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Urban greeningIt is often referred to as a way of tackling both heatwaves in cities and floods in them. This includes green roofs, living walls and vegetated urban spaces.
But our latest research showsUrban greening can subdue floods in most cities around the globe. Or mitigate heat. It is usually impossible to do both in the same city.
As the climate changes, cities around world are more likely to experience heatwaves or floods. Perth, for example, endured a record heatwave last month. Six days straight over 40℃. Perth had its 40th birthday just a few months ago. It was the wettest July in decadesWith 18 consecutive days of rain.
Our findings allow us to plan urban greening projects in a more effective way to best suit cities. So let’s take a closer look at these findings, and the benefits Australia can derive from urban greening.
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What we found
Temperatures in cities are often several degrees higher than rural areas, due to the “urban heat island” effect, where the predominance of concrete and steel absorb and retain heat, and there is a lack of cooling by water evaporating from plants.
These same heat-intensifying features are also often responsible for flash flooding in cities, as sealed surfaces can’t act like a sponge to soak up and store rain, unlike the soil they’ve replaced.
Read more:
What drove Perth’s record-smashing heatwave – and why it’s a taste of things to come
To determine whether urban greening has cooling and flood prevention benefits, we analysed global weather models and information from 175 cities across the globe. We used 15 years worth of daily observations dating back to 2000.
Our results published in the journal Nature CommunicationsThe areas with the greatest cooling potential are those that have abundant rainwater. This allows plants to transpire water vapour during photosynthesis. This is common in many cities around the Equator as well as in northern Europe.
The cooling potential of urban greening varies with the seasons – it’s more effective if periods of higher rainfall coincide with summer.
Authors. See more detail here, Author provided
In contrast, the greatest potential for water retention by soils, which is crucial for flood prevention, occurs in drier areas where there’s plenty of energy from sunshine, but rainfall is more limited. These areas are found in North Africa, Australia, and the Middle East.
These areas have higher water retention over the long-term, and their potential for water runoff is lower from season to season. Large rainfall events that exceed soil storage capacity and cause water runoff are less frequent.
What does this mean for Australia
While our findings suggest urban greening can’t reduce both flooding and heat in many, if not most, of the world’s cities, parts of southeast Australia are among the rare exceptions. This includes Hobart, Melbourne, and other parts.
Melbourne, for example can endure. urban heat island-induced temperature increases of 3℃. Greening the cityInitiatives are an important way of reducing heat.
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Urban greening can save species and cool cities. It can also make us happier.
On the other hand, Canberra, Adelaide, Perth and Brisbane are “water-limited”, which means urban greening is ineffective at reducing the urban heat island effect. Large-scale urban greening initiatives in these cities can help reduce flash flooding because of the dry climate that is prevalent in Australia.
Brisbane, for example, has a lot of sunshine in summer which provides ample energy for evaporation. This often exceeds the amount available summer rains.
Darwin is Australia’s only state capital that, accOrding to our modelling, would not derive strong stormwater orUrban greening offers cooling benefits
Darwin is situated in an area where the tropical climates of the north and south transition between the dry Australian interior. It doesn’t benefit from the high cooling or water retention performance that comes with either extreme.
Where are you going?
While it seems we can’t assume urban greening can mitigate cooling and flooding at the same time, it’s still an excellent strategy to address either in many places.
Urban greening can also be beneficial Other positive benefits – it provides habitat, filters air and has demonstrable effects on people’s well-being.
These types of schemes have important cost-benefits that should be considered, both economically and environmentally.
Continue reading:
To be healthy and more liveable, higher density cities require greening
Urban spaces are costly and many greening strategies require more complicated engineering than traditional buildings. The cooling benefits are only significant in certain areas if irrigation is used. This is not possible in many parts of the globe.
All policymakers can Use our resultsThis guide is intended to be used as a guide for further local feasibility studies on urban gardening. While it’s a crucial planning and climate change adaptation tool, urban greening has to be understood within specific local conditions – one size does not fit all.
Before embarking on urban greening projects, planners, developers, and councils need to be aware of the potential pitfalls and benefits.
