{"id":3494149,"date":"2022-12-21T15:59:58","date_gmt":"2022-12-21T15:59:58","guid":{"rendered":"https:\/\/www.resilience.org\/?p=3494149"},"modified":"2022-12-21T15:59:59","modified_gmt":"2022-12-21T15:59:59","slug":"the-cooling-cleansing-power-of-canopies-urban-tree-planting-takes-root","status":"publish","type":"post","link":"https:\/\/www.resilience.org\/stories\/2022-12-21\/the-cooling-cleansing-power-of-canopies-urban-tree-planting-takes-root\/","title":{"rendered":"The cooling, cleansing power of canopies \u2013 urban tree planting takes root"},"content":{"rendered":"

From the London borough of Hackney and Barcelona in Spain, to Freetown in Sierra Leone,\u00a0increasing the number of trees<\/a>\u00a0in cities has been shown to be an important, low-cost, and rapid way to cut pollution, improve health and well-being, and make cities less vulnerable to extreme weather.<\/p>\n

Barcelona\u2019s Trees for Life Master Plan<\/a>\u00a0is making good progress towards its goal of covering 30% of the city with trees by 2037, prioritising citizen engagement to help shape the project. And in Hackney, London,\u00a0former Hackney councillor Jon Burke<\/a>, who now advises cities on decarbonisation and climate change adaptation, oversaw an increase in the street ten-year canopy cover from 21% to 32% in the space of two years as part of the street tree programme. This was the largest urban canopy expansion a UK local authority has ever achieved.<\/p>\n

In the very different setting of Sierra Leone\u2019s capital, Freetown, the\u00a0#FreetownTheTreeTown<\/a>\u00a0campaign has successfully combined the powers of modern technology with local citizens\u2019 knowledge to plant and grow a million trees by the end of 2022, increasing the city\u2019s vegetation cover by around 50%.<\/p>\n

\"\"<\/p>\n

Image of main pedestrian street in Barcelona paved with trees (credit: Photo by Ronni Kurtz on Unsplash)<\/em><\/p>\n

Cities are on the frontline of twin crises: climate and air pollution. Many of the effects of climate change that are being felt around the world, such as extreme heat and flash flooding, are felt most strongly in cities\u2014especially amongst the most vulnerable communities. This happens because of the \u2018heat island\u2019 effect caused by many artificial hard surfaces, and due to the lack of natural ways for rainwater to soak away. During a heatwave in 2003,\u00a0mortality rates shot up by 142% in the city of Paris compared to the summer average<\/a>.<\/p>\n

As densely populated places,\u00a0home to 4.4 billion people worldwide<\/a>, cities are also struggling to deal with pernicious issues like air pollution, which\u00a0impacts 2.5 billion people globally and led to 1.8 million excess deaths across cities in 2019<\/a>. A more recent estimate puts the annual number of\u00a0deaths due to air pollution from burning fossil fuels at over 8 million<\/a>. Long-term exposure to air pollution can cause a wide range of health problems and medical conditions, from\u00a0heart disease<\/a>\u00a0to a\u00a0higher chance of getting dementia.<\/a><\/p>\n

[slide-anything id=’3472166′]<\/p>\n

Despite the many issues blighting modern cities, urban tree planting appears to be a perfect part of the antidote. In terms of rapid transition, increasing canopy cover in cities is an easy and low-cost climate win-win. But it\u2019s important to get it right in the first place, and that means engaging communities, empowering citizens, and considering factors such as species diversity, well adapted native varieties, and urban biodiversity.<\/p>\n

Lessons for rapid transition<\/h3>\n
    \n
  1. Evidence-led policy that brings together exports and communities alike:<\/b>\u00a0by bringing local people into the fold, and giving them a say over which trees go where, urban tree programmes can pollinate.<\/li>\n
  2. Community leaders and politicians that walk the talk, understand the scale and urgency of action, and are backed by national governments<\/b>: local leaders and politicians that stand up for their communities\u2019 needs and understand the urgency and severity of climate action are essential to getting urban tree planting programmes off the ground, but they need support and resources from national governments.<\/li>\n
  3. Capturing the co-benefits:<\/b>\u00a0while planting trees in cities comes at a cost, the (co)benefits abound. Communicating these to communities, fostering knowledge of trees amongst citizens and embedding these benefits into local authority planning metrics, will help urban canopies spread while empowering citizens.<\/li>\n<\/ol>\n

    The big picture<\/h3>\n

    When it comes to human-induced climate change, cities are both a prime source of the problem and an indispensable solution. Home to half the world\u2019s population, cities produce\u00a0around 75% of the world\u2019s greenhouse gas emissions<\/a>\u00a0and\u00a080% of global GDP<\/a>. Cities\u2019 density and resource-intensity are only set to rise in the coming decades. By 2050,\u00a0up to 80% of humanity could be living in cities<\/a>, with roughly 40 million people moving to them every year. Climate change and urbanisation could accelerate hand-in-hand without drastic intervention.<\/p>\n

    \"\"<\/p>\n

    Central Tokyo crossing busy with people (credit: Photo by Denys Nevozhai on Unsplash)<\/em><\/p>\n

    Promisingly, though,\u00a025 of the world\u2019s cities account for 52% of total annual urban greenhouse gas emissions<\/a>. This means that deep and far-reaching decarbonisation, targeted across a number of cities, could have a dramatic impact on cumulative global emissions and future warming trajectories. Cities are key to deciding humanity\u2019s future.<\/p>\n

    As hubs for both human life and economic activity, cities are at greater risk of facing disturbances and disruptions at the hands of climate change. Due to their proximity to coastal areas, many of the world\u2019s largest cities will find themselves threatened by sea-level rise. By 2050,\u00a0one-third of the Thai capital, Bangkok, could be permanently submerged<\/a>.<\/p>\n

    Around the world, cities face a much higher probability of experiencing extreme weather events that, depending on their locations, will bring violent storms,\u00a0severe drought<\/a>, freak snowfall,\u00a0water shortages<\/a>,\u00a0more wildfires<\/a>\u00a0and\u00a0gruelling, lethal heat waves<\/a>. As with many of the other cruelties of climate change, the brunt of these impacts will fall on the\u00a0most vulnerable and marginalised citizens<\/a>: the poor, the elderly, minorities, and those who lack the means to adapt. In the tropical city of Jakarta, where citizens battle heat and humidity today,\u00a0the rate of excess deaths among elderly urban citizens will increase by up to 15 times by 2050<\/a>\u00a0under a high-emissions scenario.<\/p>\n

    While many cities are leading in progressive approaches to\u00a0decarbonising transport<\/a>,\u00a0kicking out cars<\/a>\u00a0and\u00a0increasing clean energy generation<\/a>, they cannot adapt quick enough to the changing climate.\u00a0One study looking at over 570 cities across Europe<\/a>\u00a0concluded that they will face an entirely new climate within the next 30 years, punctuated by more heat, drought and flooding.<\/p>\n

    \"\"<\/p>\n

    Image from NYC March for Science (credit: Photo by Chris Boese on Unsplash)<\/em><\/p>\n

    By their nature, urban infrastructures are unwieldy, inert, and built for a climate that no longer exists. Research suggests that updating urban infrastructure can be done at an\u00a0adaptation rate of 3% per year<\/a>. In a similar vein, the\u00a0European Commission predicted that it would take a minimum of 50 years to revamp buildings so they are fit for purpose in a warmer world<\/a>. Given the protracted processes and lengthy lead-up times, it is essential to capture, accelerate, and scale up climate solutions like increasing urban tree cover that can deliver adaptation, resilience, and a range of co-benefits at pace, while simultaneously decarbonising transport, energy, and the built environment. Urban trees alone are no silver bullet, but they are a remarkably generous gift of nature that can make urban life more tolerable and pleasant.<\/p>\n

    Context and background<\/h3>\n

    Trees have never been strangers to cities and have long played a part in the process of urbanisation around the globe,\u00a0from inspiring the first columns in Roman cities to helping clean up existing metropolises, like New York\u2019s Central Park<\/a>. Street trees were a common sight during the reign of the\u00a0Tang Dynasty in China from 618 to 907 AD<\/a>\u00a0and were relatively ubiquitous in the\u00a015th century cities in Italy, France, Germany, and the UK<\/a>. The city of\u00a0Curitiba in Brazil\u00a0<\/a>was planned around its nature and ecology.<\/p>\n

    Yet the role of trees, and more specifically, street trees, in helping cities adapt to the climate and environmental crises is a more recent phenomenon. And that\u2019s because our understanding of their functions as part of diverse urban ecosystems is improving week on week as the impacts of climate change become ever more apparent.<\/p>\n

    Urban trees are a thing of magic. A study conducted in Manchester, UK, found that\u00a0street trees reduced surface temperatures by an average of 12C<\/a>. In some contexts, this would be the difference between life and death. Street trees are perhaps the most energy-efficient urban cooling devices we currently have, with\u00a0one US-based study showing<\/a>\u00a0that a single healthy street tree produces the same atmospheric cooling effect as around five room-sized air conditioning units operating for 20 hours a day. But the quality of cooling generated by trees is far superior to its mechanical counterparts because the leaves of trees transpire and evaporate water vapour, which reduces humidity and cools air temperatures. Urban trees are also shown to reduce nighttime temperatures in cities by reducing the amount of solar radiation absorbed at ground level.<\/p>\n

    \"\"<\/p>\n

    A lake surrounded by trees in Hamarikyu Gardens, Tokyo (credit: Photo by Egor Myznik on Unsplash)<\/em><\/p>\n

    Trees also tackle another urban blight: air pollution.\u00a0By intercepting and absorbing pollutants at street level<\/a>, urban trees help withdraw particulate matter (PM), nitrogen dioxide, sulphur dioxide and carbon monoxide. PM is a dangerous affliction for urban populations, linked with\u00a0exacerbating respiratory illnesses<\/a>,\u00a0heart disease<\/a>, and\u00a0inflammation<\/a>.\u00a0Global estimates for deaths associated with long-term exposure to PM outdoors could be in the region of 8.9 million a year<\/a>. Prolonged exposure for children is particularly worrying, as\u00a0it is proven to impact neurodevelopmental and cognitive ability, can trigger asthma<\/a>\u00a0and also\u00a0increase the risk of developing disease in adulthood<\/a>. The social impacts of this are severe: air pollution in cities can shape the life chances of young children. By reducing surface temperatures, street trees can also reduce the risk of ozone pollution peaking,\u00a0which is much more likely on hot days<\/a>.<\/p>\n

    Flash flooding from heavy rainfall, which is becoming more frequent and severe due to climate change, can be somewhat ameliorated with urban trees. Cities are built on concrete, tarmac, and other impervious surfaces that not only create heat islands, but cause rainwater to run off rather than soak into the soil. Urban trees can\u00a0curtail the intensity of rainwater runoff by slowing rainfall through canopy cover<\/a>, holding water in their leaves that evaporates into the air, and providing soil and roots that can soak up rainfall.\u00a0Researchers in Canada found that urban trees could intercept between 50% and 60% of rainfall<\/a>, greatly reducing the risk of flooding and the subsequent cost of cleaning up flood damage.<\/p>\n

    There are some even more surprising benefits of urban trees that can help boost local economies and save public spending. A\u00a02015 study on crime in a city in the state of Connecticut, USA, found that greater tree canopy cover was associated with lower rates of violent crime and property crime<\/a>. This\u00a0study<\/a>\u00a0suggests that for every ten percent increase in canopy cover, violent crime drops by 15% and crime towards property falls by 14%. Trees are not usually known for their crime fighting powers. Access to and even just the sight of urban nature has immediate and measurable health and well-being benefits. The past few years have seen an explosion of research finding concrete links between increased exposure to nature and not just improved physical health but better\u00a0mental health<\/a>\u00a0as well.<\/p>\n

    \"\"Group of people enjoying time in nature in New York (credit: Photo by Ben Duchac on Unsplash)<\/em><\/p>\n

    So, trees make us feel good too. Urban vegetation has been scientifically shown to improve\u00a0physical<\/a>,\u00a0mental, and social wellbeing<\/a>. A\u00a0recent study found that a high number of street trees<\/a>\u00a0close to the home is associated with lower levels of antidepressant prescriptions in the local area, even when controlling for factors such as deprivation. Trees help non-human lifeforms too. Studies show\u00a0a direct correlation between tree species richness within cities and a high diversity of bird species in the area<\/a>.<\/p>\n

    But for the benefits of trees to really stack up, scale is key. In cities, canopy cover needs to be at\u00a0around 40% before citizens begin to feel the cooling benefits, but ideally it should be higher<\/a>\u00a0as this is when higher marginal returns kick-in. As Jon Burke points out, if you hit 45% canopy cover, the surface temperatures will fall even further. Also important is the diversity of urban tree species. A 2022 study exploring urban trees in Westminster, London,\u00a0found that a higher diversity of tree species resulted in reduced vulnerability and susceptibility to pests and disease<\/a>. For this reason, it is critical to learn from the successes of street tree programmes in order to implement similar strategies in other cities to achieve both scale and diversity.<\/p>\n

    Enabling factors<\/h3>\n

    Evidence-led policy that brings together exports and communities alike<\/h4>\n

    Engaging local people in decision-making processes is a prerequisite for successful policy implementation, but it must be combined with evidence-led policy and action. Street trees are no different.<\/p>\n

    By bringing together experts, such as trained arborists, and local citizens, schemes that work for the community will flourish. Implementing street trees can be quite a technical endeavour, so citizen growers may not be well suited. But that isn\u2019t to say that engaging and involving citizens doesn\u2019t work; it does. The\u00a0Trees for Cities<\/a>\u00a0programme shows that by engaging citizens in tree planting and green infrastructure schemes, diverse communities and demographics can be engaged, creating citizen ambassadors for wider green infrastructure pushes. Urban tree planting programmes may provide a useful conduit for engaging with constituencies that the mainstream climate movement has failed to reach.<\/p>\n

    Barcelona\u2019s Trees for Life programme has an onus on engaging local citizens in the process of designating and designing urban green spaces. This\u00a0process of co-creation initially brings together local residents and district representatives<\/a>\u00a0to identify the needs of the surrounding community and how street trees and green spaces can help meet them. There is also a dedicated workstream to communicate the benefits of street trees to the population of Barcelona as a way of improving public participation. The strategy also has the ambitious goal of ensuring that\u00a0all primary school children can identify the trees in their neighbourhood<\/a>. Improving the relationship between urban populations and trees will lead to more trees, as the closer people live to green spaces, the more inclined they are to support green infrastructure and climate policy.<\/p>\n

    \"\"<\/p>\n

    Group of people planting trees (credit: Photo by Rafael Idrovo Espinoza on Unsplash)<\/em><\/p>\n

    Community leaders and politicians that walk the talk, understand the scale and urgency of action, and are backed by national governments<\/h4>\n

    Climate action needs real leadership to flourish. Behind every urban tree planting success around the world are determined and tenacious local politicians that understand the scale and severity of the climate crisis, as well as how local action can trigger big changes. And this needn\u2019t be about lecturing communities in a paternalistic manner. As the former Hackney councillor, Jon Burke, reflects, \u201cit is about putting the structures in place to allow society to flourish.\u201d<\/p>\n

    Erecting structures requires funds \u2013 and local ambition needs to be backed by resources from central governments. Given the strategic importance of green infrastructure and how it directly links to things like increased rainfall patterns, leadership and guidance should emanate from the national level. This could take the form of urban tree mandates, where local authorities have their funding matched or doubled to scale up their canopy cover with minimum quotas needing to be met. What\u2019s vital, however, is that national leadership actually leads to action \u2013 green infrastructure and street trees should not be an excuse for yet another select committee.<\/p>\n

    Backing local leaders with national resources could also tackle inequality. In cities, vulnerable and marginalised communities are much more likely to live in areas with limited or no green space. Urban tree planting could therefore be a platform to help the vulnerable and marginalised become more resilient in the face of extreme weather by bringing free cooling to their neighbourhoods. This is exactly what happened in Hackney, where after mapping the street trees of the borough, tree planting began in the poorest neighbourhoods.<\/p>\n

    Capturing the co-benefits<\/h4>\n

    The co-benefits of trees are as extensive as they are empirically proven. But communicating these co-benefits and monitoring their effects on urban populations is an essential part of scaling urban tree programmes across contexts and countries. Doing this requires\u00a0effective advocacy that can snowball<\/a>. Conversely, the failure to do so can see trees devalued, and even removed as a result of crude cost-cutting by outsourced street maintenance firms \u2013 a short-sighted form of urban self-harming.<\/p>\n

    Open data and cross-council collaboration can dramatically improve how co-benefits are communicated. Across the UK already, many local authorities capture data on trees on systems such as Ezytreev and Confirm, but very few are openly sharing this data across local government authorities or using it as a way of stimulating competition between councils.<\/p>\n

    What\u2019s more, data should be used to develop alternative ways of measuring the value of trees. For example, metrics on how urban trees help local businesses or how they improve learning environments for children. Local communities are much more likely to grasp the value of trees by demonstrating how they improve their lives and livelihoods.<\/p>\n

    \"\"<\/p>\n

    Credit: Photo by CHUTTERSNAP on Unsplash<\/em><\/p>\n

    It also requires including the co-benefits of urban trees into local councils\u2019 and authorities\u2019 asset and risk management frameworks. Trees are often included, but as an ongoing operational and maintenance cost, rather than incorporating the role trees play in flood prevention and microclimate regulation, which helps cut costs elsewhere. But the cost-saving benefits of trees need to be stressed to local communities, especially given that council budgets will likely face further cuts to public spending and other demands from citizens.<\/p>\n

    References<\/h3>\n