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By Charli Shield
Not too long ago, many people weren't sure if trees had a place in cities. People, cars, houses and buildings made up urban areas — there wasn't much room for nature.
Trees now have a fundamental place in many big cities around the world, says Sonja Dümpelmann, landscape historian at the University of Pennsylvania — though in most of them, they are still vying for space.
If we want to reap the benefits of urban treescapes, ecologists say it's vital trees are seen as more than just an aesthetic addition to cities. That's especially true now that half the world's population live in cities and a further 2.5 billion are projected to live in them by 2050.
As Cities Evolve, Trees Keep Us Grounded.
Trees are powerhouses when it comes to regulating city microclimates — filtering air pollution, providing shade, absorbing CO2, helping prevent flash flooding, as well as acting as an important antidote to the urban heat island effect that makes cities far hotter than surrounding rural areas.
"Trees can make a huge difference to a city's temperature," says Tobi Morakinyo, an urban climatologist whose research into the cooling effect of trees in Akure, southwest Nigeria, showed using trees to shade buildings could cool them down by up to five degrees Celsius.
In hot sub Saharan African cities like Akure — where average maximum summer temperatures can reach 38 degrees (100 degrees F) — Morakinyo says trees' cooling effect is an important tool councils can wield against both heat stress and cooling costs.
Alongside the eco-services urban trees provide, there are also the qualities "that we can't put monetary value on," adds Cris Brack, a forest ecologist from the Australian National University and director of the National Arboretum in Canberra.
Those are "biodiversity, aesthetics and our visceral, gut-need to experience nature," Brack told DW, referring to the concept of 'biophilia' — the idea that humans have an innate desire to connect with nature. Mounting evidence shows that people who live in places with more trees experience lower levels of stress and mental illness, even when controlling for socio-economic factors.
Trees Make Us Feel Good — Do We Return the Favor?
Though our need for trees in cities appears to only be becoming greater, they often battle oppressive urban environments. Street trees are "in a constant struggle" for space in cities, says Brack, where below ground their root systems can be choked by water pipes, roads and underground car parks, and above ground by pollution, power lines and traffic.
They also face mechanical damage from cars, battering from increasingly extreme weather conditions and regular uprootings to make way for construction sites.
Perhaps the most damaging modern challenge for city trees, though, says Somidh Saha, urban forest ecologist at the Karlsruhe Institute of Technology in Germany, is drought. Following Europe's unprecedented heatwave in 2018, a study co-authored by Saha found 30% of the trees planted in Karlsruhe in southwestern Germany over the previous four years had died — both directly and indirectly because of a lack of water.
"Without enough water, trees become weak and that makes them vulnerable to disease," Saha told DW. At the same time, declining city populations of birds and arboreal mammals, such as bats, leaves insect populations unchecked, and local trees susceptible to their growing numbers.
Seeing the Forest for the Trees
Ambitious greening projects have cropped up in several megacities around the globe in recent years. New York City planted a million trees between 2007 and 2015, London mayor Sadiq Khan hopes to green more than half the capital by 2050 to make the world's first "National Park City," while Paris announced it would build four inner-city urban forests throughout 2020.
But outside the Global North, in places such as Saha's native India and Morakinyo's native Nigeria, where they cite a lack of resources and political will as big barriers to making urban greenery a priority, trees in cities are much scarcer.
As climate change brings hotter temperatures and unpredictable downpours, cities are demanding a new kind of resilience from urban trees. For many cities in the world, ecologists say that means planting more exotic species of trees.
While many people are opposed to the idea of planting non-native species, ecologists Brack and Saha say alternative species are usually better adapted to the artificial environment of a city — especially in the face of increasing heatwaves.
The three-toothed Maple, native to China, Korea and Japan, is one species that could appear in greater numbers in other parts of the world as temperatures rise.
There's also an important distinction to be made between "exotic" trees, which just means they aren't local, and "invasive" trees, which are harmful — spreading very quickly and dominating the environment. As for local wildlife, while ongoing studies are being carried out in places like Germany by Saha's team, Brack says in his local Canberra, where almost all tree species in the city are exotic, birds happily eat fruit from non-natives and mammals alike find homes wherever there is an appropriate hollow.
Citizens Pitch In
One solution to preserving city trees that's grown in popularity in recent years is citizen involvement in urban tree caretaking. New York City's citizen pruner program allows city dwellers to take classes to become official city tree carers, and Berlin — a place that has typically excluded citizens from looking after urban flora — is now allowing residents to apply for permits to maintain tree pits and has proposed that they water city trees in summer.
Involving citizens has its pros and cons, Dümpelmann says, and these kinds of programs may or may not be effective depending on the culture of the city – but even watering trees alone "has been shown to be a really relevant maintenance effort."
While planting trees in urban spaces is an effective and fairly efficient way to adapt to climate change, Dümpelmann stresses that it isn't a holistic solution.
"It's something we should work on while at the same time addressing the root causes of climate change," she said.
Beyond using trees as geo-engineering fix, urban ecologists point out that more trees in cities could change perspectives on urban living and give people a greater understanding of how to value nature as part of a sustainable, livable city – not separate from it.
That means seeing trees as living, growing beings, Brack says – not fixed in time, or immune to the stressors of living in harsh urban environments.
Reposted with permission from Deutsche Welle.
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The speed and scale of the response to COVID-19 by governments, businesses and individuals seems to provide hope that we can react to the climate change crisis in a similarly decisive manner - but history tells us that humans do not react to slow-moving and distant threats.
A Game of Jenga<p>Think of it as a game of Jenga and the planet's climate system as the tower. For generations, we have been slowly removing blocks. But at some point, we will remove a pivotal block, such as the collapse of one of the major global ocean circulation systems, for example the Atlantic Meridional Overturning Circulation (AMOC), that will cause all or part of the global climate system to fall into a planetary emergency.</p><p>But worse still, it could cause runaway damage: Where the tipping points form a domino-like cascade, where breaching one triggers breaches of others, creating an unstoppable shift to a radically and swiftly changing climate.</p><p>One of the most concerning tipping points is mass methane release. Methane can be found in deep freeze storage within permafrost and at the bottom of the deepest oceans in the form of methane hydrates. But rising sea and air temperatures are beginning to thaw these stores of methane.</p><p>This would release a powerful greenhouse gas into the atmosphere, 30-times more potent than carbon dioxide as a global warming agent. This would drastically increase temperatures and rush us towards the breach of other tipping points.</p><p>This could include the acceleration of ice thaw on all three of the globe's large, land-based ice sheets – Greenland, West Antarctica and the Wilkes Basin in East Antarctica. The potential collapse of the West Antarctic ice sheet is seen as a key tipping point, as its loss could eventually <a href="https://science.sciencemag.org/content/324/5929/901" target="_blank">raise global sea levels by 3.3 meters</a> with important regional variations.</p><p>More than that, we would be on the irreversible path to full land-ice melt, causing sea levels to rise by up to 30 meters, roughly at the rate of two meters per century, or maybe faster. Just look at the raised beaches around the world, at the last high stand of global sea level, at the end of the Pleistocene period around 120,0000 years ago, to see the evidence of such a warm world, which was just 2°C warmer than the present day.</p>
Cutting Off Circulation<p>As well as devastating low-lying and coastal areas around the world, melting polar ice could set off another tipping point: a disablement to the AMOC.</p><p>This circulation system drives a northward flow of warm, salty water on the upper layers of the ocean from the tropics to the northeast Atlantic region, and a southward flow of cold water deep in the ocean.</p><p>The ocean conveyor belt has a major effect on the climate, seasonal cycles and temperature in western and northern Europe. It means the region is warmer than other areas of similar latitude.</p><p>But melting ice from the Greenland ice sheet could threaten the AMOC system. It would dilute the salty sea water in the north Atlantic, making the water lighter and less able or unable to sink. This would slow the engine that drives this ocean circulation.</p><p><a href="https://www.carbonbrief.org/atlantic-conveyor-belt-has-slowed-15-per-cent-since-mid-twentieth-century" target="_blank">Recent research</a> suggests the AMOC has already weakened by around 15% since the middle of the 20th century. If this continues, it could have a major impact on the climate of the northern hemisphere, but particularly Europe. It may even lead to the <a href="https://ore.exeter.ac.uk/repository/handle/10871/39731?show=full" target="_blank" rel="noopener noreferrer">cessation of arable farming</a> in the UK, for instance.</p><p>It may also reduce rainfall over the Amazon basin, impact the monsoon systems in Asia and, by bringing warm waters into the Southern Ocean, further destabilize ice in Antarctica and accelerate global sea level rise.</p>
The Atlantic Meridional Overturning Circulation has a major effect on the climate. Praetorius (2018)
Is it Time to Declare a Climate Emergency?<p>At what stage, and at what rise in global temperatures, will these tipping points be reached? No one is entirely sure. It may take centuries, millennia or it could be imminent.</p><p>But as COVID-19 taught us, we need to prepare for the expected. We were aware of the risk of a pandemic. We also knew that we were not sufficiently prepared. But we didn't act in a meaningful manner. Thankfully, we have been able to fast-track the production of vaccines to combat COVID-19. But there is no vaccine for climate change once we have passed these tipping points.</p><p><a href="https://www.weforum.org/reports/the-global-risks-report-2021" target="_blank">We need to act now on our climate</a>. Act like these tipping points are imminent. And stop thinking of climate change as a slow-moving, long-term threat that enables us to kick the problem down the road and let future generations deal with it. We must take immediate action to reduce global warming and fulfill our commitments to the <a href="https://www.ipcc.ch/sr15/" target="_blank" rel="noopener noreferrer">Paris Agreement</a>, and build resilience with these tipping points in mind.</p><p>We need to plan now to mitigate greenhouse gas emissions, but we also need to plan for the impacts, such as the ability to feed everyone on the planet, develop plans to manage flood risk, as well as manage the social and geopolitical impacts of human migrations that will be a consequence of fight or flight decisions.</p><p>Breaching these tipping points would be cataclysmic and potentially far more devastating than COVID-19. Some may not enjoy hearing these messages, or consider them to be in the realm of science fiction. But if it injects a sense of urgency to make us respond to climate change like we have done to the pandemic, then we must talk more about what has happened before and will happen again.</p><p>Otherwise we will continue playing Jenga with our planet. And ultimately, there will only be one loser – us.</p>
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