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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 type

The 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 strategically

The 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 forms

The 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.

Reference

H. 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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The study, published in Energy Research & Social Science, was completed shortly before the Winter Fuel Payment vote was taken, by researchers from the University of Cambridge and Delft University of Technology (TU Delft).

The researchers raise particular concerns about the impact of the policy on pensioners with annual incomes of between £11,300–£15,000 for single pensioners and £17,300–£22,000 for couples.

Drawing on data from the English Housing Survey, which sampled nearly 12,000 households across all income groups, the study investigated how income, energy efficiency, home size, household type, and tenure status impacted on energy expenditure.

The researchers found that an increase of £1 per year in income (after housing costs, tax and welfare payments) was associated with a marginal increase in heating spending of about one-tenth of a penny.

The study also found that just a small energy efficiency improvement – a one-point increase in the SAP12 rating (The Government's Standard Assessment Procedure for Energy Rating of Dwellings – had a major impact on households in energy poverty, offering an average reduction in annual heating costs of £21.59 per year.

Floor area also had an impact. The researchers found that a one square metre increase was associated with an annual increase in heating spend of £5.04 per year, for households facing energy poverty, making this the worst affected group. This compares with £4.18 per year for high-income households, £3.65 per year for low-income households, and £2.99 per year for very low-income households not in energy poverty.

“When low-income households receive more income, they generally spend a little more to warm their homes. But these households often have to spread any extra money they have across other essential needs including food,” said lead author, Dr Ray Galvin, affiliated with Cambridge Institute for Sustainability Leadership (CISL).

“A reduction in income like the loss of the Winter Fuel Payment could force low-income pensioners to cut back not only on heating but also on other basic necessities. This poses a significant risk to people who are particularly vulnerable to the effects of living in cold homes.”

Energy efficiency

Across all household types, the researchers found that the energy efficiency of the dwelling had by far the biggest impact on heating expenditure.

“The most effective strategy to warm up the homes of people living energy poverty is to increase the energy performance of their dwellings,” said Professor Minna Sunikka-Blank, from Cambridge’s Department of Architecture.

Specifically, the authors advise that the SAP12 rating of homes need to be increased to at least 72.

Each increase in SAP12 energy efficiency rating corresponds to a reduction in heating costs of around £20 per year, meaning that for households in energy poverty, with an average SAP12 rating of 59.48, increasing the rating to the level of low-income households not in energy poverty, 71.45, could reduce heating costs by about £240 per year.

The authors make clear that energy savings would not be sufficient to pay for these energy-efficiency upgrades. They would require targeted financial support.

Dr Galvin said: “Government and society may well find that these costs are paid back to the country through co-benefits of fewer days off work, longer lives and less strain on the health service.”

While this would not improve the finances of households in energy poverty as much as the direct monetary allowances such as the Winter Fuel Payment, it would, the authors argue, make a substantial, direct impact on cold, unhealthy homes.

Dr Galvin said: “There needs to be extra focus on developing policies for the long-term solution of retrofitting energy-inefficient homes. This can provide enduring reductions in energy bills while also improving thermal comfort. This approach may also align better with the goal of reducing carbon emissions and tackling climate change.”

Prof Minna Sunikka-Blank said: “Without retrofit initiatives, energy poverty will persist in the UK, because in low-income households immediate needs often take precedence over thermal comfort, even when incomes increase.”

Home size

The study found that households in energy poverty have a 7.3% larger average floor area than low-income households not in energy poverty, and that floor area makes a substantial difference to heating energy costs (about half to two-thirds the impact that the SAP12 energy efficiency rating has).

Tijn Croon, from TU Delft said: “These findings suggest that inability to downsize may be a significant driver of energy poverty in the United Kingdom. Low-income households can save money and stay warmer living in smaller homes but downsizing is not always easy for older households whose dependants have left home and who find themselves with a large, older home that is very expensive to keep warm.”

One-person households spend less on heating

A surprising finding in the study is that across all income groups, one-person households tend to spend less on energy. And for households in energy poverty the reduction (£36.77 per year) was twice as large as for low-income households not in energy poverty (£15.65 per year).

Tijn Croon, from TU Delft, said: “This might suggest that many one-person households are able to control their energy consumption more strategically than a multi-person household can.”

Dr Galvin said: “Our study controlled for other factors so this is not just a case of low-income households living in smaller homes. Further research could survey one-person households to find out if they have skills and practices that could be transferred to multi-person households.”

Mitigating impact of Winter Fuel Payment cuts

The authors suggest several potential solutions. Expanding Pension Credit eligibility to align with the government's low-income threshold would be the most comprehensive fix, though this may be financially unfeasible. Alternative measures could include a temporary application process for the Winter Fuel Payment for those just above the Pension Credit threshold or providing tax credits or rebates for low-income pensioners, which could be more easily managed since HMRC already holds income data.

While much attention has been given to the risk of energy poverty among pensioners, the authors also note that families with children and young adults are often equally vulnerable and may face even greater challenges in the housing market compared to pensioners.

The authors are currently working on a follow-up research paper that will explore the recent reforms to the government’s Warm Home Discount scheme.

Reference

R. Galvin, M. Sunikka-Blank, T. Croon, ‘Juggling the Basics: How Much Does an Income Increase Affect Energy Spending of Low-Income Households in England?’, Energy Research & Social Science (2024). DOI: 10.1016/j.erss.2024.103766

The UK Government’s policy to scrap Winter Fuel Payments could disproportionately affect low-income pensioners in England, new analysis suggests. But the same study argues that the energy inefficiency of homes and challenges involved in downsizing will have an even more harmful effect this winter.

Without retrofit initiatives, energy poverty will persist in the UKMinna Sunikka-BlankGioconda Beekman via Flikr under a CC licenseRooftops view from Totterdown to Bristol Centre

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An international group of researchers, led by the Copenhagen Business School, the University of Basel and the University of Cambridge, surveyed 4,000 people from Denmark, India, Nigeria and the United States about inequality in personal carbon footprints – the total amount of greenhouse gases produced by a person’s activities – within their own country.

Although it is well-known that there is a large gap between the carbon footprint of the richest and poorest in society, it’s been unclear whether individuals were aware of this inequality. The four countries chosen for the survey are all different in terms of wealth, lifestyle and culture. Survey participants also differed in their personal income, with half of participants belonging to the top 10% of income in their country.

The vast majority of participants across the four countries overestimated the average personal carbon footprint of the poorest 50% and underestimated those of the richest 10% and 1%.

However, participants from the top 10% were more likely to support certain climate policies, such as increasing the price of electricity during peak periods, taxing red meat consumption or subsidising carbon dioxide removal technologies such as carbon capture and storage.

The researchers say that this may reflect generally higher education levels among high earners, a greater ability to absorb price-based policies or a stronger preference for technological solutions to the climate crisis. The results are reported in the journal Nature Climate Change.

Although the concept of a personal carbon or environmental footprint has been used for over 40 years, it became widely popularised in the mid-2000s, when the fossil fuel company BP ran a large advertising campaign encouraging people to determine and reduce their personal carbon footprint.

“There are definitely groups out there who would like to push the responsibility of reducing carbon emissions away from corporations and onto individuals, which is problematic,” said co-author Dr Ramit Debnath, Assistant Professor and Cambridge Zero Fellow at the University of Cambridge. “However, personal carbon footprints can illustrate the profound inequality within and between countries and help people identify how to live in a more climate-friendly way.”

Previous research has shown widespread misperceptions about how certain consumer behaviours affect an individual's carbon footprint. For example, recycling, shutting off the lights when leaving a room and avoiding plastic packaging are lower-impact behaviours that are overestimated in terms of how much they can reduce one’s carbon footprint. On the other end, the impact of behaviours such as red meat consumption, heating and cooling homes, and air travel all tend to be underestimated.

However, there is limited research on whether these misperceptions extend to people’s perceptions of the composition and scale of personal carbon footprints and their ability to make comparisons between different groups.

The four countries selected for the survey (Denmark, India, Nigeria and the US) were chosen due to their different per-capita carbon emissions and their levels of economic inequality. Within each country, approximately 1,000 participants were surveyed, with half of each participant group from the top 10% of their country and the other half from the bottom 90%.

Participants were asked to estimate the average personal carbon footprints specific to three income groups (the bottom 50%, the top 10%, and the top 1% of income) within their country. Most participants overestimated the average personal carbon footprint for the bottom 50% of income and underestimated the average footprints for the top 10% and top 1% of income.

“These countries are very different, but we found the rich are pretty similar no matter where you go, and their concerns are different to the rest of society,” said Debnath. “There’s a huge contrast between billionaires travelling by private jet while the rest of us drink with soggy paper straws: one of those activities has a big impact on an individual carbon footprint, and one doesn’t.”

The researchers also looked at whether people’s ideas of carbon footprint inequality were related to their support for different climate policies. They found that Danish and Nigerian participants who underestimated carbon footprint inequality were generally less supportive of climate policies. They also found that Indian participants from the top 10% were generally more supportive of climate policies, potentially reflecting their higher education and greater resources.

“Poorer people have more immediate concerns, such as how they’re going to pay their rent, or support their families,” said first author Dr Kristian Steensen Nielsen from Copenhagen Business School. “But across all income groups, people want real solutions to the climate crisis, whether those are regulatory or technological. However, the people with the highest carbon footprints bear the greatest responsibility for changing their lifestyles and reducing their footprints.”

After learning about the actual carbon footprint inequality, most participants found it slightly unfair, with those in Denmark and the United States finding it the most unfair. However, people from the top 10% generally found the inequality fairer than the general population, except in India. “This could be because they’re trying to justify their larger carbon footprints,” said Debnath.

The researchers say that more work is needed to determine the best ways to promote fairness and justice in climate action across countries, cultures and communities.

“Due to their greater financial and political influence, most climate policies reflect the interests of the richest in society and rarely involve fundamental changes to their lifestyles or social status,” said Debnath.

“Greater awareness and discussion of existing inequality in personal carbon footprints can help build political pressure to address these inequalities and develop climate solutions that work for all,” said Nielsen.

The study also involved researchers from Justus-Liebig-University Giessen, Murdoch University and Oxford University. The research was supported in part by the Carlsberg Foundation, the Bill & Melinda Gates Foundation, the Quadrature Climate Foundation and the Swiss National Science Foundation.

Reference:
Kristian S Nielsen et al. ‘Underestimation of personal carbon footprint inequality in four diverse countries.’ Nature Climate Change (2024). DOI: 10.1038/s41558-024-02130-y 

The personal carbon footprint of the richest people in society is grossly underestimated, both by the rich themselves and by those on middle and lower incomes, no matter which country they come from. At the same time, both the rich and the poor drastically overestimate the carbon footprint of the poorest people.

SolStock via Getty ImagesA father and two sons running on a beach

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