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.


 

 
 
 
 

Ducks and the City: how birds thrive in urban spaces

A mandarin duck, possibly a distant relative of New York’s Hot Duck. Image: Stephen Jorgenson-Murray.

New York may be well known one of the most diverse, cosmopolitan places on Earth, but the arrival of one East Asian migrant in October 2018 still managed to surprise and delight the city. One lonely male mandarin duck – a gorgeous rust-red duck streaked with white and blue, native to Japan, Korea and East China – somehow found its way to Central Park and settled down on one of the ponds among the mallards and wood ducks to become the media sensation “Hot Duck”. Although not strictly wild in the birdspotting sense as it likely escaped from someone’s collection, the duck lives as free as, well, a bird among the skyscrapers of Manhattan.

A few months later, the mandarin’s native territory was graced by a rare visitor of its own when a European robin ended up in the heart of Beijing. Having shown up just when Britain was falling deeper into political crisis, Chinese birdspotters nicknamed it “Brexit refugee” and raced in from across the country to see what Brits would probably consider an incredibly ordinary bird.

A rash of unusual birds have hit the headlines after landing in cities lately – other recent examples include Melbourne’s “Goth Duck” (a tufted duck, a mainly northern European species never before seen in Australia) and the eagle owl that divebombed bald men in Exeter – but when they do, it’s always their rarity that makes them newsworthy, along with the incongruity of seeing a beautiful wild animal among concrete and litter. Normally cities aren’t home to anything more interesting than a dirty pigeon or a bloodthirsty seagull.

Right?

Moving in

Popular myth says London’s first ring-necked parakeets were released in Carnaby Street by Jimi Hendrix. It’s probably not true, but it’s one hell of a story. Image: Stephen Jorgenson-Murray.

Rome wasn’t built in a day, and nor was any other city. Thousands of years ago, wild birds discovered new opportunities on the edges of the first villages. Today the house sparrow is ubiquitous in just about every urban area in the world, but before the first house was built it lived in the dry grasslands of the Middle East, picking seeds out of the sandy soil. Then humans came along and started farming wheat; and whenever a grain fell from a mill or blew from a market stand, a sparrow was there to pick it up. As the technology of farming spread around the world, sparrows came along, too.

Other birds didn’t come by choice but were dragged in by humans. Thousands of rock doves, plump grey-striped birds that nest on cliffs, were caged up and brought into the new cities for their eggs, meat and uncanny ability to find their way home. Naturally, a few of these escaped, but quickly discovered that the walls of buildings were just as good for nesting as natural cliffs. The familiar pigeon was born.

More recently, many species of ducks and geese found a home in cities for the same reason, as have pets-gone-wild like the Indian ring-necked parakeets that brighten up London’s parks and the Javan mynas that chatter in Singapore’s streets.

Bohemian waxwings mainly live in the forests of Scandinavia, but in cold winters they will fly across the sea to British parks and gardens to feast on garden berries. No prizes for guessing where this one is. Image: Stephen Jorgenson-Murray.

As cities have changed, so too have the birds that lived there. Back when most meat was butchered in shops and markets, piles of skin and bone attracted huge flocks of scavengers like ravens and red kites. Now city streets are mostly free of scrap meat thanks to bin lorries, supermarkets and industrial meat processing; both species fled into the countryside, where they found themselves persecuted by farmers and gamekeepers, the red kite almost to the point of extinction. Now both birds are making a slow comeback.

On the other hand, parks and gardens have lured new species out of the woods and into the town with their sweet berry bushes and seed-filled bird feeders. Blue tits – tiny birds that in the forest prefer to pick spiders off oak trees – adapted especially well to garden life: in the days of milk rounds, the birds learned how to peck open bottle caps and sip at the cream inside. The birds’ behaviour has recently changed again because of the rise of supermarkets and the fall of dairy delivery, and it certainly won’t be the last time.

What do city birds think of us?

Herring gulls are as happy in a Latvian bus station as they are on a windswept beach. Happier, maybe. Image: Stephen Jorgenson-Murray.

If you walk in a forest you might well find yourself absolutely surrounded by bird song but unable to see where it’s coming from. Birds are shy and, unless they grew up on a desert island, they will fly away and hide at the slightest hint of a threat. They almost behave like programmed characters from a video game – they draw an imaginary circle around themselves (known as the “flight zone”) and if anyone enters that circle, they flee.

Urban birds consistently have a much smaller flight zone and will tend to let humans get much closer to them; and the longer a species has been urbanised, the more this radius shrinks. In the most extreme instance, urban birds will hop right up to someone who might feed them and even land on their hand. (In one of the best birding moments of my life, a parakeet in Hyde Park snatched a peanut from a tourist then landed right on my shoulder to eat it, staying there long enough to pose for a selfie).

If one bird invades another’s territory, things can get messy. Here, two magpies chase off a buzzard as its partner watches. Image: Stephen Jorgenson-Murray.

Then again, not all birds are that friendly. Many are very territorial, especially in the nesting season. Even medium-sized birds like vicious Australian magpies can cause eye injuries to people passing their nests; really big birds like swans can seriously injure people who get too close. Others, like the larger species of gulls, are just greedy and will attack people to steal their food.

Most birds aren’t quite that bold, but living close to humans has still affected their behaviour. Many species of birds are very intelligent – European magpies might be the cleverest non-mammal on the planet – and they’ve worked out how many of the systems of the city work. Pigeons can hop on-board trains for a lazier way to travel between feeding spots. Seagulls understand how to open automatic doors in order to raid branches of Greggs. Crows use passing cars to crack tough nuts, and will even wait at traffic lights to swoop in when the cars stop.

What do we make of city birds?

The robin was voted Britain’s favourite bird in a recent poll, which just goes to show what being small, cute and surprisingly aggressive can do for you. Image: Stephen Jorgenson-Murray.

Although we share our cities with a whole menagerie of wildlife, most of it is either shy and nocturnal, or prefers the dark, dirty places where humans rarely venture. Birds by contrast are inescapable – on any day on any city street you can expect to at least see a few pigeons flying overhead, or hear something singing from a nearby bush. For some people, this constant awareness has morphed into affection; for others, jealousy at sharing urban spaces with other species.


Even setting aside the risk of attack, birds can come into conflict with humans. Their droppings are not only unpleasant, but they can damage buildings and cause nasty lung diseases. Not every bird has a beautiful song either – a great tit squeaking away outside your bedroom window at 5am is bad enough, but spare a thought for the Australians who have kookaburras scream-laughing on their balconies. If waking you up wasn’t antisocial enough, big birds like herring gulls and Australian white ibises (better known as “bin chickens”) will rip open bin bags and fling the rubbish across your garden. The birds guilty of these indiscretions are generally classed as pests and many cities are fighting back – either by killing the birds or by taking eggs from their nests.

Herons eat fish from ponds and occasionally birds of prey will attack small pets. Urban pigeon keepers, angry after having a prize bird attacked by a sparrowhawk, occasionally try to poison or set cruel traps to kill hawks; but in general cities actually provide a safe haven for birds of prey. Scottish sparrowhawks seem to breed significantly better in cities, likely because there are so many other birds there to hunt.

In fact, many city councils are encouraging birds of prey as a natural way to control the population of pigeons and rats. Peregrine falcons – the fastest birds on the planet – are given protected nesting sites on church spires and skyscrapers and their every move is streamed on webcams. Harris hawks – native to American deserts – have been brought across the Atlantic to scare birds away from the tennis courts at Wimbledon.

Smaller, cuter birds don’t have any such image problems, and millions of Brits put bird seed in their gardens or feed the ducks at their local park. (I should add: if you do, please don’t give them bread, which lacks the vitamins birds need and causes a horrible disease called “angel wing”; seeds, vegetable peel or little bits of fruit are better.) Cities are increasingly recognised as places where you can spot interesting birds – right now, the bird tracking portal eBird lists no fewer than 289 species that have been seen in London – and the last couple of years have seen guides such as David Lindo’s How to be an Urban Birder and even scientific journals such as the Journal of Urban Ecology dedicated to the life of the town.

Save the birds

An American robin has a rest in Boston Common. American robins are in a completely different family to European robins, in case you ever wondered why the robin in Mary Poppins looked so messed up. Image: Stephen Jorgenson-Murray.

Although cities offer food and shelter, they also contain many threats. Glass windows are invisible death to birds flying at full speed – the exact number killed isn’t clear, but it might be as many as 30 million a year in the UK alone. Vehicles can also kill, especially in suburban areas where dense gardens meet busy streets.

Although city birds are protected from some of the predators that they would encounter in the countryside, there are still plenty of animals looking for a meaty meal – not least pet cats, which the RSPB estimates kill 55 million birds in the UK every year. 


These threats aren’t necessarily having an effect on bird populations as a whole – most birds lay more eggs than needed, and if one young bird is killed by a cat a sibling can take its place. The bigger risks come from changes to the environment itself. Pesticides, patios and over-neat lawns have reduced the number of insects crawling around, and therefore the amount of food available for birds like thrushes, starlings and sparrows.

In spite of how easy they are to observe, urban birds tend to be understudied compared to their rural cousins. The fact pigeons are so widespread means researchers often overlook them, but their ubiquity means that observing the birds can help scientists to track environmental changes and to compare cities that otherwise have little in common. Citizen science can help here – the bird tracking apps Birdtrack and eBird let anyone submit their bird sightings, and actually need more coverage of urban and suburban areas.

Thankfully, the idea of creating urban bird sanctuaries is now being taken seriously. Parks have a role to play, but many birds actually prefer the wild roughness of building sites and industrial land, where bare soil crawls with bugs and wildflowers grow gloriously high – ironically, brownfield sites can be as important to the ecosystem as pristine green belt. Perhaps the most spectacular example is the London Wetland Centre in Barnes. Just across the Thames from Hammersmith, this Victorian waterworks has been converted into marsh land and attracts huge flocks of water birds, many of which can’t be found anywhere else in London. In fact thanks to the reserve, a few birds such as the reed-dwelling bittern – which almost went extinct in the UK – are now easier to spot in London than in the countryside it.

Flying into the future

This blackbird probably doesn’t understand its rural cousins. Image: Stephen Jorgenson-Murray.

In his book Darwin in the City, the biologist Menno Schilthuizen suggests that we’ve been looking at blackbirds all wrong. European blackbirds were originally forest-dwellers eating berries and bugs from the ground. For this, they needed long, probing beaks and the ability to migrate in the winter when the soil froze hard. However, a few blackbirds – possibly initially those living in the hills around Rome – made their way into cities and found plentiful supplies of food year round.

Since they no longer needed to pry into the earth or the bark of trees, their beaks started to get shorter. Because food was available year round, their migration instinct was switched off. And because they needed to compete with traffic and the other noises of city life, their songs got louder. The city dwelling birds became incompatible with their forest dwelling ancestors; the changes to their beaks meant that their songs changed too, until they were effectively speaking different languages. There is a compelling case to be made that there isn’t just one species of blackbird, but two: the forest blackbird, Turdus merula, and the city blackbird, Turdus urbanicus.

Where the blackbird has led, other birds are sure to follow. British great tits are evolving bigger beaks that help them dig around in garden bird feeders and many urban birds have started singing the dawn chorus earlier to avoid traffic and aircraft noise and to take advantage of artificial streetlighting. City-dwelling pigeons even seem to be evolving darker feathers, probably because the dark pigment captures the toxic elements pigeons accidentally ingest when they peck at paint.

Nesting in coated metal gutters like this exposes pigeons to dangerous chemicals in the paint, and this pigeon’s dark feathers are likely an evolutionary response to that threat. Image: Stephen Jorgenson-Murray.

Birds are no longer just accidental wanderers into cities, nor are they just greedy opportunists: they are an integral part of urban ecosystems. Not only do cities need their birds – Increasingly, birds need their cities.

Stephen Jorgenson-Murray tweets at @stejormur. Many of the birds mentioned in this article tweet in a tree near you.