Extreme weather events are on the rise, increasingly impacting cities and their urban populations. In response, urban greening and nature-based solutions (NbS) have emerged as key approaches for reducing risks from multiple types of extreme climate and weather events while making a positive impact on urban social and environmental inequities. NbS interventions are high on urban agendas worldwide, but in practice, they often are hyper-local and contain novel ecological entities, with unknown capacity to deal with different pressures and disturbances. Thus, there is an urgent need to build knowledge around how, when, and under what circumstances different NbS can be expected to perform their functions as intended. One step towards building, and then constantly updating, such knowledge is to establish practices for monitoring and evaluating NbS.

In this study, we showcase a novel approach based on wireless sensor technology that harnesses hyperlocal data in real time to understand the direct impact of NbS on the local climate across seasonal variation and under extreme weather conditions. We aimed to quantify to what extent NbS are contributing to ecosystem services such as cooling.

To answer this, we installed eighteen microsensor weather stations across the biggest and most recent sustainable urban development in Sweden - Stockholm Royal Seaport. We investigated five distinct types of NbS - forest parks, green courtyards, rain gardens, green roofs, and lawns, during the summer of 2021 to examine whether real-time temperature changes varied between NbS site types. Despite large differences in vegetation and urban landscape, we did not observe a clear trend in air temperature differences between sites, even for experimental reference sites. Our analysis reveals that forest parks are the coolest and the green roofs are the warmest green places overall. The largest differences in daytime temperatures reached up to 2 °C difference between sites in summer, which gradually disappeared during cooler months. Our results suggest that regional weather dynamics dominate over the Stockholm Royal Seaport's micro-climate, leading to a relative similarity in NbS cooling performances. Though the district overall may be too homogeneous to affect air temperature variation and local NbS too small to alter the regional weather patterns, we nonetheless conclude that ecosystem services of NbS should not be taken for granted. Results suggest that NbS interventions, almost regardless of type, need to be considered and implemented at larger district scales to add up to the substantial total green cover needed to impact local and regional temperatures.