January and February are often the coldest months here in the UK, and it feels like this year it will be no exception. Those of us who live in our larger towns and cities, though, benefit from sometimes noticeably higher temperatures than the surrounding countryside. Called the urban heat island effect, this can be a welcome upside of living alongside lots of other people. But what exactly are urban heat islands? And do they really help or hinder us and our wildlife?
Urban Heat Islands and Their Causes
Simply put, urban heat islands (UHI) are where the relative temperatures of larger towns and cities are higher than those of the surrounding countryside. In some urban centres, average temperatures can be as much as 5°C higher than those in nearby rural areas due to the UHI effect. There are a number of causes. One of the main factors, however, is the way that humans have altered surfaces within urban areas. Towns and cities, unsurprisingly, consist of lots of hard, impermeable surfaces, from concrete buildings to tarmacked roads and footpaths. These manmade substances retain heat much easier and at higher levels than soil or the surrounding air. Dark surfaces, in particular, absorb more heat. The effect is most noticeable at night when these manmade substances trap heat at lower levels, preventing it from escaping into the surrounding atmosphere.
Another major contributing factor is the energy emitted in cities. This includes from heating, air conditioning, lighting and vehicles. Humans themselves also give off heat, and where lots of people are concentrated together, this has a marked effect. Other contributing factors include the way cities are designed. Narrower gaps between tall buildings, so-called urban canyons, impede the cooling effects of wind. Tall buildings also have more surfaces to absorb the sun’s heat. Reduced green areas, meanwhile, have an impact because plants have evaporative cooling properties. How much greenery there is just outside a city likewise has an effect, with less vegetated surroundings contributing to higher urban temperatures. While climate change doesn’t cause UHIs, it can exacerbate the situation by causing more frequent and intense heat waves.
The Impacts of Urban Heat Islands
Without the heat increases caused by climate change, there is an argument that there are positive aspects of UHIs. Humans might benefit, for instance, from slightly raised temperatures compared to nearby rural areas in winter, reducing the cost of heating homes and offices. Likewise, wildlife can benefit in similar ways in winter. For example, ectotherms such as insects that can’t regulate their own body temperatures, are less vulnerable to freezing. Some take advantage by not hibernating as they would in rural areas. This in turn might allow them to breed earlier in the spring. Species that feed in water, such as otters and herons, are less likely to encounter frozen ponds and rivers. Warmer temperatures also mean that pioneer species that are already good at spreading into our urban centres, find it easier to expand their ranges into latitudes that might otherwise be inhospitable.
Climate change, though, increases the negative impact of UHIs. Heat waves are more frequent and more intense. City heat waves are already becoming increasingly deadly. Extreme heat also leads to more energy use as we try to keep cool, further intensifying the UHI effects. In addition, intensified UHI effects mean that spring is arriving earlier in our cities. One consequence of this is that breeding birds, especially migratory ones, are not able to advance their nesting to take advantage of the consequent earlier caterpillar emergence. This mismatch has already been recorded in connection with climate change in general. The increased temperatures of UHIs are likely to make these processes even more out of sync. Ultimately, as this University of Utah study shows, urban biodiversity is likely to decline as temperatures rise and the UHI effect of a city increases.
Combatting the Effects of Urban Heat Islands
The United Nations predicts that nearly 70% of the world’s human population will live in urban areas by the year 2050. Meanwhile, some models project that, without significant work to reduce carbon emissions, global temperatures could be 2.5°C higher by the same date. These two scenarios mean that before too long, nearly three quarters of humans are going to be struggling to cope with the vastly increased effects of UHIs. Better urban planning and design, though, can counter the effects of urban heat islands. Lighter coloured building materials reflect more sunlight, storing less heat. Cool roofs and cool pavements use materials with different heat absorption properties, as well. Any newly developed areas can also reduce the effects of UHI by incorporating wider thoroughfares with less densely packed buildings.
Adding green spaces is also key. Plants give off water vapour through their leaves in a process called transpiration. This leads to evaporative cooling of both the plant and the surrounding air. The more plants an urban space has, the more it benefits from this process. This temperature map of London, for example, shows that vegetated areas such as Hampstead Heath are much cooler (coloured blue) than surrounding built up zones. More vegetation also helps to improve air quality which tends to decline in UHIs. Where possible, parks and similar green spaces are, of course, hugely beneficial. Even where space is limited, though, there are solutions, including the installation of green roofs and walls. Another option is to reverse the modern trend for replacing gardens with concreted car ports or stone patios.
Urban Heat Islands: The Future
Densely populated areas create the urban heat island effect. And climate change is intensifying this process. As a result, and as humans become an increasingly urban species, we will need to create new ways of designing our towns and cities. If not, our warming world will make cities harder to live in for both humans and wildlife alike.