a net benefit to cities, according to Barber, are educated guesses. Although they know a lot about the paint and how it works, there are certain things we won’t know until the paint is tested in a more large-scale way out of the lab.
But their goals really are that broad — to someday have global application. While urban heat solutions are a viable application of Purdue’s research, they initially pursued the idea with the goal of combating climate change.
Paint Your Roof, Solve Climate Change?
The way Jeremy Munday, a UC Davis professor who researches clean energy, put it, the Earth both absorbs and emits heat. The Earth is currently heating up because greenhouse gasses like carbon dioxide trap heat in Earth’s atmosphere, causing it to take in more energy than it is able to send back out to space.
“If the Earth absorbed less energy or emitted more energy, it would cool down,” Munday said. “So can we get the earth to emit more energy, so that it can start cooling itself down, or at least not heat up anymore?”
That’s the question that Purdue’s lab, along with other researchers across the world, are trying to answer.
We can theoretically stop the Earth from heating up, Munday said. But in order to do so, he estimates that we would have to coat 1-2 percent of the Earth’s surface in ultra-white paint — or in other words, approximately the entire United States: cornfields, Rocky Mountains, Tower Bridge and all. And as long as carbon dioxide levels are rising, we’d need to paint more and more surfaces with the reflective paint in order to keep up with the amount of energy or heat Earth is absorbing.
It’s not exactly a practical plan to execute. And even if it was, Munday warned that if certain regions suddenly became a couple of degrees cooler, there may be some hard-to-predict effect on weather patterns and precipitation. So what would a practical application of this new material look like?
A reasonable place to start, Munday said, would be to cover rooftops with the new paint, and then begin to consider other surfaces where a large amount of sunlight commonly reflects, and cover those.
Reflective paint, or any other super-reflective surface, won’t remove carbon dioxide from the atmosphere — that was never what they were intended to do. Munday said he sees that as almost a separate problem from the one that Purdue’s paint takes on.
“The real solution is that we have to stop burning fossil fuels,” Munday said. “And we probably need to get rid of the CO2 that we’ve already put up in the atmosphere, too. In the meantime, we need something as a short-term fix.”
Even if reflective cooling via paint is employed as a solution by cities like Sacramento in the future, it might be just one of a combination of tools used to beat the heat, Munday said. But one of the main benefits the paint offers is that if it doesn’t work, or has unexpected side effects, there’s a simple solution: “Paint your roof back to a different color.”
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