As global warming threatens transportation systems and other vital infrastructure, engineers are developing cool countermeasures.
By Thomas K. Grose
The most dire predictions of climate change effects focus on the coming decades—but global warming has already begun to take its toll. Extreme heat wreaked havoc on transportation systems worldwide this past summer, especially in Europe. Roads softened, highways buckled, tracks deformed. In July alone, Britain’s Luton Airport closed a runway after high temps caused a panel to lift; a main train line between London and Edinburgh shut down for hours due to overheated rails; and long delays on the Amtrak service between New York and Philadelphia resulted from heat-related speed restrictions.
Transportation networks aren’t the only systems at risk. Cities are plagued by urban heat islands as pavements and buildings soak up sunlight and warm the surrounding air. Temperatures in urban hot spots can be up to 7ºF higher in daytime and 5ºF higher at night than outlying regions, which contributes to heat-related deaths and illnesses. Intolerable temperatures are also causing surges in demand for air conditioning. The International Energy Agency estimates AC use could triple by 2050, causing “one of the most critical energy issues of our time.” AC and electric fans constitute 10 percent of global electricity consumption.
Engineers are tackling the problems. But solutions often come at a cost or with unanticipated effects.
Cracks and Buckles
In the US, about 95 percent of all roads are either fully or partly paved with asphalt, so how the material handles temperature extremes matters greatly. Media reports last summer often referred to “melting” asphalt. “Melting is not a bad analogy, but it’s not exactly what happens,” explains Jo Sias, a civil and environmental engineering professor at the University of New Hampshire who is an expert on pavements and asphalt. “The hotter it gets, the softer it gets.” Asphalt is a relatively stable material, but once it gets too hot, roads can become bumpy and rutted—increasing the likelihood of hydroplaning. “Once it ruts, it can’t un-rut; you’ve got to go out and fix it,” Sias says, which increases highway maintenance expenses. Just as too much sunlight can rapidly age skin, higher temperatures speed the aging process of asphalt, “which makes it brittle, so it cracks easier.”
The standard formulation for asphalt is a mix of coarse and fine aggregates bound with bitumen, the by-product of refining crude oil into petroleum. Different grades have been developed for different temperatures. “You choose the right material and structure for the conditions you expect to have,” Sias says. “Traditionally, highways were designed using historical data, which may not be the best approach anymore,” given the onset of extreme hot spells. But changing an asphalt formula to accommodate soaring mercury can be pricey, she notes. Highway departments will have to weigh the risk of an occasional extreme event against that cost. “It might happen, but is it worth spending extra to account for it?”
Concrete won’t melt under a blazing sun, but it’s not impervious to extreme heat. Buckling is the main problem, says Steve Muench, a professor of civil and environmental engineering at the University of Washington who studies transportation infrastructure. Buckling, he points out, “happens when temperatures are significantly hotter than normal.” Concrete expands when it gets hot and contracts when it cools. The joints between paving slabs are designed to accommodate that movement. But, he says, over time debris can clog a joint because its sealing has deteriorated, leaving no space for expansion. “That causes the pavement to pop up at that joint.” The buckling can range from slight bumps to foot-high protuberances.
Joints are typically spaced 16 feet apart. If you shrink that down to 10 feet, Muench explains, the slabs will expand and contract less. But changing a concrete highway’s design to better withstand higher temperatures “will cost you more”; use of additional joints requires more of the steel dowel bars inserted into them.
Like asphalt roads, concrete highways have been designed using historical temperature data. “But historical data is not the best predictor for what could happen in the future,” Muench says. “We need to stop using that and rely more on predictive models to enhance the design process.”
Stressed Out
Steel rails also expand under heat—but the rail ties restrict that movement, explains Kangkang Tang, a senior lecturer in civil and environmental engineering at London’s Brunel University and a railway construction researcher. So stress builds up and compression occurs, causing the rails to buckle or bend. When train drivers see misshapen rails, Tang says, they understandably hit the brakes. Hard. “That applies extra compression to the track, which makes matters worse.” Tang suspects sudden braking is the cause of most derailments.
Track designers rely on a benchmark called stress-free temperature (SFT), which is based on an area’s historical temperatures and involves pre-stressing the steel to set the level of heat it can tolerate. The higher the SFT, the more heat the rails can withstand. Britain’s SFT is 27ºC (80.6ºF). “At this temperature, there’s basically no tension, no compression,” Tang says. But several times last summer temperatures in parts of the UK rose to near or just over 40ºC (104ºF). Using a higher SFT is certainly an option; it’s what rail networks in warmer climes do. But in countries and regions that experience cold winters, there’s a catch. Setting the benchmark higher allows the steel to remain trouble-free in the summer. But, Tang says, “in the winter, when the temperatures drop, this can release a huge force that can shear off the anchorage,” or the steel clips used to fasten rails to sleepers (crossties in the US).
Most of the fixes to protect steel rails from hot weather are low-tech and temporary. White paint can be applied—but it quickly wears off—or tracks can be sprayed with cooling water. Reducing both the number of trains in service and their speeds until the heat abates also can provide relief.
Operators, however, are getting some high-tech help. Network Rail, which owns and maintains most of the UK’s rail lines, plans to use data gleaned from sensors embedded along routes to create digital twins of train lines. Operators would use the digital representations of rail networks to simulate what would happen to tracks under varying temperature conditions, then use those predictions to take action—for example, applying white paint or rerouting traffic—in advance of heat waves. “This digital re-creation,” Tang says, “will allow us to plan for the worst.”
Lighten Up
Additional tech solutions are under development to combat urban heat islands. Cool-pavement technologies are cousins to white roofs and walls that reflect solar heat into the upper atmosphere. The solutions lighten paved surfaces either by covering them with a polymer-based coating or using light-colored aggregates in the mix (usually concrete). Paving materials can also be made to be porous so the ground below soaks up more rain water, which cools the pavement when it evaporates. But as Ronnen Levinson, head of the Heat Island Group at the US Department of Energy’s Lawrence Berkeley National Laboratory, says, porous pavement is “only good when it’s someplace that gets rain in the summer.”
Dark paving materials soak up most of the sunlight that strikes them, then reemit it into the air as heat. Making the pavement more reflective lowers both its temperature and that of the outside air, Levinson notes. According to an MIT study, cool pavement can reduce the surrounding air temperatures by more than 2.5ºF, easing the need for AC.
However, location matters. If cool pavement is close to buildings—think narrow urban streets—the reflected light can increase temperatures in those structures. One solution, Levinson says, is to make the exterior walls of those buildings white, too, so they also reflect the sunlight. However, he adds, houses in many neighborhoods are set back from pavements, so it’s not a major concern.
Levinson was part of a 2017 study that discovered another problem. Over a 50-year lifespan, the extra energy and emissions linked to manufacturing, installing, and disposing of cool pavement were often greater than the amounts of energy and emissions curbed. According to Levinson, the discrepancy may be more of an accounting issue than an environmental one. Because asphalt is a by-product of oil refining, it doesn’t count toward the embedded carbon or energy. But the energy used and the carbon released to manufacture polymer coatings are factored in. Employing cement, which is carbon-intensive, as a binder to lighten roads can create a “real issue, but it’s not an insurmountable obstacle,” Levinson says. Instead of cement, researchers are developing binders from two other by-products—slag and fly ash—and getting superior results, because they are even more reflective.
Cool Tech
White roofs have grown in popularity in recent decades. Lighter building materials reflect sunlight, so buildings absorb less solar heat, which keeps their inside temperatures cooler.
While most heat bounces off the Earth’s atmosphere and eventually returns to the ground, infrared radiation between the wavelengths of 8 and 13 micrometers can pass through the atmosphere like water through a sieve and journey out into space. “The atmosphere of the Earth is opaque, but it’s only opaque to visible light,” explains Yi Zheng, an expert on nanoscale thermal transport and an associate professor of industrial and mechanical engineering at Northeastern University. “It’s totally transparent to the infrared light.”
This effect, deemed passive radiative cooling, occurs naturally at nighttime. But in the daytime, it’s stymied by the sun, which creates more heat than can be shot into space. Researchers have in recent years made breakthroughs in materials that can make radiative cooling work during the day, even under direct sunlight. The technologies are particularly efficient at absorbing the infrared light that falls within the 8–13 micrometer wavelength and releasing it into space.
Among them: a “cooling paper” Zheng invented. Applied to the roof of a building, it can drop the inside temperature 10ºF lower than the outside temperature. The paper not only reflects sunlight away from the building but also siphons heat from the building’s interior “and radiates it out into the universe.” Zheng says the technology can reduce the need for AC and cut electricity costs by 20–25 percent. In developing countries, where electricity is often unreliable or nonexistent, it can be used in lieu of AC.
Cooling “paper” is a bit of a misnomer, however. The first iteration was a material made from paper and coated with Teflon. But concerns arose that it would be flammable, and it wasn’t terribly robust. The new version is a sturdy paperlike material spun out of hydroxyapatite, an inorganic fiber used for bone repairs and in toothpaste. It’s fireproof and hydrophobic, so it’s self-cleaning. Zheng says it’s also easy to fabricate, entirely recyclable, and “not expensive at all.” He’s created a spin-off company, Planck Energies, but his invention isn’t quite market-ready. Zheng’s still working on a way to turn off its cooling effect when outdoor temperatures drop. He’s optimistic he’s close to a solution to infuse the paper with phase-change microparticles that will act like an off switch, so when outside temperatures turn colder, the paper stops cooling and insulates instead.
‘Life Support’
Countries are enacting a multitude of strategies to mitigate climate change, but whether current steps will be enough to avoid devastating effects is increasingly uncertain. As United Nations Secretary-General António Guterres told attendees of the Economist Sustainability Summit in March, the most ambitious goal of the Paris agreement—to keep global warming below 1.5° Celsius compared with pre-industrial levels—“is on life support. It is in intensive care.” Even the less ambitious 2°C target may be difficult to reach, he said, warning of “climate catastrophe” without more urgent action.
Even if the international community meets one of those targets, we’ll still be living in a hotter world—last year’s Global Climate Report from the National Oceanic and Atmospheric Administration measured temperatures at 0.84°C (1.51°F) above the 20th-century average. With global warming come extreme heatwaves, frequent droughts, and stronger storms, all of which will continue to affect aspects of our lives from transportation to housing. Engineers have already developed some technologies to help us cope with these changes. But as the Earth’s temperature continues to rise, it’s clear that their efforts have only just begun.
Thomas K. Grose is Prism’s chief correspondent.
Design by Toni Rigolosi