Urban heat mitigation by green and blue infrastructure: Drivers, effectiveness, and future needs

Abstract

The combination of urbanization and global warming leads to urban overheating and compounds the frequency and intensity of extreme heat events due to climate change. Yet, the risk of urban overheating can be mitigated by urban green-blue-grey infrastructure (GBGI), such as parks, wetlands, and engineered greening, which have the potential to effectively reduce summer air temperatures. Despite many reviews, the evidence bases on quantified GBGI cooling benefits remains partial and the practical recommendations for implementation are unclear. This systematic literature review synthesizes the evidence base for heat mitigation and related co-benefits, identifies knowledge gaps, and proposes recommendations for their implementation to maximize their benefits. After screening 27,486 papers, 202 were reviewed, based on 51 GBGI types categorized under 10 main divisions. Certain GBGI (green walls, parks, street trees) have been well researched for their urban cooling capabilities. However, several other GBGI have received negligible (zoological garden, golf course, estuary) or minimal (private garden, allotment) attention. The most efficient air cooling was observed in botanical gardens (5.0 ± 3.5°C), wetlands (4.9 ± 3.2°C), green walls (4.1 ± 4.2°C), street trees (3.8 ± 3.1°C), and vegetated balconies (3.8 ± 2.7°C). Under changing climate conditions (2070-2100) with consideration of RCP8.5, there is a shift in climate subtypes, either within the same climate zone (e.g., Dfa to Dfb and Cfb to Cfa) or across other climate zones (e.g., Dfb [continental warm-summer humid] to BSk [dry, cold semi-arid] and Cwa [temperate] to Am [tropical]). These shifts may result in lower efficiency for the current GBGI in the future. Given the importance of multiple services, it is crucial to balance their functionality, cooling performance, and other related co-benefits when planning for the future GBGI. This global GBGI heat mitigation inventory can assist policymakers and urban planners in prioritizing effective interventions to reduce the risk of urban overheating, filling research gaps, and promoting community resilience.

Keywords: climate change; heat mitigation; heat stress; nature-based solutions; sustainable development goals; urban cooling.

Graphical abstract

Figure 1

The literature availability across the 10 primary types of GBGI and their 51 subcategories The number of (A) identified, (B) screened, (C) eligible, and (D) included publications for meta-analysis, and (E) the percentage of included publications for each of the 51 GBGI sub-categories (shown at the y axis of A), falling under the 10 main GBGI categories (shown as bold text in A). A detailed list of the GBGI main and sub-categories is listed in Table S3.

Figure 2

Geographical distribution of reviewed papers based on the number of GBGI categories, their location (latitude and longitude) by continent and number of publications by year The number in the magnetic disk and bar plot shows the number and percentage of GBGI sub-categories and types in each continent.

Figure 3

Scatterplot of GBGI cooling efficiency Scatterplot of GBGI cooling efficiency in different climate zones and against population density (A), area of city (B), ratio: area of GBGI/area of city (C), altitude (D), and temporal scale of cooling (E).

Figure 4

Number of studies under each GBGI category (A) A six-point-scale evidence-based classification of the number of studies under each GBGI category, (B) co-benefits, (C) dis-benefits, and (D) multiple interventions of GBGI within the main category of GBGI for heat mitigation. Gray cells indicate that there was no evidence found in the online database (Table S3).

Figure 5

Performance of all GBGI sub-categories (A) Scatter representation showing the performance of all GBGI sub-categories assessed from 202 papers through the following methodologies in the reviewed publications: (B) in situ monitoring, (C) numerical modeling, (D) monitoring and numerical modeling (MM), (E) RS, and (F) the overall performance with and without RS (average of a-d) for each of the GBGI categories. The error bars in all plots represent 95% LCIs and UCIs as computed using the t-test. The CI is not applicable for GBGI sub-categories with very low publication availability. The data presented above from individual studies are summarized in Table S3.

Figure 6

Köppen-Geiger climate conditions Each GBGI’s performance is classified into Köppen-Geiger climate conditions (A, E, I), followed by categorization into spatial scales (B, F, J). Cooling performance in the same climate zone at three spatial scales is linked to the location (left: C, D, G, H, K, L, N) and surroundings (right: C, D, G, H, K, L, N) of the GBGIs. The location IDs (1: front, 2: inside, 3: inside and near, 4: inside and outside, 5: inside and top, 6: inside, outside, and near, 7: inside, outside, and top, 8: near, 9: not reported, 10: outside, 11: outside and top, and 12: top) represent where the cooling performance was calculated. The surrounding conditions (13: built-up area, 14: mixed [built-up with nature], 15: nature, and 16: not reported) describe the environment around the GBGIs.

Figure 7

Efficiency of various GBGI types for urban heat mitigation (A) A summary of the overall performance of different GBGI types from all studies, (B) heatmap showing GBGI performances from for different methods and the average values, and (C) overall average of GBGI efficiency for urban heat mitigation. The error bars in all plots represent 95% LCIs and UCIs as computed using the t-test. The Average and Average∗ values represent the average of all study types with and without RS data, respectively. M&M denotes combined monitoring and modeling studies. The color gradient represents the performance, with gray cells representing studies that did not consider either monitoring, modeling, M&M, or RS. The figure uses a boxplot representation with the median indicated by a thick vertical black line, the mean represented by blue dots, and the upper and lower quartiles indicated by the box boundaries. The circle with a vertical line represents the GBGI categories with only one publication. All numerical data presented is provided in Table S6.

Figure 8

Base maps are Köppen-Geiger classifications, and the point are location of ten GBGI categories (A) The present-day map (1991–2020) and (B) the future map (2071–2100) under the RCP8.5 scenario.

Figure 9

A conceptual framework outlining the implementation of GBGI to mitigate urban overheating A conceptual framework outlinning the GBGI implementation for heat mitigation through four stage processes: co-planning, design, and management, full-scale development, and nine sub-processes.