For decades, we’ve known that urban environments modify climate. The “heat island” effect caused by hard surfaces boosts city temperatures, and with global warming adding its contribution to heat woes, urban heat stress becomes a very real phenomenon.
Just as we see climate responsive buildings that can save energy on lighting, heating and cooling through clever design that works with the surrounding environment, so urban landscape architecture should be climate responsive. To achieve this, says Wieke Klemm, a researcher at Wageningen University in the Netherlands, landscape architects need situational knowledge of the specific location and its unique characteristics as well as general knowledge spanning a wide range of topics including methods to influence microclimate, promote biodiversity, and facilitate recreation.
What is Urban Green Infrastructure?
Klemm sees Urban Green Infrastructure as embracing the entire network of green spaces in urban environments. They include public and private gardens, street trees, and parks, and together, they provide ecosystem services to urbanized areas. The vision would be to design and manage these spaces so that they regulate city microclimates resulting in greater comfort and reduced thermal stress.
The Cooling Effect of Urban Greenery
Urban greenery has two broad impacts on thermal comfort. On an objective level, it reduces ambient temperatures and the physical stress associated with radiated light and heat. You could be up to 1.7° C cooler in a cluster of trees or under a tree with a large canopy.
When under smaller trees, the temperature isn’t lower, but you have the comfort of shade and reduced glare so you feel cooler. Meanwhile, the shaded and vegetated surfaces don’t absorb as much heat and won’t have as much heat to give off later on.
There’s definitely a subjective effect. It’s possible that we all perceive thermal comfort differently, but our behavior is certainly influenced by urban greenery and its microclimate-regulating effects. The simplest example is moving into the shade of a tree on a hot day.
But most people don’t consciously gravitate to urban parks to get cooler. They enjoy the recreational, aesthetic, and psychological benefits of green spaces too. Nevertheless, Klemm points to research that shows a preference for green spaces with shade and water features.
Design Guidelines Needed: Theory and Practice Meet
For landscape architects and urban planners to obtain the best combination of benefits from urban green infrastructure, design guidelines are needed.
Klemm sees the development of these guidelines as consisting of a process in which real-life contexts are used to evaluate urban green infrastructure impacts on microclimate. This information feeds into evidence-based design guidelines which can be implemented in practice, evaluated, and further refined.
Green Infrastructure Guidelines: Using What we Know
By compiling accepted knowledge and information, Klemm and her research team formulated a simple set of design guidelines for urban green infrastructure. The guidelines covered cities as a whole, parks, and streets. A team of landscape architects and a team of Master’s Degree students were each supplied with the guidelines, and their responses were recorded.
In general, both groups saw the importance of considering microclimate in landscape design – not only in determining site conditions, but in altering them for greater comfort.
However, landscape architecture professionals expressed concern about implementing the guidelines for use at city and street level while approving the guidelines generated for use at park level. Participants felt that the city level guidelines, which include expanding green spaces to form interconnected corridors, were self-evident, but could be difficult to implement.
Naturally, thermal adaptation isn’t just about keeping cool. The guidelines for parks recommends a diversity of shaded, half-shaded and sunny areas with multiple potential functions and park benches allowing park visitors to choose how much sun they want to get.
At street level, tree selection and placement play a vital role. Deciduous trees allow sun through in the winter, and the researchers recommend species with wide canopies for streets subject to high sun exposure. But thermal perception also matters, and the guide suggests using greenery at different heights or levels to improve it – a measure that may be difficult to implement on many existing urban streets.
Where to From Here?
According to Klemm, climate responsive green infrastructure design isn’t getting the attention it should. She argues that global warming may soon make it a priority, even in mild climates like that of the Netherlands.
Microclimate enjoys some attention as a design consideration, but improved consciousness could lead to more focused implementation of the design guidelines, a greater volume of scientific evidence, and a refined set of green infrastructure guidelines for the cities of tomorrow.