Evidence shows cities can reduce heat impacts and protect residents
By James Goldie.
Cities across Southeast Asia and Australia are turning to cooler rooftops, green spaces and other innovative solutions in the face of record heatwaves over 2022.
As the year closed, 2022 was equal fourth warmest year on record, despite natural climate drivers like La Niña dragging global temperatures down.
With more than 15,000 dead from heat this year in Europe alone, cities are now grappling with an uncomfortable truth: hotter and longer heatwaves are now a fact of life.
Extreme heat takes more lives in Australia than all other natural hazards combined, and because of the high level of urbanisation, most of those deaths are in cities.
Cities aren’t entirely at the mercy of the climate, though. Evidence shows that a variety of urban planning interventions can not only reduce the impact of extreme heat but benefit cities in other ways.
Switch air conditioning for fans
Air conditioning, the weapon of choice in developed nations, is getting complicated.
“In face of urban warming, air conditioning remains almost the standard response in more developed nations, whatever the weather,” says Wanyu Shih, a researcher from the Ming-Chuan University in Taiwan.
Air conditioners and fans today account “for nearly 20 percent of the total electricity used in buildings around the world,” according to a report by the International Energy Agency. But that figure is set to explode in the coming decades as more people in India, China and Indonesia are able to buy air conditioners.
One way to contain that energy use is to focus on making air conditioners more efficient. But fans could replace air conditioning up to 19 out of 20 hot days in the countries where cooling is needed most.
Climate change and rising household incomes will create the desire and the opportunity for more air-conditioner use. Reducing dependence on air-conditioners to cope with hot weather will require a radical change of mindset – using fundamental principles of human physiology and thermodynamics.
Across the developed world, cooling people is entrenched in the notion that the air that surrounds a hot person must be cooled and kept still. To maintain comfort, the typical air temperature target is between 20 to 23˚C. This ensures the temperature difference between the skin and the surrounding air is sufficient to transfer heat from the body without breaking a sweat.
But heat transfer can also be strongly influenced by the speed at which air passes across the body. Moving warmer air faster can produce the same level of cooling – and thermal comfort – as still, but cooler air. In experiments, people with fans on them have judged air temperature to be 3 to 4℃ cooler than it was.
Read more: All you need to know about heat waves.
Fans require less than 5 percent of the electricity required by air-conditioners. If air is moved using fans, the thermostat of air-conditioning units in buildings can be set at a higher temperature without sacrificing the thermal comfort of the occupants. This simple technique means air-conditioners can be used for shorter durations, turning on later and turning off earlier in the day. On some days, fan-use alone may be sufficient to maintain thermal comfort and air-conditioners may not be needed at all. Current cooling practices might have them running for hours.
In a first investigation of its kind, an Australian case study found that ‘fan-first’ cooling could reduce annual electricity demand from air-conditioning by more than 70 percent. The cost of implementing fan-first cooling relative to the volume of greenhouse gas emissions would yield a net benefit better than the government-supported switch from incandescent to LED home lighting.
Fan-first cooling is also relevant to people trying to keep cool during summer in countries facing energy insecurity due to recent geopolitical events. During the 2022 summer, governments in Italy, Greece, and Spain all mandated minimum thermostat set-point temperatures of around 27˚C to conserve energy and reduce reliance on Russian gas. Combined with greater air-flow the equivalent to a typical pedestal fan on medium, this temperature is still within a comfortable thermal range.
Electric fans offer an affordable and accessible cooling solution for the most vulnerable during extreme heat events, who typically cannot afford air-conditioning. Using more fans and fewer air-conditioners during heatwaves could also help reduce stress on overwhelmed power grids and lower the risk of wide-scale power outages.
Read more: Preparing to beat the heat.
Yet, the effectiveness of fans in extreme heat conditions has been contentious. Major public health agencies including the World Health Organization, the US Centers for Disease Control and the UK National Health Service, currently advise that fans should not be used above 35˚C because they can supposedly “speed the onset” of heat exhaustion. This notion probably arises from the understanding that, when hot, human skin temperature stabilises at around 35˚C. So, when air temperatures rise above this level fans may force more heat into the body. This is the same science behind fan-forced ovens.
However, unlike a roasting turkey, humans sweat. Fans greatly enhance the evaporation of sweat, producing a cooling effect. Body heating will only be accelerated when extra dry heat gain from the fan exceeds extra cooling from sweat evaporation. There is no evidence that the ‘tipping point’ for this is 35˚C.
Human physiologists conducting studies in climate chambers that replicate heatwave conditions have observed the protective effect of fans up to 42˚C in young healthy adults. There is a definite limit, beyond which fans can worsen heat stress, especially when humidity is low and most sweat freely evaporates even in still air. In very hot (47˚C) and dry (less than 15 percent relative humidity) conditions, fans worsen physiological heat strain, with body temperatures rising faster, dehydration accelerated, and the cardiovascular system needing to do more work to prevent blood pressure dropping rapidly.
As people age beyond 65 years, their ability to sweat declines. Since the benefits of fans depend on sweating, the temperatures at which fans are effective become lower with age. The actual temperature at which fans should be turned off because they are harmful depends on humidity. But recent work proposes a simplified fan-use temperature guide irrespective of humidity, to enable effective public health messaging: 39˚C for young adults, 38˚C for older adults, and 37˚C for people on specific kinds of medication (anticholinergics).
Replace roofs to protect homes
Homes in developed nations might decide between fans and air conditioning in a heatwave, but for many people they aren’t even options.
Hot metal roofing amplifies heatwaves in slums, and residents escape by spending more time outside. They also take more baths and showers, placing further stress on drinking water supplies.
Authorities in the Indian city of Ahmedabad are trialling a programme to replace these hot roofs in the slums with cooler alternatives, lowering indoor temperatures by 2–5 degrees Celsius.
Cool cities down with plants
Cities are reaching for another solution beyond the home: plants. Although high-rise ‘sky gardens’ dominate public attention, parks and gardens at ground level also play a massive role in cooling cities.
It takes more than planting a few trees to protect cities, though. Green spaces can’t protect us from extreme heat unless they are resilient to heat themselves, and they need to be sited carefully to have the right effect.
“The trees left to protect the hotter cities of the future will depend on the planning actions we make today,” said Manuel Esperon-Rodriguez and Sally Power of Western Sydney University.
Take advantage of other green space benefits
Climate change is a famously wicked problem beset with cascading failures, but the solutions can also help us in unexpected ways.
Focusing on plants has reduced crime and supported local crafts in the Indonesian city of Surabaya, while Danish researchers have found access to green space improves kids’ mental health later in life.
But green space needs to be within a short walk in order for people to use and benefit from it, cautioned Kristine Engemann, a researcher at Aarhus University. “Sometimes we’re not so good at doing the things that are actually good for us.”
Read more: Feeling the heat.
The co-benefits of green space aren’t just limited to social benefits: they can also help to protect us from other aspects of climate change.
China’s sponge cities, built from porous materials, combat flood risk and urban heat at the same time.
Describing one of these sponge cities, Bao-Jie He of Chongqing University said, “Zhuhai has constructed more than 115 square kilometres of sponge city infrastructure since 2016… The green-blue infrastructure development has been no impediment to the city’s continued growth.”
Options like these are attractive to cities looking to protect residents and take ownership of climate change adaptation. But they only work for so long, Elspeth Oppermann from Ludwid-Maximilians University and Jamie Cross from the University of Edinburgh reported.
“The shade of a tree, appropriate use of fans, or improved ventilation of homes only works as long as outdoor conditions remain survivable. This might not be the case in many regions by 2050.”
For many cities, the race to keep cities cool will come as the world attempts to keep global temperatures below 1.5 degrees Celsius of warming.
Written by James Goldie. Originally published under Creative Commons by 360info.