Why are cities so much hotter than the surrounding areas?

Scorchio! London. Image: pic fix/Flickr/creative commons.

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.

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 Ontari/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.

Laying a cool roof. Image: NNSANews/Flickr/creative commons.

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.

The ConversationAs 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.

Gerald Mills, Senior Lecturer in Geography, University College Dublin.

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

 
 
 
 

How collecting food waste could slow climate change – and save us money

Cleaning up. Image: Getty.

Food waste is a global problem, and one that’s driving climate change. Here in the UK, the country’s biodegradable waste goes to a landfill, where it breaks down to produce methane, a gas that is roughly 30 times as bad as carbon dioxide.

And yet there’s a simple solution. With the exception of garden waste, which often contains lignin from woody matter, all biodegradable materials, including much of our food waste, could instead be processed in anaerobic digesters. This decomposition in an atmosphere devoid of oxygen produces biogas, which can then be used to generate heat and electricity.

This is more or less the same process that takes place in landfill sites, except that the biogas can’t escape from an anaerobic digester as it can from landfill – meaning the breakdown of the organic matter takes place in an environment that is enclosed and controlled.

The result is biogas consisting of 60 per cent methane and 40 per cent carbon dioxide, which can be burnt in order to generate heat or used as a fuel for vehicles. It could also be used to generate electricity after the biogas has been scrubbed, which can then either power the anaerobic digester or be exported to the national grid. The process also produces digestate, a solid and liquid residue that can be returned to farmland as a soil conditioner. The amount of biogas and the quality of digestate varies according to what feedstock is used in the digester.

This process is already widely used both across Europe – particularly in Denmark, Sweden, Germany and Austria – and elsewhere globally, particularly in India and Thailand. What's more, this move towards separate food waste collection is already happening in countries outside the UK, and its momentum is increasing according to the World Biogas Association. Already, major cities, including New York, Paris, Oslo, Copenhagen, Auckland, San Francisco, Mexico City and many others are regularly collecting food waste from their citizens. The decisions to do so are usually taken at city level, but enabling legislation from national governments assist in this.


At present the UK is lagging behind. Only 109 local authorities in England, about 33 per cent of the total number, collect food waste as of May 2018, according to the Anaerobic Digestion and Bioresources Association (ADBA). Yet making a separate food waste collection mandatory across the UK and running the food waste through anaerobic digesters, could supply enough biogas to generate 36 per cent of UK electricity, according to a 2007 Friends of the Earth report. This percentage could be increased again if food waste from restaurants, cafeteria and retailers was also collected. 

ADBA’s research also suggests that universal separate household food waste collections would trigger the construction of around 80 new anaerobic digester plants for food waste processing. This would add an extra 187 megawatts equivalent (MWe) of capacity, powering 285,000 extra homes – representing all the homes in a city the size of Glasgow. Data from WRAP suggests that further food waste collection from businesses would add around a further 10 per cent, depending on the quality of the feedstock collected and what exemptions were applied (for example, it might only apply to businesses collecting more than 50 kilograms per week or the lower threshold of 5kg).

A 38 per cent improvement in food waste collection from flats in Ealing alone could generate £26,000 of annual savings for the London borough, £28,000 in revenue for a local anaerobic digester (based on electricity sales to the national grid) and reductions in carbon dioxide emissions of around 270 tonnes, found Londoners Lab, a collaborative project consisting of Greater London Authority, University College London, Ferrovial Services Centre of Excellence for Cities and Future Cities Catapult.

ADBA has been campaigning on this issue for a while, but the good news is that the government finally signalled its intention to introduce separate food waste collections in its forthcoming Resources & Waste Strategy, which will ensure that all homes and suitable businesses in England will have access to food waste collections by 2023. The next step, following the government announcement, is a consultation, but it is widely acknowledged that additional funding would be needed by local authorities to achieve this, as the business case isn’t currently strong enough.