Nature-based solutions (NBS) offer significant potential for enhancing ecosystem services in cities, mitigating climate change impacts, improving biodiversity, and promoting liveability. Studies highlight the benefits of NBS in reducing heatwave effects, managing stormwater, and providing recreational spaces. However, urban densification has led to a loss of private green spaces without adequate replacement in the public realm, resulting in decreased biodiversity, increased flood risk, open space shortages, and vulnerability to urban heat island effects. This has spurred the adoption of ambitious municipal NBS strategies with bold targets for tree planting, vegetation cover, and tree canopy. The challenge lies in delivering these targets at scale by retrofitting NBS into existing urban environments where public space is highly contested. The urgent need for large-scale urban NBS delivery is driven by global policy drivers such as the Sustainable Development Goals (SDGs), the need to mitigate increasingly frequent and severe climate events, and the role of cities in biodiversity conservation and addressing environmental injustices. The concept of a 'green recovery' supported by NBS delivery has gained momentum post-COVID-19. Despite the importance of large-scale NBS delivery, it remains largely unrealised. Optimistic discourses often fail to acknowledge the significant land use change necessary. The authors highlight the challenge of finding sufficient land for NBS, using the example of Melbourne, Australia's Elizabeth Street Catchment, which requires 65 ha of de-paved or permeable land by 2030 to address extreme flood risk. Finding this much land in a dense urban area necessitates targeting existing land uses for systematic replacement. Urban land is expensive and subject to competing uses; hence, identifying the most viable opportunities for large-scale change is crucial. The study focuses on a promising trade-off: converting street parking into biodiverse green space. Streetscapes cover vast areas in many city centers, with a significant portion allocated to on-street parking. The existence of substantial off-street parking capacity in many cities, often due to outdated urban planning regulations, suggests a potential for duplication. High vacancy rates in off-street parking, especially in central city garages, further highlight this underutilization. The paper investigates the opportunity to consolidate on-street parking into nearby garages with redundant capacity, creating space for NBS. This could be achieved through parking management mechanisms like centralized parking facilities or peer-to-peer parking apps. While this approach involves no net loss of parking, it requires addressing potential concerns such as the loss of familiarity with street parking, the additional cost of garage parking, and public perception regarding the necessity of free street parking. The study aims to minimize these concerns by focusing only on street parking very close to vacant garages.

The introduction extensively reviews existing literature on the importance of Nature-Based Solutions (NBS) in urban environments, citing numerous studies that demonstrate their effectiveness in addressing climate change impacts, improving biodiversity, and enhancing the overall liveability of cities. The review highlights the challenges associated with the implementation of NBS at scale, particularly the scarcity of available land in dense urban areas. The authors point to several global policy initiatives that underscore the urgent need for large-scale NBS deployment, including the Sustainable Development Goals and the post-COVID-19 'green recovery' initiatives. The literature also shows the under-utilization of off-street parking in many cities and the potential for repurposing this space to benefit urban sustainability. The existing literature, however, lacks a focused examination of the potential for converting redundant street parking into green space as a scalable NBS strategy. This gap is what the current study aims to address.

The study employs a two-phase methodology. Phase 1 involves GIS analysis to identify redundant street parking spaces based on proximity to underutilized off-street parking. Twelve scenarios are modeled, varying assumptions regarding the type of destination garages (commercial, non-commercial, or both), vacancy levels (high or low), and maximum distance between on-street and off-street parking (100 m and 200 m). This identifies thousands of potentially redundant parking spaces, while acknowledging that some on-street spaces are irreplaceable (disability and delivery parking). Phase 2 models the sustainability benefits of replacing redundant parking with biodiverse green space. The benefits are quantified for three ecosystem services: (1) increased tree canopy cover, (2) stormwater interception, and (3) improved ecological connectivity. A modular green space design, informed by Water Sensitive Urban Design (WSUD) and Biodiversity Sensitive Urban Design (BSUD) principles, forms the basis of the modeling. This design integrates a street tree, habitat resources (understory plants), stormwater infiltration (a sunken 'raingarden'), and de-paving of asphalt. The models are calibrated using existing data from the City of Melbourne to estimate the increase in tree canopy (using allometric analysis and linear regression to project growth), the improvements in ecological connectivity (using a geometric measure based on effective mesh size), and the amount of stormwater interception (using the MUSIC model). Vacancy rates in off-street garages are estimated based on existing data and informed assumptions. Residential vacancy rates are based on previous studies. For commercial and office parking, higher vacancy rates are assumed, reflecting potential changes in work patterns post-COVID-19. The methodology explicitly acknowledges conservative assumptions, such as the 'no net loss' of parking availability (only relocation, not removal). GIS analysis using location-allocation methods identifies optimally placed on-street parking for consolidation. The modelling considers design constraints like overhead cables, underground services, and existing canopy cover. Three design variations are developed to account for different site conditions (commercial areas, standard kerbside parking, and median parking).

The study reveals substantial opportunities to convert redundant on-street parking into green space. Across twelve scenarios, the number of redundant on-street spaces identified ranged from 3146 to 11,668, representing 12.7% to 47% of the total on-street spaces (covering roughly 50 ha). The potential increase in tree canopy cover ranges from 31 to 59 ha at maturity, with a significant contribution in intermediate years (11-22 ha). This represents a substantial addition to the existing public realm tree canopy, even with the conservative choice of tree species prioritizing habitat over canopy optimization. Ecological connectivity improved significantly, creating habitat stepping stones and reducing fragmentation for two focal animal species, the Blue-banded Bee and the New Holland honeyeater. The study identified 6.6–24.5 ha of parking that could be de-paved, creating permeable green space. A significant portion of this de-paving opportunity (2.7–7.7 ha) lies within the flood-prone Elizabeth Street Catchment. The raingarden design effectively intercepted stormwater, capturing a significant amount of gross pollutants (up to 27 tons), sediment (up to 202 tons), and nutrient pollutants (phosphorus and nitrogen). Comparing the results to City of Melbourne's sustainability targets, the study finds that this single strategy could meet sediment and phosphorus interception targets and contribute significantly to the city's tree canopy cover target (up to one-third of the required change by 2040). The de-paving in the Elizabeth Street Catchment represents 4-12% of the catchment's overall de-paving target, illustrating the need for complementary measures. The integrated approach considering tree canopy, biodiversity, and stormwater is highlighted as a valuable approach.

The findings demonstrate the substantial potential of converting redundant street parking into biodiverse green space for delivering integrated ecosystem service improvements at a scale necessary to address significant urban sustainability challenges. This is particularly relevant in highly urbanized areas with histories of minimum parking requirements. The study showcases how a systematic reallocation of space in streetscapes can produce benefits at the scale required for effective climate change adaptation and biodiversity conservation. The thousands of redundant spaces in central Melbourne represent a chance to replace considerable asphalt with green space, significantly increasing tree canopy cover and contributing to heat mitigation. The stormwater treatment results are also promising, meeting and exceeding targets for sediment and nutrient pollutants. The approach offers benefits for biodiversity by creating habitat linkages. The integrated focus on canopy, biodiversity, and stormwater is highlighted as rare, with the study quantifying only a fraction of the possible benefits (physical activity, mental health benefits, aesthetic appeal etc. are omitted, as are potentially substantial economic benefits such as job creation). The conservative assumptions used in the analysis likely underestimate the actual benefits. Further potential benefits exist through optimized designs tailored to specific locations. The study also acknowledges the political and social context surrounding street space allocation, including potential conflicts over changes to parking arrangements. The implementation costs and practicalities of a large-scale conversion are acknowledged; however, these are not seen as insurmountable with sufficient political will and public support. The authors emphasize the importance of evidence-based narratives and the need to focus on the significant gains from such urban greening initiatives.

This study demonstrates the significant potential for repurposing redundant urban car parking spaces into biodiverse green infrastructure as a scalable nature-based solution to address major urban sustainability challenges. The findings show this single intervention can deliver substantial benefits across multiple ecosystem services, contributing significantly towards established city sustainability targets. While political and logistical hurdles exist, the scale of potential benefits underscores the importance of exploring and implementing this approach. Future research could focus on refining the design for diverse urban contexts, developing comprehensive cost-benefit analyses, and investigating strategies for effective public engagement and policy implementation.

The study's analysis relies on several conservative assumptions which likely underestimate the actual benefits of converting redundant parking to green space. The modeling assumes a ‘no net loss’ of parking spaces, only relocating parking rather than removing it entirely. The vacancy rates used in the modeling, particularly for commercial and private parking, involve estimations which may not fully capture the dynamic nature of parking demand in the city. The standardized design for green spaces may not fully capture the potential for more nuanced, site-specific designs that could deliver even greater benefits. The ecological connectivity analysis focuses on two target species, and additional species may respond differently to the changes in habitat connectivity. The study also does not fully account for additional, intangible benefits associated with increased access to green spaces.