The Urban Heat Island, explained: Why are cities warmer than the countryside?

A child plays cools down in a fountain in Brive-la-Gaillarde, southwestern France, during last summer's heatwave. Image: Getty.

In cities, the air, surface and soil temperatures are almost always warmer than in rural areas. This effect is known as the Urban Heat Island – a term which first came into use in the mid-20th century.

Until the 1980s, this effect was considered to have relatively little practical significance. In fact, given that most studies were done in cities with cold winter climates, a warmer temperature was seen as a potential benefit, because it reduced the need for heating. But since then, we’ve found a number of reasons to be concerned.

For one thing, it became clear that the Urban Heat Island (UHI) effect of cities was influencing air temperature records, which are used to assess climate change. In other words, it became important to remove urban “contamination” from weather station records to ensure their accuracy.


What’s more, as populations in warm and hot cities have increased, so too has the demand for indoor cooling – typically met by air conditioning. This even applies in colder climates, where changing building uses has increased the demand for cooling; for example, in office buildings, to offset the heat generated by computers.

In these situations, the UHI adds to the heating burden: ironically, cooling buildings with air conditioners increases outdoor air temperatures.

Heatwaves have the power to kill; for example, during the 2003 heatwave in Europe, 70,000 additional deaths were recorded, making it one of the region’s deadliest natural disasters of the last 100 years. The UHI makes city dwellers more vulnerable to the dangerous effects of extreme weather events like this.

The potential medical impact is perhaps the most significant issue related to UHI, especially against the backdrop of continued climate change and global warming. For all these reasons, it’s crucial to understand how the UHI works, so that we can find ways to mitigate and adapt to its effects.

Understanding the UHI

The UHI is strongest during dry periods, when the weather is calm and skies are clear. These conditions accentuate the differences between urban and rural landscapes.

Cities are distinguished from natural landscapes by their form: that is, the extent of the urban land cover, the construction materials used, and the geometry of buildings and streets. All of these factors affect the exchanges of natural energy at ground level.

Much of the urban landscape is paved and devoid of vegetation. This means that there is usually little water available for evaporation, so most available natural energy is used to warm surfaces. Construction materials are dense, and many – particularly dark-coloured surfaces like asphalt – are good at absorbing and storing solar radiation.

The urban jungle. Image: mdalmul/Flickr/creative commons.

Meanwhile, the shape and positioning of buildings in the city slows the movement of air near the ground, creates complex patterns of shade and sunlight and limits natural energy exchanges. Urbanisation is also associated with the emission of waste heat from industry, transport and buildings, which contributes directly to the UHI.

There are, however, different types of UHIs, with different dominant causes.

Keeping our cool

“Surface UHI” refers, unsurprisingly, to warmer urban temperatures at the Earth’s surface. Typically, this type of UHI is measured using satellites with a plan view of the city, so that the temperature of roofs and roads (but not walls) can be measured. From this perspective, the surface UHI is highest during the daytime, when hard urban surfaces receive solar radiation and warm quickly.

Another type of UHI is based on observations of air temperature, which are made close to the ground; in the city, this means placing the instruments below roof height.

This UHI is usually strongest at night, as street surfaces and the adjacent air cool slowly. Above the roof level, the contributions of streets and building roofs together warm the overlying urban atmosphere. In some conditions, this warming can be detected up to 1km to 2km above the surface.

The geography of the Urban Heat Island. Image: Jamie Voogt/University of Western Ontario/Author provided.

The geography of the UHI is relatively simple – it’s magnitude generally increases from the urban outskirts towards the city centre. However, it also contains many micro-climates – for example, parks and green spaces appear as cool spaces.

The UHI is an inevitable outcome of the landscape changes that accompany urbanisation. But its magnitude and impacts can be managed by modifying some physical aspects of our cities. This can include increasing vegetative cover and reducing impermeable cover; using lighter coloured materials, designing urban layouts to allow for better ventilation through the streets and buildings, and managing urban energy use.

Of course, these solutions need to be tailored to the type of UHI. For example, a focus on building cool or green roofs will have an impact on the overlying air and the top floor of buildings, but may have little impact on the UHI at street level. Similarly, trees may be an effective means of providing street shade, but if the canopy encloses the street, then it can trap traffic emissions, resulting in poor air quality.

As a first step, many cities have completed UHI studies to identify the “hot-spots”, where design interventions could have greatest effect. But what most cities need is a coherent climate plan, which addresses interrelated environmental issues including flooding and air quality, as well as surface and air temperatures. The Conversation

Gerald Mills is senior lecturer in geography at University College Dublin.

This article was originally published on The Conversation. Read the original article.


 

 
 
 
 

In South Africa's cities, evictions are happening despite a national ban

An aerial view shows a destroyed house in Lawley, south of Johannesburg, on April 20, 2020. The city has been demolishing informal structures on vacant land despite a moratorium on evictions. (Marco Longari/AFP via Getty Images)

On the morning of 15 July, a South African High Court judge ruled that the city of Cape Town’s Anti-Land Invasion Unit had illegally evicted a man when it destroyed the shack where he was living.

That afternoon, the Anti-Land Invasion Unit was out again, removing shacks in another informal settlement.

Evictions were banned in South Africa for nine weeks, after the national government placed the country under a strict Covid-19 lockdown in late March. At present, eviction orders are automatically suspended until the country moves to a lower “alert level” and can only be carried out with a special order from a judge.

Yet major cities including Cape Town, Johannesburg and eThekwini (created through the merger of Durban with several surrounding communities), have continued to use municipal law enforcement agencies and private security companies to remove people from informal housing. In many cases those operations have been conducted without a court order – something required under regular South African law.

Around 900 people were evicted from three informal settlements in eThekwini during the eviction ban, according to the Church Land Programme, a local NGO. Its director, Graham Philpott, says it’s also aware of evictions in other informal settlements.

While evictions aren’t a “new experience” in these communities, the NGO released a report on lockdown evictions because they were “so explicitly illegal”. “There was a moratorium in place,” Philpott says, “and the local municipality acted quite flagrantly against it. There’s no confusion, there’s no doubt whatsoever, it is illegal. But it is part of a trend where the eThekwini municipality has acted illegally in evicting the poor from informal settlements.”

Evictions also took place in Cape Town and Johannesburg during so-called “hard lockdown” according to local activists. In eThekwini and other municipalities, the evictions have continued despite restrictions. In Cape Town, authorities pulled a naked man, Bulelani Qholani, from his shack. That incident, which was captured on video, drew condemnation from the national government and four members of the Anti-Land Invasion unit were suspended. 


The cities say they’re fighting “land invasions” – illegal occupations without permission from the land owner.

“Land invasions derail housing and service projects, lead to the pollution of waterways, severely prejudice deserving housing beneficiaries and cause property owners to lose their investments over night,” Cape Town’s executive mayor, Dan Plato said in a statement. (Plato has also claimed that Qholani did not live in the shack he was pulled from and that he disrobed when municipal authorities arrived.)

South African municipalities often claim that the shacks they destroy are unoccupied. 

If they were occupied, says Msawakhe Mayisela, a spokesman for the eThekwini municipality, the city would get a court order before conducting an eviction. “Everything we’re doing is within the ambit of the law,” Mayisela says. But “rogue elements” are taking advantage of Covid-19, he added.

“We fully understand that people are desperately in need of land, but the number of people that are flocking to the cities is too much, the city won’t be able to provide housing or accommodation for everyone overnight,” he says. 

While eThekwini claims to be a caring city, local activists say the evictions show otherwise.

In one case, 29 women were evicted from shacks during the hard lockdown. With nowhere to go, they slept in an open field and were arrested by the South African Police Service for violating the lockdown, Philpott says.

“These evictions are dehumanizing people whose dignity is already compromised in many ways,” says S’bu Zikode, the president of Abahlali baseMjondolo, a community organization whose Zulu name translates to “the people of the shacks”. 

“It has reminded us that we are the people that do not count in our society.”

Municipal law enforcement and private security contractors hired by cities regularly fire rubber bullets, or even live ammunition, at residents during evictions. Some 18 Abahlali baseMjondolo activists have been killed since the organization was founded in 2005, Zikode says, most by the eThekwini Land Invasion Unit and Metro Police.

(Mayisela says that if city employees have broken the law, Abahlali baseMjondolo can file a complaint with the police. “There is no conclusive evidence to the effect that our members have killed them,”  he says.)

Other Abahlali baseMjondolo activists have been killed by what Zikode calls “izinkabi,” hitmen hired by politicians. Two eThekwini city councillors were sentenced to life in prison 2016 after they organized the killing of Thuli Ndlovu, an Abahlali baseMjondolo organizer. A member of the Land Invasion Unit who is currently facing a charge of attempted murder after severely injuring a person during an eviction remains on the job, Zikode says.

South Africa’s 1996 constitution is intended to protect the public from arbitrary state violence and guarantees a right to housing, as well as due process in evictions. But for Zikode, the South African constitution is a “beautiful document on a shelf”.

“For the working class and the poor, it’s still difficult to have access to court. You’ve got to have money to get to court,” he says. 

The actions by municipal law enforcement are breaking down social trust, says Buhle Booi, a member of the Khayelitsha Community Action Network, a community group in the largest township in Cape Town.

“There’s a lack of police resources and those very few police resources that they have, they use to destroy people’s homes, to destroy people’s peace, rather than fighting crime, real criminal elements that we see in our society,” Booi says.

For him, it’s a continuation of the practices of the colonial and apartheid governments, pushing poor people, most of whom are Black, to the periphery of cities.

Around one-fifth of South Africa’s urban population live in shacks or informal dwellings, according to a 2018 report by SERI. Many more live in substandard housing. City governments maintain that the shacks destroyed during anti-land invasion operations are unfinished and unoccupied. But Edward Molopi, a research and advocacy officer at SERI, says that this claim is an attempt to escape their legal obligations to get a court order and to find alternative accommodation for affected people. 

The roots of the current eviction crisis go back to apartheid, which barred non-white people from living in cities. Between the 1940s and 1970s, tens of thousands of people were forcibly relocated from neighbourhoods like Johannesburg’s Sophiatown and Cape Town’s District Six to remote townships.

In the 26 years following the end of apartheid, deepening economic inequality and rampant unemployment have limited access to formal housing for millions of South Africans. Government housing programs have mostly focused on building small stand-alone homes, often on the peripheries of cities far from jobs and amenities.

While these well-intentioned projects have built millions of homes, they’ve failed to keep up with demand, says Marie Huchzermeyer, a professor at the Centre for Urbanism & Built Environment Studies at the University of the Witwatersrand in Johannesburg. Government-funded housing projects “will never on it’s own be enough,” she says. “It has to be accompanied by land release.”

Government policies call for the “upgrading” of informal settlements and the formalization of residents’ occupation. But “there are still very, very, very few projects” of that nature in South Africa, Huchzermeyer says. “Even if it’s an informal settlement that’s been around for 20 years, there still seems to be a political wish to punish people for having done that.” The government wants people to go through the formal process of being given a house, she says – and for them to be thankful to the government for providing it.

At the municipal level, change will require “real leadership around informal settlement upgrading and around ensuring that land is available for people to occupy,” she says. 

Despite the end of enforced racial segregation, spacial apartheid remains a factor in South Africa. There are few mixed-income neighbourhoods. Those who can afford to often live behind walls in sprawling low-density suburbs, while the poor live in overcrowded slums and apartment buildings.

The creation of the apartheid city “didn't happen by chance,” says Amira Osman, a professor of architecture at the Tshwane University of Technology. “It was a deliberate, structured approach to the design of the city. We need a deliberate, structured approach that will undo that.”

Since last fall, Johannesburg’s Inclusionary Housing Policy has required developments of 20 or more units to set aside 30% of those units for low-income housing.

The policy, which faced significant opposition from private developers, won’t lead to dramatic change, says Sarah Charlton, a professor at the Centre for Urbanism and Built Environment Studies, but it is “an important and significant step.”

Zikode isn’t optimistic that change will come for shack dwellers, however.

“People in the high positions of authority pretend that everything is normal,” he says. “They pretend that everyone is treated justly, they pretend that everyone has homes with running water, that everyone has a piece of land – and hide the truth and the lies of our democracy.”

Jacob Serebrin is a freelance journalist currently based in Johannesburg. Follow him on Twitter.