Cities, or thousands of years, have thrived by attracting people with diverse backgrounds and talents. But as climate change slowly warms up our planet, urban areas are attracting something less desirable: heat.
Roads, buildings, pavements, car parks and other urban infrastructure absorb large amounts of heat energy from the sun. This drastically raises temperatures way above those in surrounding rural areas which are less developed and have more greenery such as grasslands, trees and waterways.
This “urban heat island effect” may vary widely, not only from neighbourhood to neighbourhood, but from city block to city block and even house to house. Due to the erratic nature of the phenomenon, it is difficult to factor into predictions as a person’s actual experience of heat may not be in line with the local weather forecast. And with climate change, it’s increasingly difficult to keep vulnerable, rapidly growing populations safe during extreme heat events.
Wired’s Matt Simon spoke to Jonathan Porter, chief meteorologist at AccuWeather, the online weather forecasting service who said that he often refers to heat as being a “silent killer” and that “it doesn't get the headlines that tornadoes, flooding, and blizzards get.”
“We think heat-illness deaths are woefully under-counted because people often times have other medical conditions that yes, may have contributed to or caused their death, but it was aggravated by a heat situation,” Porter said.
Predicting extreme heat is key in assisting public health officials in preparing. But weather forecasts can only go so far. AccuWeather, for instance, mashes together over 170 forecast models that are run by meteorologists with decades of experience.
They can partially account for the heat island effect by incorporating land use data that shows which parts of a city have vegetation, which helps cool things off. That’s because plants “sweat” as they photosynthesise, releasing water vapour and cooling the air.
Heavily industrialised areas, on the other hand, tend to be devoid of vegetation. “You've got a lot of surfaces that are basically just re-radiating heat, a lot of concrete and particularly asphalt because it's dark,” says Carl Parker, a meteorologist and climate specialist.
“Then on top of that, you've got these artificial sources of heat, including air conditioning units, and then cars that are also putting excess heat into the atmosphere.”
Other extra-granular factors contribute to making hotspots hard to predict. For example, different kinds of building materials, like brick or wood, absorb and release heat differently. And tall buildings block winds that would otherwise cool a landscape. Weather models consider wind generally, but there’s no way for them to model how air moves between every building in a city.
There is also the fact that you, standing on a street, will experience heat differently than a weather station would. These stations include sensors for temperature, wind, and pressure, among other indicators, and are usually located at airports, military bases or research stations far from urban centres in South Africa.
“The standard for taking temperature measurements, believe it or not, is in the shade,” says Porter.
“Those are all temperatures that are being taken outside of the direct impact of sunlight, which understandably is done that way, because depending upon the time of the day, that can have a huge impact on what the temperature is,” he says.
In order to create a more representative value of what it actually feels like outside, AccuWeather produces a metric called RealFeel. It uses an algorithm which incorporates factors, like cloud cover, the angle of the sun, wind speed and humidity. Factoring in humidity does get complicated. The wetter the air, the less efficiently we perspire, which is why 25°C in Durban would feel much hotter than 25°C in Pretoria.
According to Edith de Guzman, director and co-founder of the Los Angeles Urban Cooling Collaborative at the University of California, “it is vital to know how heat actually feels because people forced to be outside such as construction workers, delivery workers, the homeless and city workers are among those most at risk of heat illness or death.”
If public health officials were aware that a given neighbourhood is due to experience particularly hot weather because of the heat island effect, they can focus resources there such as handing out water and getting people to cooling centres, which are typically public buildings with air conditioning such as libraries or community halls.
The heat island effect continues even when the sun goes down. This is due to the built environment slowly releasing absorbed heat throughout the night, keeping temperatures high.
“The body doesn't even necessarily have an opportunity to just chill out, literally, in the evening,” says de Guzman. “And that, too, is problematic, because it's just a longer period of stress for the body.”
A person’s long-term exposure to heat increases their chances of heat-related illness and death, with children and the elderly being most at risk since their bodies cannot cool themselves as efficiently.
According to the South African Institute for Security Studies, just 11 fatalities have been officially attributed to severe heat in South Africa. Yet researchers studying excess deaths through temperature correlations in South Africa found a very high mortality burden associated with warmer weather. Nearly 30 000 deaths in the country were directly attributable to increased temperatures from 1997 to 2013, pointing to severe under-reporting.
While long-term and climate change adaptation plans acknowledge heat as a significant threat to South Africa, this isn’t reflected in city preparedness or emergency response plans. Risk assessments are based on historical records, and if these are missing, they become almost invisible to disaster managers.
Low-income neighbourhoods, as usual, will bear the brunt of increased urban temperatures as these often lack air conditioning to bring cool air in and proper insulation to keep warm air out.
These neighbourhoods also have less tree cover, which, in wealthier neighbourhoods, helps attenuate the heat.
A 2021 study that strengthens the case for tree planting to help cities adapt to global warming showed that the cooling effect of trees reduced surface temperatures of European cities in the summer by up to 12°C with green spaces which were lacking trees having a negligible effect.
Perhaps it is time to think of heat as a treatable disease that affects a city, not just its people.
Working with community leaders, cities can develop more green spaces, for example.
Researchers simulated heat events in Los Angeles, with and without strategies in place to mitigate the heat island effect, and calculated what those improvements would mean for human mortality.
“We tried these different dosages, so to speak, of trees plus reflective surfaces,” said de Guzman. “Then through a process that involves creating algorithms and understanding actual mortality numbers that were collected and reported by the state, we're able to see how many of the excess deaths that we saw would have been prevented.”
They found that making simple tweaks to the built environment, primarily adding trees and painting roofs light colours, would save one in four lives that would normally be lost to heat events.
“Even though we're in a very dire situation,” says de Guzman, “and we're seeing changes occurring faster than most scientists even though, there are very practical solutions that we can implement at the local level.”
A better understanding of how heat islands form within urban areas is key to providing mitigating solutions within South Africa’s rapidly growing urban areas. We cannot prevent hurricanes or earthquakes, floods or volcanic eruptions. But we can ensure that both people and communities are better prepared and more resilient.