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The science of urban greening

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People want action on rising temperatures in Australian cities. Can urban trees help?

The catastrophic bushfires are a terrible reminder that adapting to climate change won't be enough. Here's how urban trees can help stop it.


The dangers of extreme heat waves have never been more apparent. In addition to the deadly risks for those directly in the path of the fires, excessive heat loads are serious threats to human health. In the 2009 bushfires in Victoria, more people died from excessive heatloads than in the fire itself. Right now, vulnerable populations like the elderly are in high risk for health problems in the hottest urban areas. And rising temperatures will only increase the demand for air conditioning, which increases outdoor heat loads, leading to even more energy use and greenhouse gas emissions.


By all indications, heat waves in Australia and elsewhere will keep coming, and they will get worse. Given the magnitude of the problem, can planting more trees in cities really help? As I write this, temperatures in Penrith, New South Wales, a western Sydney suburb, reached a record-breaking 48.9oC (120oF). How much can trees really offset that much excess heat?


The carbon sequestration potential of urban trees is very small, because cities emit enormous amounts of CO2 while occupying only a tiny fraction of the earth's land surface. However, urban trees do significantly affect local air temperatures at different scales and by different mechanisms. In the next few months, I'll be reviewing the current evidence for specific tree cooling effects with a group led by Australian scientists. So we'll be reporting on the details of how cooling mechanisms work later. For now, we'll look at just a few ways that urban trees might be used for climate adaptation - coping with climate change - but also climate mitigation, which avoids climate change by offsetting or reducing greenhouse gas emissions.


The localized effects of trees

This small tree doesn't provide much shade. But when planted as a barrier for cars, it has an outsized impact on fossil fuel emissions.

If you've ever sat under a tree to get some shade on a hot day you know that trees intercept sunlight and keep you cooler. This effect can be quite localized, so it may seem trivial at first. If there are enough trees, shading effects can scale up to cool cities by a few degrees (3oC on average in one modeling study of the U.S., albeit with large regional differences). But shade, and its effect on human comfort outdoors, is also a pretty powerful lever of human behavior. At home in Salt Lake City I usually walk to work, and in the summer I'll walk well out of my way to stay in the shade and out of the blazing Utah sun. When it gets really hot I take the bus, because luckily my local bus stop is under the shade of a tree canopy. In hot cities, shading of bus stops and pedestrian walkways is a tool in the arsenal of ways to get people out of their cars, and one that needs to be fairly distributed to low income, and not just wealthy, neighborhoods.


Speaking of cars, urban trees are known to be effective for traffic calming - slowing down traffic - which makes streets safer for pedestrians and further discourages driving. This isn't a cooling effect, but all of these tools in urban and landscape design influence human behavior around transportation and energy use - the very behaviors that caused climate change in the first place.


Effects on indoor air temperature


Trees that are strategically placed around buildings can lower the temperature indoors, where most heat-related mortality occurs. This is partially a shading effect and partially due to evaporative cooling (more on the details of urban evapotranspiration later). If we want to affect building microclimate, we have to carefully locate trees in ways that affect air temperatures at larger scales than the cooling you experience from standing or sitting under a tree. Which means that we can't just plant trees in any old place. Exactly where, how many, and what types of trees are located near buildings matters: trees most effectively shade buildings on their south and west sides; these trees need to be relatively large; and the particular species will determine the amount of evaporative cooling.


The indoor and outdoor environment of buildings are closely related - another tool to reduce energy use.

The devil is in the details, but if we can design sites and landscapes that passively cool buildings, we can both adapt to and mitigate climate change at the same time. By lowering building temperatures, we'll lower the demand for air conditioning and burn less fossil fuel. This will help avoid and not just "cope" with climate change.


The effects of trees on our own choices


In my opinion, the most effective way to include urban forests in climate change strategies is to acknowledge that while human actions greatly influence trees and forests, trees also greatly influence us. Whether we choose to spend time outdoors, where and how we move around, whether we invest in air conditioning, how we adjust the thermostat, and whether we walk, drive, or take the bus are all influenced by our environment, including the plants, landscapes, and trees around us. There are limits to how much urban trees and greenspace can compensate for these new, extreme heatwaves through direct cooling mechanisms. We should try to maximize direct cooling - but we can do even more. Trees and urban nature are part of the day-to-day experience of living in a city, shaping our perceptions and decisions. And so many of our decisions influence energy use. We can strategically incorporate these relationships into plans that transition cities away from cars and fossil fuels.


Urban tree and forest stewards are well aware of the close and often complicated relationships between trees and human behavior. We'll look at some out-of-the-box thinking about human-tree relationships in the next post, when we visit the Australian School of Urban Forestry at the University of Melbourne.

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