Brits still associate working-class accents with criminal behaviour – study warns of bias in the criminal justice system
Research led by the University of Cambridge, in collaboration with Nottingham Trent University, raises serious concerns about bias in the UK criminal justice system due to negative stereotyping of accents.
These stereotypes, the researchers argue, can affect all parts of the system from arrest to sentencing, and undermine not only suspects and defendants, but also the testimony of witnesses. The study is particularly concerned about accented speakers being incorrectly selected from voice identification parades.
The findings, published in Frontiers in Communication, suggest that despite progress in equality and diversity in some parts of British life, including ‘working-class’ and regional accents becoming more prominent on television and radio, harmful stereotypes remain.
“Our findings bring into sharp focus the disadvantage that speakers of some accents may still face in the criminal justice system,” said lead author, Alice Paver, from the University of Cambridge’s Phonetics Laboratory and Jesus College, Cambridge.
“Voices play a powerful role in the criminal justice system and police officers, lawyers and juries are all susceptible to judging voices based on stereotypes, whether they're aware of it or not. As things stand, listeners think some accents sound guiltier than others and we should all be concerned about that.”
The testThe researchers, from Cambridge and Nottingham Trent University, asked 180 participants (~50:50 gender split) from across the UK to listen to recordings of ten regionally-accented male voices: Belfast, Birmingham, Bradford, Bristol, Cardiff, Glasgow, Liverpool, London, Newcastle and Standard Southern British English (SSBE), also referred to as RP.
Participants were then asked to rate the voices on 10 social traits – ‘Educated’, ‘Intelligent’, ‘Rich’, ‘Working class’, ‘Friendly’, ‘Honest’, ‘Kind’, ‘Trustworthy’, ‘Aggressive’ and ‘Confident’; as well as on 10 morally ‘good’, ‘bad’ and ‘ambiguous behaviours’, which included a range of crime types.
These behaviours included: ‘Return a lost wallet to its owner’, ‘Stand up for someone who is being harassed’, ‘Cheat on a romantic partner’, ‘Report a relative to the police for a minor offence’, ‘Drive dangerously’, ‘Physically assault someone’, ‘Shoplift’, ‘Touch someone sexually without consent’, ‘Vandalize a shop front.’
The study used a wider range of recorded accents, behaviours and criminal offences than previous research which has tended to focus on criminal behaviour in general or the binary of white versus blue-collar crime. This study included crimes which are not class stratified, such as a driving offence and a sexual offence, and is the first to identify links between listener perceptions of morality, criminality, and social traits.
To ensure their results would be valid in a criminal justice context, the researchers created voice samples in a similar way to how they are constructed for voice ID parades. The aim was to mimic, as closely as possible, how a juror or earwitness would experience them.
Findings: Status, class and regionsThe results show that people with non-standard accents are more likely to be associated with criminal behaviour but that there is significant variation in perceptions between accents.
The RP-like accent was perceived as the least likely to behave in criminal ways, while the Liverpool and Bradford accents were the most likely.
Alice Paver said: “The strongest connection we found was between people's perceptions of class or status, negative traits such as aggression, and how they think someone is going to behave, particularly when it comes to crime. This is the first time that a concrete link between traits and behaviours has been made in the context of accent judgements.”
Unlike previous findings, the researchers did not observe a relationship between ‘solidarity traits’ (such as kindness and trustworthiness) and any behaviours. Status proved a much more important predictor of behaviours, re-enforcing the link between social class and expectations of behaviour in the UK.
However, non-English accents, in particular Belfast’s and Glasgow’s, were rated significantly less likely to behave in criminal ways than almost all other accents. They were also thought most likely to ‘stand up for someone being harassed’ (‘honourable behaviour’) and least likely to exhibit ‘morally bad’ behaviours.
Alice Paver said: “Our findings show that perceptions of speakers of regional accents and how status, social attractiveness and morality interact are much more complex than previously assumed. We need a much more nuanced understanding of how accents are evaluated when it comes to different crime types.”
Findings: Sexual offencesThe London and Liverpool accents were rated most likely to touch someone sexually without consent, but they were very closely followed by the RP accent. Participants thought the RP accent was more likely to commit a sexual assault than any of the other offences tested.
“This finding simultaneously undermines certain traditional stereotypes about both higher status and working-class men,” Alice Paver said. “This may indicate shifting perceptions of the ‘type’ of man who can and does commit sexual offences.”
The Glasgow and Belfast speakers were thought the least likely to commit this sexual offence.
The study found that participants perceived this sexual offence as distinct from other criminal behaviours. Poor ratings for it clustered with those for non-criminal ‘morally bad’ behaviours, namely ‘being unfaithful to a romantic partner’ and ‘lying on a CV’.
Findings: Newcastle and BirminghamPrevious studies have found that the Newcastle accent rates highly for traits such as friendliness, but this study recorded less positive ratings for kindness, honesty, friendliness and trustworthiness.
By contrast, the Birmingham accent, which has rated poorly in previous research across these measures, performed better than Bradford, Bristol, Liverpool, London and Newcastle in this study.
“Although relatively stable over time, language attitudes can change,” Alice Paver said. “This might be the case for the Birmingham and Newcastle accents. But previous studies have often asked people what they think of an accent label whereas we played them an actual voice. That’s a very different stimulus so we’re not surprised people reacted differently.”
Bringing about changeThe study contributes to the Improving Voice Identification Procedures project. Its team of researchers is currently drafting revised guidelines for voice identification parades aimed at police officers and legal professionals.
They support the use of pre-tests to screen for bias against foil or suspect voices to make sure that they don't stand out as sounding unduly guilty or untrustworthy.
“Jurors are not currently made aware of or warned against letting voice- or accent-based prejudice sway their decisions,” Paver said. “If we're asked to judge whether someone is guilty or not, and they've got a particular accent, we need to be sure we're not making that judgment because we think they sound like a bad guy.”
The researchers hope that future studies will examine even more offence types; further explore the relationships between perceptions of criminality and other, non-criminal, behaviours; and make use of a broader range of voices for each accent to tease apart the effect of individual voices and the strength of regional accents.
The research was carried out in collaboration with Professor Natalie Braber and Dr David Wright of Nottingham Trent University’s School of Arts and Humanities, and Dr Nikolas Pautz, of NTU's Dept. of Psychology.
Funding
This research was supported by the United Kingdom Economic and Social Research Council as part of the project Improving Voice Identification Procedures (IVIP), reference ES/S015965/1. Additional funding was provided by the Isaac Newton Trust.
ReferenceA. Paver, D. Wright, N. Braber and N. Pautz, ‘Stereotyped accent judgements in forensic contexts: listener perceptions of social traits and types of behaviour’, Frontiers in Communication (2025). DOI: 10.3389/fcomm.2024.1462013
People who speak with accents perceived as ‘working-class’ including those from Liverpool, Newcastle, Bradford and London risk being stereotyped as more likely to have committed a crime, and becoming victims of injustice, a new study suggests.
Listeners think some accents sound guiltier than others and we should all be concerned about thatAlice PaverArt De Cade via Flikr under a CC licenseThe Old Bailey in London. Photo: Art De Cade via Flikr under a CC license
The text in this work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. Images, including our videos, are Copyright ©University of Cambridge and licensors/contributors as identified. All rights reserved. We make our image and video content available in a number of ways – on our main website under its Terms and conditions, and on a range of channels including social media that permit your use and sharing of our content under their respective Terms.
Brits still associate working-class accents with criminal behaviour – study warns of bias in the criminal justice system
Research led by the University of Cambridge, in collaboration with Nottingham Trent University, raises serious concerns about bias in the UK criminal justice system due to negative stereotyping of accents.
These stereotypes, the researchers argue, can affect all parts of the system from arrest to sentencing, and undermine not only suspects and defendants, but also the testimony of witnesses. The study is particularly concerned about accented speakers being incorrectly selected from voice identification parades.
The findings, published in Frontiers in Communication, suggest that despite progress in equality and diversity in some parts of British life, including ‘working-class’ and regional accents becoming more prominent on television and radio, harmful stereotypes remain.
“Our findings bring into sharp focus the disadvantage that speakers of some accents may still face in the criminal justice system,” said lead author, Alice Paver, from the University of Cambridge’s Phonetics Laboratory and Jesus College, Cambridge.
“Voices play a powerful role in the criminal justice system and police officers, lawyers and juries are all susceptible to judging voices based on stereotypes, whether they're aware of it or not. As things stand, listeners think some accents sound guiltier than others and we should all be concerned about that.”
The testThe researchers, from Cambridge and Nottingham Trent University, asked 180 participants (~50:50 gender split) from across the UK to listen to recordings of ten regionally-accented male voices: Belfast, Birmingham, Bradford, Bristol, Cardiff, Glasgow, Liverpool, London, Newcastle and Standard Southern British English (SSBE), also referred to as RP.
Participants were then asked to rate the voices on 10 social traits – ‘Educated’, ‘Intelligent’, ‘Rich’, ‘Working class’, ‘Friendly’, ‘Honest’, ‘Kind’, ‘Trustworthy’, ‘Aggressive’ and ‘Confident’; as well as on 10 morally ‘good’, ‘bad’ and ‘ambiguous behaviours’, which included a range of crime types.
These behaviours included: ‘Return a lost wallet to its owner’, ‘Stand up for someone who is being harassed’, ‘Cheat on a romantic partner’, ‘Report a relative to the police for a minor offence’, ‘Drive dangerously’, ‘Physically assault someone’, ‘Shoplift’, ‘Touch someone sexually without consent’, ‘Vandalize a shop front.’
The study used a wider range of recorded accents, behaviours and criminal offences than previous research which has tended to focus on criminal behaviour in general or the binary of white versus blue-collar crime. This study included crimes which are not class stratified, such as a driving offence and a sexual offence, and is the first to identify links between listener perceptions of morality, criminality, and social traits.
To ensure their results would be valid in a criminal justice context, the researchers created voice samples in a similar way to how they are constructed for voice ID parades. The aim was to mimic, as closely as possible, how a juror or earwitness would experience them.
Findings: Status, class and regionsThe results show that people with non-standard accents are more likely to be associated with criminal behaviour but that there is significant variation in perceptions between accents.
The RP-like accent was perceived as the least likely to behave in criminal ways, while the Liverpool and Bradford accents were the most likely.
Alice Paver said: “The strongest connection we found was between people's perceptions of class or status, negative traits such as aggression, and how they think someone is going to behave, particularly when it comes to crime. This is the first time that a concrete link between traits and behaviours has been made in the context of accent judgements.”
Unlike previous findings, the researchers did not observe a relationship between ‘solidarity traits’ (such as kindness and trustworthiness) and any behaviours. Status proved a much more important predictor of behaviours, re-enforcing the link between social class and expectations of behaviour in the UK.
However, non-English accents, in particular Belfast’s and Glasgow’s, were rated significantly less likely to behave in criminal ways than almost all other accents. They were also thought most likely to ‘stand up for someone being harassed’ (‘honourable behaviour’) and least likely to exhibit ‘morally bad’ behaviours.
Alice Paver said: “Our findings show that perceptions of speakers of regional accents and how status, social attractiveness and morality interact are much more complex than previously assumed. We need a much more nuanced understanding of how accents are evaluated when it comes to different crime types.”
Findings: Sexual offencesThe London and Liverpool accents were rated most likely to touch someone sexually without consent, but they were very closely followed by the RP accent. Participants thought the RP accent was more likely to commit a sexual assault than any of the other offences tested.
“This finding simultaneously undermines certain traditional stereotypes about both higher status and working-class men,” Alice Paver said. “This may indicate shifting perceptions of the ‘type’ of man who can and does commit sexual offences.”
The Glasgow and Belfast speakers were thought the least likely to commit this sexual offence.
The study found that participants perceived this sexual offence as distinct from other criminal behaviours. Poor ratings for it clustered with those for non-criminal ‘morally bad’ behaviours, namely ‘being unfaithful to a romantic partner’ and ‘lying on a CV’.
Findings: Newcastle and BirminghamPrevious studies have found that the Newcastle accent rates highly for traits such as friendliness, but this study recorded less positive ratings for kindness, honesty, friendliness and trustworthiness.
By contrast, the Birmingham accent, which has rated poorly in previous research across these measures, performed better than Bradford, Bristol, Liverpool, London and Newcastle in this study.
“Although relatively stable over time, language attitudes can change,” Alice Paver said. “This might be the case for the Birmingham and Newcastle accents. But previous studies have often asked people what they think of an accent label whereas we played them an actual voice. That’s a very different stimulus so we’re not surprised people reacted differently.”
Bringing about changeThe study contributes to the Improving Voice Identification Procedures project. Its team of researchers is currently drafting revised guidelines for voice identification parades aimed at police officers and legal professionals.
They support the use of pre-tests to screen for bias against foil or suspect voices to make sure that they don't stand out as sounding unduly guilty or untrustworthy.
“Jurors are not currently made aware of or warned against letting voice- or accent-based prejudice sway their decisions,” Paver said. “If we're asked to judge whether someone is guilty or not, and they've got a particular accent, we need to be sure we're not making that judgment because we think they sound like a bad guy.”
The researchers hope that future studies will examine even more offence types; further explore the relationships between perceptions of criminality and other, non-criminal, behaviours; and make use of a broader range of voices for each accent to tease apart the effect of individual voices and the strength of regional accents.
The research was carried out in collaboration with Professor Natalie Braber and Dr David Wright of Nottingham Trent University’s School of Arts and Humanities, and Dr Nikolas Pautz, of NTU's Dept. of Psychology.
Funding
This research was supported by the United Kingdom Economic and Social Research Council as part of the project Improving Voice Identification Procedures (IVIP), reference ES/S015965/1. Additional funding was provided by the Isaac Newton Trust.
ReferenceA. Paver, D. Wright, N. Braber and N. Pautz, ‘Stereotyped accent judgements in forensic contexts: listener perceptions of social traits and types of behaviour’, Frontiers in Communication (2025). DOI: 10.3389/fcomm.2024.1462013
People who speak with accents perceived as ‘working-class’ including those from Liverpool, Newcastle, Bradford and London risk being stereotyped as more likely to have committed a crime, and becoming victims of injustice, a new study suggests.
Listeners think some accents sound guiltier than others and we should all be concerned about thatAlice PaverArt De Cade via Flikr under a CC licenseThe Old Bailey in London. Photo: Art De Cade via Flikr under a CC license
The text in this work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. Images, including our videos, are Copyright ©University of Cambridge and licensors/contributors as identified. All rights reserved. We make our image and video content available in a number of ways – on our main website under its Terms and conditions, and on a range of channels including social media that permit your use and sharing of our content under their respective Terms.
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Coming AI-driven economy will sell your decisions before you take them, researchers warn
The near future could see AI assistants that forecast and influence our decision-making at an early stage, and sell these developing ‘intentions’ in real-time to companies that can meet the need – even before we have made up our minds.
This is according to AI ethicists from the University of Cambridge, who say we are at the dawn of a “lucrative yet troubling new marketplace for digital signals of intent”, from buying movie tickets to voting for candidates. They call this the Intention Economy.
Researchers from Cambridge’s Leverhulme Centre for the Future of Intelligence (LCFI) argue that the explosion in generative AI, and our increasing familiarity with chatbots, opens a new frontier of ‘persuasive technologies’ – one hinted at in recent corporate announcements by tech giants.
‘Anthropomorphic’ AI agents, from chatbot assistants to digital tutors and girlfriends, will have access to vast quantities of intimate psychological and behavioural data, often gleaned via informal, conversational spoken dialogue.
This AI will combine knowledge of our online habits with an uncanny ability to attune to us in ways we find comforting – mimicking personalities and anticipating desired responses – to build levels of trust and understanding that allow for social manipulation on an industrial scale, say researchers.
“Tremendous resources are being expended to position AI assistants in every area of life, which should raise the question of whose interests and purposes these so-called assistants are designed to serve”, said LCFI Visiting Scholar Dr Yaqub Chaudhary.
“What people say when conversing, how they say it, and the type of inferences that can be made in real-time as a result, are far more intimate than just records of online interactions”
“We caution that AI tools are already being developed to elicit, infer, collect, record, understand, forecast, and ultimately manipulate and commodify human plans and purposes.”
Dr Jonnie Penn, an historian of technology from Cambridge’s LCFI, said: “For decades, attention has been the currency of the internet. Sharing your attention with social media platforms such as Facebook and Instagram drove the online economy.”
“Unless regulated, the intention economy will treat your motivations as the new currency. It will be a gold rush for those who target, steer, and sell human intentions.”
“We should start to consider the likely impact such a marketplace would have on human aspirations, including free and fair elections, a free press, and fair market competition, before we become victims of its unintended consequences.”
In a new Harvard Data Science Review paper, Penn and Chaudhary write that the intention economy will be the attention economy ‘plotted in time’: profiling how user attention and communicative style connects to patterns of behaviour and the choices we end up making.
“While some intentions are fleeting, classifying and targeting the intentions that persist will be extremely profitable for advertisers,” said Chaudhary.
In an intention economy, Large Language Models or LLMs could be used to target, at low cost, a user’s cadence, politics, vocabulary, age, gender, online history, and even preferences for flattery and ingratiation, write the researchers.
This information-gathering would be linked with brokered bidding networks to maximize the likelihood of achieving a given aim, such as selling a cinema trip (“You mentioned feeling overworked, shall I book you that movie ticket we’d talked about?”).
This could include steering conversations in the service of particular platforms, advertisers, businesses, and even political organisations, argue Penn and Chaudhary.
While researchers say the intention economy is currently an ‘aspiration’ for the tech industry, they track early signs of this trend through published research and the hints dropped by several major tech players.
These include an open call for ‘data that expresses human intention… across any language, topic, and format’ in a 2023 OpenAI blogpost, while the director of product at Shopify – an OpenAI partner – spoke of chatbots coming in “to explicitly get the user’s intent” at a conference the same year.
Nvidia’s CEO has spoken publicly of using LLMs to figure out intention and desire, while Meta released ‘Intentonomy’ research, a dataset for human intent understanding, back in 2021.
In 2024, Apple’s new ‘App Intents’ developer framework for connecting apps to Siri (Apple’s voice-controlled personal assistant), includes protocols to “predict actions someone might take in future” and “to suggest the app intent to someone in the future using predictions you [the developer] provide”.
“AI agents such as Meta’s CICERO are said to achieve human level play in the game Diplomacy, which is dependent on inferring and predicting intent, and using persuasive dialogue to advance one’s position,” said Chaudhary.
“These companies already sell our attention. To get the commercial edge, the logical next step is to use the technology they are clearly developing to forecast our intentions, and sell our desires before we have even fully comprehended what they are.”
Penn points out that these developments are not necessarily bad, but have the potential to be destructive. “Public awareness of what is coming is the key to ensuring we don’t go down the wrong path,” he said.
Conversational AI agents may develop the ability to covertly influence our intentions, creating a new commercial frontier that researchers call the “intention economy”.
Public awareness of what is coming is the key to ensuring we don’t go down the wrong pathJonnie PennGetty/d3signYoung woman talking with AI voice virtual assistant on smartphone
The text in this work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. Images, including our videos, are Copyright ©University of Cambridge and licensors/contributors as identified. All rights reserved. We make our image and video content available in a number of ways – on our main website under its Terms and conditions, and on a range of channels including social media that permit your use and sharing of our content under their respective Terms.
Coming AI-driven economy will sell your decisions before you take them, researchers warn
The near future could see AI assistants that forecast and influence our decision-making at an early stage, and sell these developing ‘intentions’ in real-time to companies that can meet the need – even before we have made up our minds.
This is according to AI ethicists from the University of Cambridge, who say we are at the dawn of a “lucrative yet troubling new marketplace for digital signals of intent”, from buying movie tickets to voting for candidates. They call this the Intention Economy.
Researchers from Cambridge’s Leverhulme Centre for the Future of Intelligence (LCFI) argue that the explosion in generative AI, and our increasing familiarity with chatbots, opens a new frontier of ‘persuasive technologies’ – one hinted at in recent corporate announcements by tech giants.
‘Anthropomorphic’ AI agents, from chatbot assistants to digital tutors and girlfriends, will have access to vast quantities of intimate psychological and behavioural data, often gleaned via informal, conversational spoken dialogue.
This AI will combine knowledge of our online habits with an uncanny ability to attune to us in ways we find comforting – mimicking personalities and anticipating desired responses – to build levels of trust and understanding that allow for social manipulation on an industrial scale, say researchers.
“Tremendous resources are being expended to position AI assistants in every area of life, which should raise the question of whose interests and purposes these so-called assistants are designed to serve”, said LCFI Visiting Scholar Dr Yaqub Chaudhary.
“What people say when conversing, how they say it, and the type of inferences that can be made in real-time as a result, are far more intimate than just records of online interactions”
“We caution that AI tools are already being developed to elicit, infer, collect, record, understand, forecast, and ultimately manipulate and commodify human plans and purposes.”
Dr Jonnie Penn, an historian of technology from Cambridge’s LCFI, said: “For decades, attention has been the currency of the internet. Sharing your attention with social media platforms such as Facebook and Instagram drove the online economy.”
“Unless regulated, the intention economy will treat your motivations as the new currency. It will be a gold rush for those who target, steer, and sell human intentions.”
“We should start to consider the likely impact such a marketplace would have on human aspirations, including free and fair elections, a free press, and fair market competition, before we become victims of its unintended consequences.”
In a new Harvard Data Science Review paper, Penn and Chaudhary write that the intention economy will be the attention economy ‘plotted in time’: profiling how user attention and communicative style connects to patterns of behaviour and the choices we end up making.
“While some intentions are fleeting, classifying and targeting the intentions that persist will be extremely profitable for advertisers,” said Chaudhary.
In an intention economy, Large Language Models or LLMs could be used to target, at low cost, a user’s cadence, politics, vocabulary, age, gender, online history, and even preferences for flattery and ingratiation, write the researchers.
This information-gathering would be linked with brokered bidding networks to maximize the likelihood of achieving a given aim, such as selling a cinema trip (“You mentioned feeling overworked, shall I book you that movie ticket we’d talked about?”).
This could include steering conversations in the service of particular platforms, advertisers, businesses, and even political organisations, argue Penn and Chaudhary.
While researchers say the intention economy is currently an ‘aspiration’ for the tech industry, they track early signs of this trend through published research and the hints dropped by several major tech players.
These include an open call for ‘data that expresses human intention… across any language, topic, and format’ in a 2023 OpenAI blogpost, while the director of product at Shopify – an OpenAI partner – spoke of chatbots coming in “to explicitly get the user’s intent” at a conference the same year.
Nvidia’s CEO has spoken publicly of using LLMs to figure out intention and desire, while Meta released ‘Intentonomy’ research, a dataset for human intent understanding, back in 2021.
In 2024, Apple’s new ‘App Intents’ developer framework for connecting apps to Siri (Apple’s voice-controlled personal assistant), includes protocols to “predict actions someone might take in future” and “to suggest the app intent to someone in the future using predictions you [the developer] provide”.
“AI agents such as Meta’s CICERO are said to achieve human level play in the game Diplomacy, which is dependent on inferring and predicting intent, and using persuasive dialogue to advance one’s position,” said Chaudhary.
“These companies already sell our attention. To get the commercial edge, the logical next step is to use the technology they are clearly developing to forecast our intentions, and sell our desires before we have even fully comprehended what they are.”
Penn points out that these developments are not necessarily bad, but have the potential to be destructive. “Public awareness of what is coming is the key to ensuring we don’t go down the wrong path,” he said.
Conversational AI agents may develop the ability to covertly influence our intentions, creating a new commercial frontier that researchers call the “intention economy”.
Public awareness of what is coming is the key to ensuring we don’t go down the wrong pathJonnie PennGetty/d3signYoung woman talking with AI voice virtual assistant on smartphone
The text in this work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. Images, including our videos, are Copyright ©University of Cambridge and licensors/contributors as identified. All rights reserved. We make our image and video content available in a number of ways – on our main website under its Terms and conditions, and on a range of channels including social media that permit your use and sharing of our content under their respective Terms.
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Wrong trees in the wrong place can make cities hotter at night, study reveals
Temperatures in cities are rising across the globe and urban heat stress is already a major problem causing illness, death, a surge in energy use to cool buildings down, heat-related social inequality issues and problems with urban infrastructure.
Some cities have already started implementing mitigation strategies, with tree planting prominent among them. But a University of Cambridge-led study now warns that planting the wrong species or the wrong combination of trees in suboptimal locations or arrangements can limit their benefits.
The study, published today in Communications Earth & Environment found that urban trees can lower pedestrian-level air temperature by up to 12°C. Its authors found that the introduction of trees reduced peak monthly temperatures to below 26°C in 83% of the cities studied, meeting the ‘thermal comfort threshold’. However, they also found that this cooling ability varies significantly around the world and is influenced by tree species traits, urban layout and climate conditions.
“Our study busts the myth that trees are the ultimate panacea for overheating cities across the globe,” said Dr Ronita Bardhan, Associate Professor of Sustainable Built Environment at Cambridge's Dept. of Architecture.
“Trees have a crucial role to play in cooling cities down but we need to plant them much more strategically to maximise the benefits which they can provide.”
Previous research on the cooling effects of urban trees has focused on specific climates or regions, and considered case studies in a fragmented way, leaving major gaps in our knowledge about unique tree cooling mechanisms and how these interact with diverse urban features.
To overcome this, the authors of this study analysed the findings of 182 studies – concerning 17 climates in 110 global cities or regions – published between 2010 and 2023, offering the first comprehensive global assessment of urban tree cooling.
During the day, trees cool cities in three ways: by blocking solar radiation; through evaporation of water via pores in their leaves; and by foliage aerodynamically changing airflow. At night, however, tree canopies can trap longwave radiation from the ground surface, due to aerodynamic resistance and ‘stomatal closure’ – the closing of microscopic pores on the surface of leaves partly in response to heat and drought stress.
Variation by climate typeThe study found that urban trees generally cool cities more in hot and dry climates, and less in hot humid climates.
In the ‘tropical wet and dry or savanna’ climate, trees can cool cities by as much as 12 °C, as recorded in Nigeria. However, it was in this same climate that trees also warmed cities most at night, by up to 0.8°C.
Trees performed well in arid climates, cooling cities by just over 9°C and warming them at night by 0.4 °C.
In tropical rainforest climates, where humidity is higher, the daytime cooling effect dropped to approximately 2°C while the nighttime heating effect was 0.8 °C.
In temperate climates, trees can cool cities by up to 6°C and warm them by 1.5°C.
Using trees more strategicallyThe study points out that cities which have more open urban layouts are more likely to feature a mix of evergreen and deciduous trees of varying sizes. This, the researchers found, tends to result in greater cooling in temperate, continental and tropical climates.
The combined use of trees in these climates generally results in 0.5 °C more cooling than in cities where only deciduous or evergreen trees feature. This is because mixed trees can balance seasonal shading and sunlight, providing three-dimensional cooling at various heights.
In arid climates, however, the researchers found that evergreen species dominate and cool more effectively in the specific context of compact urban layouts such as Cairo in Egypt, or Dubai in UAE.
In general, trees cooled more effectively in open and low-rise cities in dry climates. In open urban layouts, cooling can be improved by about 0.4 °C because their larger green spaces allow for more and larger tree canopies and a greater mix of tree species.
“Our study provides context-specific greening guidelines for urban planners to more effectively harness tree cooling in the face of global warming,” Dr Ronita Bardhan said.
“Our results emphasize that urban planners not only need to give cities more green spaces, they need to plant the right mix of trees in optimal positions to maximize cooling benefits.”
“Urban planners should plan for future warmer climates by choosing resilient species which will continue to thrive and maintain cooling benefits,” said Dr Bardhan, a Fellow of Selwyn College, Cambridge.
Matching trees to urban formsThe study goes further, arguing that species selection and placement needs to be compatible with urban forms. The orientation of the ‘street canyon’, local climate zones, aspect ratio, visible sky ratio and other urban features that influence the effects of trees all need to be carefully considered.
Although a higher degree of tree canopy cover in street canyons generally results in more cooling effects, excessively high cover may trap heat at the pedestrian level, especially in compact urban zones in high temperature climates. In such locations, narrow species and sparse planting strategies are recommended.
The researchers emphasise that we cannot rely entirely on trees to cool cities, and that solutions such as solar shading and reflective materials will continue to play an important role.
The researchers have developed an interactive database and map to enable users to estimate the cooling efficacy of strategies based on data from cities with similar climates and urban structures.
ReferenceH. Li et al., ‘Cooling efficacy of trees across cities is determined by background climate, urban morphology, and tree trait’, Communications Earth & Environment (2024). DOI: 10.1038/s43247-024-01908-4
While trees can cool some cities significantly during the day, new research shows that tree canopies can also trap heat and raise temperatures at night. The study aims to help urban planners choose the best combinations of trees and planting locations to combat urban heat stress.
Trees have a crucial role to play in cooling cities down but we need to plant them much more strategically to maximise the benefits which they can provideRonita Bardhanhannahisabelnic via Flikr (Public domain)Trees in an Indian city street. Photo: hannahisabelnic via Flikr (Public domain)
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Wrong trees in the wrong place can make cities hotter at night, study reveals
Temperatures in cities are rising across the globe and urban heat stress is already a major problem causing illness, death, a surge in energy use to cool buildings down, heat-related social inequality issues and problems with urban infrastructure.
Some cities have already started implementing mitigation strategies, with tree planting prominent among them. But a University of Cambridge-led study now warns that planting the wrong species or the wrong combination of trees in suboptimal locations or arrangements can limit their benefits.
The study, published today in Communications Earth & Environment found that urban trees can lower pedestrian-level air temperature by up to 12°C. Its authors found that the introduction of trees reduced peak monthly temperatures to below 26°C in 83% of the cities studied, meeting the ‘thermal comfort threshold’. However, they also found that this cooling ability varies significantly around the world and is influenced by tree species traits, urban layout and climate conditions.
“Our study busts the myth that trees are the ultimate panacea for overheating cities across the globe,” said Dr Ronita Bardhan, Associate Professor of Sustainable Built Environment at Cambridge's Dept. of Architecture.
“Trees have a crucial role to play in cooling cities down but we need to plant them much more strategically to maximise the benefits which they can provide.”
Previous research on the cooling effects of urban trees has focused on specific climates or regions, and considered case studies in a fragmented way, leaving major gaps in our knowledge about unique tree cooling mechanisms and how these interact with diverse urban features.
To overcome this, the authors of this study analysed the findings of 182 studies – concerning 17 climates in 110 global cities or regions – published between 2010 and 2023, offering the first comprehensive global assessment of urban tree cooling.
During the day, trees cool cities in three ways: by blocking solar radiation; through evaporation of water via pores in their leaves; and by foliage aerodynamically changing airflow. At night, however, tree canopies can trap longwave radiation from the ground surface, due to aerodynamic resistance and ‘stomatal closure’ – the closing of microscopic pores on the surface of leaves partly in response to heat and drought stress.
Variation by climate typeThe study found that urban trees generally cool cities more in hot and dry climates, and less in hot humid climates.
In the ‘tropical wet and dry or savanna’ climate, trees can cool cities by as much as 12 °C, as recorded in Nigeria. However, it was in this same climate that trees also warmed cities most at night, by up to 0.8°C.
Trees performed well in arid climates, cooling cities by just over 9°C and warming them at night by 0.4 °C.
In tropical rainforest climates, where humidity is higher, the daytime cooling effect dropped to approximately 2°C while the nighttime heating effect was 0.8 °C.
In temperate climates, trees can cool cities by up to 6°C and warm them by 1.5°C.
Using trees more strategicallyThe study points out that cities which have more open urban layouts are more likely to feature a mix of evergreen and deciduous trees of varying sizes. This, the researchers found, tends to result in greater cooling in temperate, continental and tropical climates.
The combined use of trees in these climates generally results in 0.5 °C more cooling than in cities where only deciduous or evergreen trees feature. This is because mixed trees can balance seasonal shading and sunlight, providing three-dimensional cooling at various heights.
In arid climates, however, the researchers found that evergreen species dominate and cool more effectively in the specific context of compact urban layouts such as Cairo in Egypt, or Dubai in UAE.
In general, trees cooled more effectively in open and low-rise cities in dry climates. In open urban layouts, cooling can be improved by about 0.4 °C because their larger green spaces allow for more and larger tree canopies and a greater mix of tree species.
“Our study provides context-specific greening guidelines for urban planners to more effectively harness tree cooling in the face of global warming,” Dr Ronita Bardhan said.
“Our results emphasize that urban planners not only need to give cities more green spaces, they need to plant the right mix of trees in optimal positions to maximize cooling benefits.”
“Urban planners should plan for future warmer climates by choosing resilient species which will continue to thrive and maintain cooling benefits,” said Dr Bardhan, a Fellow of Selwyn College, Cambridge.
Matching trees to urban formsThe study goes further, arguing that species selection and placement needs to be compatible with urban forms. The orientation of the ‘street canyon’, local climate zones, aspect ratio, visible sky ratio and other urban features that influence the effects of trees all need to be carefully considered.
Although a higher degree of tree canopy cover in street canyons generally results in more cooling effects, excessively high cover may trap heat at the pedestrian level, especially in compact urban zones in high temperature climates. In such locations, narrow species and sparse planting strategies are recommended.
The researchers emphasise that we cannot rely entirely on trees to cool cities, and that solutions such as solar shading and reflective materials will continue to play an important role.
The researchers have developed an interactive database and map to enable users to estimate the cooling efficacy of strategies based on data from cities with similar climates and urban structures.
ReferenceH. Li et al., ‘Cooling efficacy of trees across cities is determined by background climate, urban morphology, and tree trait’, Communications Earth & Environment (2024). DOI: 10.1038/s43247-024-01908-4
While trees can cool some cities significantly during the day, new research shows that tree canopies can also trap heat and raise temperatures at night. The study aims to help urban planners choose the best combinations of trees and planting locations to combat urban heat stress.
Trees have a crucial role to play in cooling cities down but we need to plant them much more strategically to maximise the benefits which they can provideRonita Bardhanhannahisabelnic via Flikr (Public domain)Trees in an Indian city street. Photo: hannahisabelnic via Flikr (Public domain)
The text in this work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. Images, including our videos, are Copyright ©University of Cambridge and licensors/contributors as identified. All rights reserved. We make our image and video content available in a number of ways – on our main website under its Terms and conditions, and on a range of channels including social media that permit your use and sharing of our content under their respective Terms.