Why urban greening is not a panacea for extreme weather conditions under climate change
Urban greening is often presented as a means of combating both heat waves and flooding in cities. This includes green roofs, green walls, green urban spaces, private and community gardens, habitat corridors, bushland and parks.
Corn our latest research shows that, for most cities in the world, urban greening can either control flooding Where reduce the heat. He usually can’t do both in one city.
As the climate changes, cities around the world are experiencing both heat waves and flooding more frequently. Perth, for example, suffered a record heat wave last month, with six consecutive days of over 40℃. A few months earlier, Perth had recorded its wettest July in decades, with 18 consecutive days of rain.
Our results allow us to more effectively plan urban greening projects tailored to cities. So let’s take a closer look at these results and the benefits Australia can derive from urban greening.
What we found
Temperatures in cities are often several degrees higher than those in rural areas, due to the “urban heat islandwhere the predominance of concrete and steel absorbs and retains heat, and there is a lack of cooling by water evaporating from plants.
These same heat intensifying characteristics are also often responsible for flash flooding in cities, because sealed surfaces cannot act like a sponge to absorb and store rain, unlike the ground they replaced.
Read more: What drove Perth’s record-breaking heat wave – and why it’s a taste of things to come
To find out if the cooling and flood prevention benefits of urban greening hold true, we analyzed global climate models and weather information from 175 cities around the world over 15 years of daily observations, from 2000 to 2015.
Our results, published in the journal Nature Communication, show that the greatest cooling potential occurs where abundant rainwater is available for plants to transpire (release water vapor during photosynthesis). This is common for cities around the equator and in much of northern Europe.
The cooling potential of urban greening varies seasonally – it is most effective if periods of heavy rainfall coincide with summer.
In contrast, the greatest potential for soil water retention, which is crucial for flood prevention, occurs in drier areas where solar energy is abundant but rainfall is more limited. These areas are common in North Africa, Australia and the Middle East.
These areas have higher long-term average water retention and its potential varies less from season to season. Indeed, major rainfall events that exceed the soil storage capacity and cause water runoff are less frequent.
What do the results mean for Australia?
Although our results suggest that urban greening cannot reduce both flooding and heat in many, if not most, cities around the world, parts of southeastern Australia are among the few exceptions. This includes parts of Melbourne and Hobart.
Read more: Urban greening can save species, cool warming cities and make us happy
On the other hand, Canberra, Adelaide, Perth and Brisbane are “water constrained”, which means that urban greening is ineffective in reducing the urban heat island effect. However, as much of Australia has a relatively dry climate which is good for water retention, large-scale urban greening initiatives can help reduce flash flooding in these cities.
For example, Brisbane has plenty of sunshine in summer, providing ample energy for evaporation, which often exceeds the amount of summer rainfall available.
Darwin is the only state capital in Australia that, according to our modeling, would not draw heavy rainfall Where cooling benefits from urban greening.
This is because Darwin is in an area that transitions between Australia’s arid interior and the wetter tropical climates to the north. It doesn’t benefit from the high cooling or water retention performance that comes with either extreme.
Where to go from here?
While it seems like we can’t assume that urban greening can mitigate cooling and flooding at the same time, it’s still a great strategy for addressing either in many places.
Urban greening also has other positive benefits – it provides habitat, filters the air and has demonstrable effects on people’s well-being.
However, there are important cost-benefits of these types of projects to consider, both environmentally and economically.
Read more: High-density cities need greening to stay healthy and livable
Urban spaces are expensive and many greening strategies require more complex engineering than traditional buildings. Furthermore, cooling benefits may only be significant in certain regions if irrigation is used, which is impossible to do sustainably in many parts of the world.
Decision makers around the world can use our results as a first-pass guide for more local feasibility studies on urban greening. Although a crucial tool for planning and adapting to climate change, urban greening must be understood under specific local conditions – one size does not fit all.
Councils, governments, planners and developers should be fully aware of the pros and cons before embarking on urban greening projects.