ASLA recently published findings from a study conducted by Dr. Daniella Hirschfeld, ASLA, PhD that reveal concrete ways Landscape Architects can use Nature-based solutions (NbS) to reduce the impacts of extreme heat, specifically in urban areas. The Federal Emergency Management Agency (FEMA) defines NbS as "sustainable planning, design, environmental management and engineering practices that weave natural features or processes into the built environment to promote adaptation and resilience." Many of the solutions found boil down to increasing the overall amount of green space and creating greater connectivity between green spaces.

 
As temperatures regularly break records, affecting the global temperature seems daunting and elusive, but Urban Heat Islands (UHIs) are a phenomenon Landscape Architects can directly mitigate through their designs. In her research funded by a grant from the ASLA Fund, Dr. Hirschfeld reviewed 107 international studies published between 2007 and 2022 across many scientific fields that address NbS, heat, and designing and planning built environments. Her review shows that a greater percentage of hardscape has a positive correlation with greater urban heat, but a greater percentage of green spaces combined with blue spaces like rivers performed well in terms of reducing heat. The literature also reveals that the amount of greenness in an urban park directly and positively affected the coolness of the surrounding area, with higher tree density highlighted for its efficient cooling effects. Greater square footage of a park also proved to cool further into the urban setting, with a combination of trees and short vegetation providing better benefits over single trees. Trees were also deemed more effective at reducing heat when they were taller and placed in places of high solar radiation, as they contributed to increasing shaded areas.
Studies further showed that the way a green space is connected to its environment greatly affected its ability to reduce heat. For example, moss covering and living walls demonstrated the ability to lower external wall temperatures greater than that of shade sails. This effect also occurred on the ground, where spaces with higher moss concentration saw cooler temperatures. Furthermore, compared to evenly distributed trees, clusters of trees and layers of vegetation from ground level to rooftop proved most effective at reducing UHI. On a larger scale, green spaces distributed across a cityscape that were larger and better-connected showed a positive correlation with lower land surface temperatures. Increased complexity of the green space was also a positive contributing factor.
In her report, Dr. Hirschfeld said, "Our research demonstrates the importance of maximizing the benefits of nature-based solutions to extreme heat. And landscape architects do that every day through their critically important planning and design work."
For ASLA's announcement and Dr. Hirschfeld's original report, click on the links below.
https://www.asla.org/NewsReleaseDetails.aspx?id=65222
https://www.asla.org/uploadedFiles/CMS/Practice/Action_Research/Heat_Summary.pdf
https://www.asla.org/uploadedFiles/CMS/Practice/Action_Research/Heat_Study.pdf