Why greening our cities is key to urban climate resilience  

26 November 2024

As the world deals with rapid urbanisation and escalating climate variability, green-blue resource management offers opportunities for creating sustainable, resilient and livable cities

Urban growth comes with significant environmental costs. As natural landscapes give way to built environments, habitats fragment and biodiversity declines. The spread of heat-absorbing materials like concrete and asphalt, combined with reduced vegetation, results in localised warming, a phenomenon known as the urban heat island effect. Consequently, cities can be several degrees warmer than their rural counterparts, exacerbating both energy consumption and heat-related health risks.

Melbourne benefits from plenty of greening © Valentina Marchionni

Urbanisation also disrupts the natural water cycle. Impervious surfaces prevent water infiltration, increasing surface runoff, while reduced vegetation diminishes evapotranspiration. Urban stormwater systems, designed to rapidly channel runoff away from developed areas, can lead to increased flooding risks downstream and reduced groundwater recharge.

Climate crisis

These challenges are further exacerbated by global climate change, which is intensifying the frequency and severity of extreme weather events. Record-breaking heatwaves, with temperatures soaring above 40°C, have been straining infrastructure and threatening public health worldwide. Simultaneously, many cities face increased flooding risks, as seen in the devastating 2021 floods in Germany and Belgium, and more recent inundations in central and southern Europe. Meanwhile, prolonged droughts challenge water supplies in drier regions.

These cascading climate impacts test urban resilience, exposing vulnerabilities in infrastructure and disproportionately affecting disadvantaged communities due to their often limited access to green spaces, older infrastructure and fewer resources for climate adaptation measures. As extreme weather becomes the ‘new normal’, cities must urgently adapt to ensure livability and resilience

Urban greening

Urban greening offers a powerful solution to these challenges by strategically integrating natural elements like street trees, parks, gardens, urban forests and engineered solutions such as green roofs, green walls, biofilters and rain gardens. This comprehensive approach addresses the environmental challenges of modern urban centres, offering dual benefits of climate adaptation and mitigation.

As a key element of urban greening, trees contribute to carbon sequestration, but their mitigation potential in urban areas is often limited due to space constraints and relatively small biomass. However, their adaptation benefits are immediate and substantial: trees provide significant local cooling through shade and evapotranspiration, help manage stormwater, mitigate flood risks, improve air quality, support urban biodiversity, and offer psychophysical benefits that help communities cope with climate-related stresses.

By prioritising urban greening in city planning, municipalities can create sustainable, liveable and healthy urban environments that better withstand the pressures of climate change and rapid urbanisation. Urban vegetation improves human thermal comfort through evapotranspiration, where plants release cooling water vapour, and offer shading from the sun. However, the effectiveness of these cooling effects varies based on factors such as vegetation density, plant composition, irrigation practices, and the characteristics of the surrounding landscape and local climate. Larger, densely vegetated areas – especially those with multilayered plant communities – can reduce temperatures by as much as 4.7°C; urban parks with wide-canopy trees can lower surrounding temperatures by up to 3.5°C; while sparsely vegetated areas show more modest cooling effects.

Bioretention basin designed to supplement irrigation of sports ovals in Edinburgh Gardens, Melbourne © Valentina Marchionni

Beyond temperature regulation, green infrastructure enhances urban resilience. Green roofs and green walls insulate buildings, reducing energy consumption, while features like rain gardens manage stormwater and mitigate flooding. Urban trees intercept rainfall, minimising runoff during storms, and green spaces absorb rainwater, reducing surface flooding.

Strategies for climate resilience

Effective urban greening requires a comprehensive strategy that goes beyond simply planting trees. A crucial element is the careful selection of native and climate-resilient plant species that can thrive in local conditions now and in the future, while promoting biodiversity. Trees, as long-term investments, offer century-long benefits, but some species may struggle to endure rising temperatures. For instance, research published in Nature shows that a 2°C temperature rise could endanger 40 per cent of eucalyptus species in Australia, highlighting the risks climate change poses to biodiversity. The implementation and success of urban greening initiatives varies significantly across climatic zones and socio-economic contexts. While arid cities must prioritise drought-resistant species and water conservation, tropical cities face the challenge of managing intense rainfall and humidity. 

Moreover, there is a critical knowledge gap when it comes to understanding water requirements in the context of trees’ cooling and overall health benefits. For example, a study in the Journal of Hydrology  emphasises the role of groundwater in supporting Melbourne’s green spaces during heatwaves and dry periods. Groundwater contributes up to 40 per cent of total transpiration in the driest months and allows trees to transpire an additional 3-16 per cent during warm nights after days exceeding 35°C. These findings underscore the importance of considering soil water resources in urban planning.

Biocities: a holistic approach

Urban resilience also depends on comprehensive water management strategies. As cities confront varying climatic conditions, implementing comprehensive water sensitive urban design (WSUD) technologies and integrated urban water management practices is critical. These systems incorporate innovative solutions such as rainwater harvesting, bioswales and permeable pavements to maximise water retention and reuse, while also leveraging alternative sources like greywater and stormwater for irrigation. Success requires careful adaptation to local environmental conditions – from water conservation in arid regions to robust rainfall management in tropical areas.

The path to transform cities as we know them into resilient ‘biocities’ requires a holistic approach: investment in urban greening, green infrastructure and nature-based solutions, climate resilient species selection, advanced water management, evidence-based planning, clear policy frameworks, and active community engagement. Balancing resource allocation and interdisciplinary collaboration is key to restoring natural water flows while maintaining environmental balance. This approach ensures the longevity of urban green spaces, strengthens cities’ resilience to climate change, and creates sustainable and healthy urban environments that meet both current and future needs.

Moving forward

By embracing urban greening and integrated water management strategies, cities are not just planting trees – they're sowing the seeds of resilient, liveable urban environments capable of weathering the storms of climate change. As we move forward, the success of these initiatives depends on adapting to local conditions, leveraging technology and fostering collaboration. In doing so, cities can not only survive but thrive in spite of environmental challenges, enhancing urban life and contributing to global sustainability goals

Author: Valentina Marchionni is a research scientist with experience in academia and research institutions including researcher the European Forest Institute’s Biocities Facility. Her work spans hydrology, ecology, climatology and soil science, and aims to address environmental problems related to global change and its interaction with vegetation, soil, and water resources. The focus of her research is primarily to investigate the interactions and feedback between the water cycle and ecosystems in both natural and urban environments, with particular emphasis on soil-plant processes, hydrological and ecohydrological modeling, groundwater dependent ecosystems and climate change impacts.