If you take a trip from rural Virginia into Washington, D.C., you may notice a change in atmosphere—literally.
Scientists say the “urban heat island” effect, which can raise temperatures in cities by several degrees compared with their surroundings, could be a risk to human health as growing urban populations exacerbate the heating effects of climate change.
According to a study published last week in Environmental Research Letters, the urban heating effect may cause extra warming in many cities, on top of the warming already caused by climate change. The researchers made their estimates using a statistical model based on projections for future urban expansion.
The study suggests that, on average globally, urban heat island warming will probably be equivalent to about half the warming caused by climate change by the year 2050. In a city that experiences 2 degrees of warming from climate change, for instance, that would mean an extra degree of warming.
In some locations, the study warns, the effect could be twice as strong as the impact of global warming.
According to lead study author Kangning Huang, a doctoral student at Yale University, the urban heat island effect isn’t included in the regional warming projections produced by the Intergovernmental Panel on Climate Change. But this effect may make a big difference in the amount of warming that some cities experience.
That means it should be factored into future adaptation strategies, Huang said.
“That’s the reason we conducted this research,” he told E&E News. “It’s to try to complement the projections in the IPCC report for climate change, so we’ll have a full picture of the warming that will affect about two-thirds of the population. By 2050, about two-thirds of the world population will live in an urban area.”
It’s not the first study to suggest that urban heat islands may add to the effects of climate change.
A 2017 paper in Nature Climate Change evaluated the economic risks of climate change in large cities around the world. It found that local warming, caused by the urban heat island effect, “significantly increases temperatures as well as economic losses in addition to global warming.” In fact, the study estimated that the added effects of urban heat island warming could double the economic losses expected from human-caused climate change.
Different cities—and solutions
Although these findings seem alarming, the consequences of urban heat islands are more serious in some places than in others.
The urban heating effect is caused by a variety of factors—and not always the same factors in every city. Research suggests that the phenomenon is broadly the result of less evapotranspiration—that’s when water evaporates from plants or soil into the atmosphere, cooling the air—in cities, compared with their surrounding environments. But other influences, from the density of the city to the types of building materials and urban designs it incorporates, can also play a role.
That means every city will experience different heating effects.
How much it affects a city’s residents also varies from place to place. Studies have shown that urban heat islands tend to be hotter compared with their surroundings in colder parts of the world. But in cold climates, a warmer city isn’t always a major risk to its inhabitants, notes Scott Krayenhoff, an urban climate expert at the University of Guelph in Canada.
In general, he says the influence of heat waves in cities is probably a bigger challenge than the urban heat island effect. Heat waves are becoming more frequent, more intense and a bigger risk to human health, he pointed out. Even in cooler regions of the world, summer heat waves can cause thousands of deaths.
As populations around the world continue to grow, more and more people may be exposed to these dangerous weather events.
The authors of the new study acknowledge that urban heat islands don’t pose the same risks all over the world. But they may be a big concern in some regions.
In particular, the authors point to cities in temperate and tropical parts of the global south, including parts of Latin America, Africa and South Asia, where absolute temperatures tend to be higher and where many nations have smaller economies and more difficulties adapting to the warming climate. The researchers estimate that about half the world’s urban population will live in these regions by 2050.
There are a variety of ways urban planners can attempt to lower city temperatures, Huang noted—creating more green spaces or building reflective rooftops that beam sunlight away. But there isn’t a one-size-fits-all approach, and not every solution is appropriate for every city.
Green spaces, for instance, can be a challenge to maintain in water-scarce areas. And in colder climates, reflective surfaces may make temperatures more bearable in the summer, but can also make for even colder winters, increasing the amount of energy that residents use for heat.
Sometimes there are trade-offs to consider when it comes to fighting climate change and fighting urban heat, Huang added. Denser cities are often more carbon-efficient and climate-friendly. But they also tend to be hotter.
“I think in order to find the right balance, we need to consider both factors to determine what kind of cities we want to build in the future,” Huang said. “I think my point is that the issue is more complicated than just about increasing density and increasing green space. There are a lot of factors we need to consider in the future and how to build future cities.”