The temperature in our cities' is rising faster than the planet as a whole and the mechanisms are complex.
In 2004 I lived in the old city of Damascus, Syria, not far from the magnificent Umayyad Mosque, on a side street off the famous straight street (الشارع المستقيم) near Bab al-Jabiyah (بَابُ الْجَابِيَّةِ) referenced in the Bible.
Summer days were hot. They reached a scorching 35°C on the streets, helped by clear skies and little rain. Entering my home I was amazed how cool it was in comparison. It dropped 10°C the spaces separated by a ramshackle door and walls, both open to the sky, without running ACs. Daytime was spent in rooms adjacent to the central courtyard. The temperature contrast fascinated me and has since ignited my curiosity for solutions to combat excessive heat.
Increasingly many city dwellers struggle with dangerously high temperatures. In Shenzhen where I live, I'm almost always in AC regulated areas, at home, at work, on the train and taxis. And I'm not alone with this problem.
Did you know that city temperatures tend to be higher than their surrounding areas? This is called Urban Heat Island (UHI) effect. The urban population for the first time exceeded rural population in 2007. Currently 56% of all people live in cities. UN estimates this will grow to 68% by 2050. As man-made global warming shows no sign of abating the UHI problem will also grow.
In this article I’ll explain its causes, how it correlates to global warming, its negative effects and the solutions devised to combat it. Read and find out how it is affecting you and what you can do to protect yourself and your loved ones. Tell me what your best remedy to fix it is!
What is an Urban Heat Island?
It’s defined as the outdoor air temperature difference between an urban area and its rural surroundings.
Climate Central research and communicate climate change impact and solutions and published a 2021 summary and report on this phenomenon. The illustrates show both variations between areas and within areas for night and day. One thing this does not capture well is the fact that the temperature difference tends to be higher at night which is mentioned in my research on New York City.
It was first identified by Luke Howard in the 1810s showing that nights in London’s center were 2.1°C hotter than in the surrounding countryside. The term was coined by German meteorologist Albert Peppler in 1929 and the first quantitative analysis was published in 1969.
Let's look at 3 cities manifesting this phenomenon.
Madrid
I spent 8 months in Madrid learning spanish. The locals described the city's climate as "nueve meses de invierno, tres meses de infierno" e.g. nine months of winter, three months of hell. But for this Swede it was almost paradise. The extra dose of sunshine, after a wet and cold Swedish summer supercharged my energy level. This surrounding mountains makes this beautiful city a cauldron during the summer.
According to an UHI study by sustainable development consutancy Arup, it was rated the city with the most extreme Urban Heat Island effect in 2022 from a list of 6 cities edging out Mumbai. Temperature difference of city center to rural surroundings peaked at 8.5°C (15.3°F). Inner city temperature differences were also extreme with 8°C lower temperature in the central El Retiro Park compared to nearby areas.
This interactive city climate map from meteoblue shows the current local temperature for Madrid with the option to go back 3 days. Temperature variation illustrated with color coding will depend on the season and time of day.
For a high temperature variation I recommend setting the time between 16:00 - 19:00.
New York City
To quantify the UHI effect the California Environmental Protection Agency (CalEPA) published an Urban Heat Island Index Report in 2015 as well as Urban Heat Island Interactive Maps. To calculate the index they recorded temperatures 2 meters above ground in different urban areas and rural reference points during 182 warm season days from 2006 to 2013 once an hour. The temperature differences generate degree-hours which are aggregated and averaged over 24 hours to produce the daily average temperature difference.
The research showed New York is the US city with the highest average UHI Index at 5.3°C (9.5°F) anead of San Francisco 4.9°C (8.8°F).
The study concluded:
Higher temperatures do not equate to higher UHI index because hot surrounding areas cause smaller temperature differences
UHI index increases during heat waves which are likely to become worse in the future due to climate change
Wind and topography impacts UHI effect so while California's coastal cities are cooled by ocean water the heat they generate are carried inland
Ozone air pollution, similarily, is blown inland and disproportionally affects inland communities blanketed by this pollution
This static map from climate central shows a visualisation of the UHI Index for New York City. High population density causes high temperature difference to be felt by 6MM residents. The UHI Index is most severe during the night.
If you live in, or plan to visit, NYC during the summer, check out it's Cool It! website for information on how to stay cool.
Barcelona
Researchers for the Municipality of Barcelona generated a predictive model on UHI from 2011 to 2015 summarized here following a 10-year study from 2004 to 2013 using the airport and weather stations. It showed strong predictive ability (R²=0.8) for the winter with less predictive ability for the summer (R²=0.55).
This has great potential to help understand implications on urban climate from global warming scenarios as well as guide urban design decision making to mitigate the negative impact of UHI.
The map clearly shows significant temperature differences between downtown urban areas, less populated suburban areas and the mountainous surroundings.
What Causes Urban Heat Islands?
4 main causes:
Albedo
Albedo is the fraction of sunlight reflected by a surface. It is high for white colors and low for dark colors. High albedo contributes to keeping the ice at the earth's poles cool but causes cities many dark road and building surfaces made from asphalt, brick and concrete, to absorb a lot of energy.
Percentage of greenery
Plants and trees contain water which converts into water vapor when heated by the sun. This transformation, called evapotranspiration, removes energy without raising the temperature. Trees also provides shading which significantly lowers ground temperature. In addition, the soil under plants bind moisture which helps to reduce heat. In contrast, impermeable city surfaces like asphalt and cement do not capture moisture causing them to heat up quickly.
Population density
Humans bodies and activities generate heat. If you consume 2500 calories per day you emit an average of 120W at steady state conditions. By walking it grows to 200W. Driving a car, riding the bus or metro, or all other forms of transportation use energy. All appliances we use, machines we run, and industrial facilities that produce our goods use up energy. Keeping buildings temperatures regulated also generate heat. One fascinating study showd heat generation increasing 20% due to AC use during a heat wave.
Building height, narrow streets, city irregularity
Tall buildings provide more surfaces that reflect and absorb sunlight. They also block incoming wind, slow it down and trap it in so called urban canyons. This effect is particularly noticable during the night and the reason why nighttime temperature UHI can be as high as 12°C (22°F) compared to 3.3°F (6°F) during the day.
Narrow streets and building shapes may also contribute to "increased surface roughness" obstructing the wind flow.
This is an image of an interactive urban wind map by Meteoblue that shows Zurich's local temperature and winds at 11:00 on Friday 30-August 2024.
You can clearly see the effects on wind from open spaces and the river as opposed to buildings, courtyards and narrow streets.
Urban Heat Islands link to Global Warming
Global warming increases the number and severity of heat waves. Urban heat islands are the most negatively affected by heat waves. Meanwhile, urbanization increases the number of people affected by this trend.
Heat waves are characterized by:
Frequency: heat waves per year
Duration: days per heat wave
Season: days from first to last heat wave
Intensity: degrees above baseline
US Heat waves have become worse on all metrics. Below see how 3 metrics have changed over the last 60 years by decade in the US.
The above chart shows that the number of heat waves in the US has grown significantly over the last 60 years from 2.2 to 6.3 per decade. Meanwhile the duraction per heat wave have also grown over the period from 3 days to 4.3 days.
The above chart shows that the heat wave temperature relative the baseline temperature has also gone up for the heat waves in the US. In the 60s it was below 2°F and in 2020s it is above 2.5°F.
Negative Effects from Urban Heat Islands
4 main effects:
Health complications
Extreme heat is the top weather killer. It also increases the risk of premature births.
Serious complications include respiratory disease, emphysema, heart disease, heart attack, stroke, renal failure, hypertension, asthma and intestinal infection.
High temperatures and poor air quality can also lead to dehydration, cramps, exessive sweating, dizzyness, heatstroke, heat exhaustion and excessive sweating.
According to the US Center of Disease Control (CDC) heat caused a total of 10,527 deaths in the US from 2004 to 2018. In 59% of cases heat was the main cause. In the reminder the disease, injuries, or complications together with heat led to death.
The 2003 heatwave from 20-July to 20-August in Europe is estimated to have killed more than 70,000 people. France was particularly affected due to the high temperatures, exceeding 40°C, absence of physicians on holiday, and the elderly and physcally impaired who did not know how to react. Studies showed that the 2023 European heatwave killed 61,000.
A Guardian article from 2021 reported at least 6,500 migrant workers died in Qatar since the construction work on World Cup facilities began. Qatar officials label them as natural leaving investigators and the UN to speculate that the intense summer heat is a key factor. At least 4 months of the year workers faced significant heat stress when working outside.
Air pollution
Heat trap pollutants in the lower atmosphere. This is called stagnation. Rising temperatures exasperate the effect. Heat and sunlight can also create ground-level ozone which can cause health problems like asthma, allegies, respiratory distress and eye irritation.
Infrastructure damage
Roads, bridges, railways, power lines and water pipes can crack, buckle, melt or burst from the heat induced stress. It also increase the risk of wildfires which can destroy buildings and vegetation in addition to generating harmful emissions.
In 2019 two nuclear reactors in southwest France were shut down and another six reactors were curtailed due to the high temperatures of the rivers used as cooling water.
In 2020 Australian bushifires burned 42MM acres destroying thousands of buildings, killing dozens of people and 3 billion animals after Australia experienced the hottest and driest year in its recorded history 2019-20. It even depleted 3-5% of the ozone layer. This is significant as the ozone layer only recovers 1-3% per decade.
Chinese social media captured a video of bridge in the city of Quanzhou breaking during the 2023 heatwave due to the 40°C (104°F) temperature. See the video on CNN here.
Economic losses
Productivity losses of outdoor workers in construction, agriculture, and transportation as well as indoor workers in heat sensitive areas. Increased energy demand to provide cooling increase energy costs and power grid strains increases the risk of blackouts. Lower demand for tourism and recreational activities.
The Working on a Warmer Planet report by the International Labor Organization estimates global productivity losses of 2.2% of total working hours by 2030 if we meet the 1.5°C temperature increase which assumes that some work that is currently done during the day can be displaced to cooler periods.
The Unlivable report focused on East Asian countries by the World Bank Group presents an estimate including the worlds largest 1692 cities reducing their GDP by 1.4-1.7% by 2050 from the combined effects of global warming and the Urban Heat Island effect. This effect is most severe during extreme heat events.
Contemporary Solutions
4 strategies:
Planting trees
Trees and vegitation provide multiple benefits helping to reduce the effects of Urban Heat Islands. Evapotranspiration consumes energy by releasing water in the form of vapor avoiding direct temperature increases from the sun. They provide shade to the pavement. Check out this tree equity score report for US cities by American Forests.
Other direct benefits not related to cooling are converting CO2 into oxygen through photosynthesis. Absorbing harmful gases such as ozone and nitrogen dioxide thus improving air quality. Absorbing rainwater to reduce risk of flooding.
Green or reflective roofs
Green roofs are covered with vegitation providing evapotranspiration and cover. Reflective roofs are covered in reflective material or paint to reflect more sunlight thus lowering the energy needed to cool the roof.
Cool islands
Cooling spaces like parks and street level fountains that help to cool people down. An activity that can be enjoyed by children and adults alike.
Permeable or reflective surfaces
Permeable surfaces allow water to enter absorb less heat compared with impermeable surfaces like classic concrete or asphalt. Reflective surfaces are not restricted to roofs also keep the surface cooler. For instance, reflective pavements have many benefits however they also risk causing pedestrians to become hotter as they receive more reflected sunlight.
Personal Solutions
Beyond the structural changes to our environment each individual is incentivized to find effective ways to keep cool. While Sweden is not known for its hot climate our summers can get quite hot. Growing up my family and I would sometimes sleep in our basement during the summer because it was cooler.
For instance, the British Red Cross provided a host of recommendations to deal with expected heatwave across Europe during the 2024 holiday season.
Limit your physical activity to the cooler parts of the day
Stay hydrated by drinking plenty of water
Wear light-coloured and loose fitting clothes
Take a siesta during the peak heat hours
Keep your home and workplaces cool
Take cool showers
Cold drinks or ice desserts
Limit drinks that dehydrate and smoking
This BBC article describe strategies more used in Japan to cool down in scorching weather. But if you plan to do some Uchimizu (water sprinkling) make sure you have the concent of the person you are doing it to.
Apply cooling sunscreen
Wear iced neck rings
Carry a sun umbrella outside
Uchimizu, aka water sprinkling, the street
Rehydrate with electrolyte charged drinks
Eat salt plum candies which replentish salt levels
Going beyond these measures you can also consider installing smart thermometers to regulate indoor temperatures, window films that block ultraviolet and infrared radiation, and heat pumps which can be extremely effective at transporting heat. Since they were first used to regulate home temperatures in the 1960s they have become very popular in Norway, Sweden and Finland where they regulate the temperature in 60%, 43% and 41% of households respectively.
Ancient Solutions
I mentioned how impressed I was with the cooling of my house in Damascus. It's narrow central courtyard was in shadow most of the day. This created a column of cool air which gradually flowed into all adjacent rooms. Here I share examples of more passive technologies to help cool down our homes and cities.
Buildings in reflective material
Across the Mediterranean coast you have the beautiful view of lightly colored houses, like Mykonos in Greece, from buildings covered by in highly reflective plaster material.
Buildings made from high thermal inertia breathable material and wind tunnels
Mud is a popular building material in Africa with a bad reputation for constructions collapsing during the rainy season trapping its owners inside. But some buildings have stood for centuries. Its durability depend on material and construction quality. Mud needs less energy to make and keeps houses cooler than cement.
16th century Yemen city Shibam nicknamed "Manhattan of the Desert" has been made for the scorching desert heat with hits mud buildings with highly reflected surfaces and narrow streets covered in shadow.
The mud-brick Agadez Mosque in Niger is a beautiful example of 16th century architecture. Although it keeps cool the construction process and material has gradually been replaced by cement which in addition to be less efficient at cooling contributes to 8% of carbon emissions from its high energy intensity production.
Central courtyard common in the Mediterranean, Middle East, and Africa
These houses moderate their own microclimate providing passive cooling, e.g. without the need for electricity, to its dwellers. The cooling takes place over 3 cycles. First the cool night air fills the courtyard and the surrounding rooms until noon. Next, at noon the sun strikes the courtyard floor causing the cool air to rise and leak from surrounding rooms. This causes some air flow. Finally, in the late afternoon, the remaining cool air trapped in rooms spill out until the sun sets and the courtyard irradiates heat to the sky as temperatures fall.
It needs high, thick walls to prevent the suns rays and heat from the street to enter.
Architecture of the veil aims to look plain on the outside but beautiful on the inside to deter potential thieves while providing an earthly paradise for its residents.
Conclusion
Rising urban temperatures caused by global warming and the urban heat island effect exposes cities and citizens to risks of negative health effects, property damage, and productivity losses. To fix these problems we must act.
Strategies for urban planners to plant trees, use reflective and permeable materials, and incorporate traditional building materials and passive solutions like central courtyards can help to create cooler, more sustainable cities.
Homeowners can install heat pumps, smart thermometers, and window films to reduce energy demand and temperature.
Citizens can incorporate strategies like plan time for physical activities when its cool, choose appropriate clothes, and stay hydrated.
By reshaping our urban environments, living spaces, and habits we can stay cooler and keep energy use low, even as the global climate changes. But tread wisely, because the steps we take now will shape the path for the generations to come.
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