Cultivating nature, cooling cities: An urban future grown to last

Urbanization has surged in recent decades, especially in regions with arid climates like the Middle East, where the population in cities has nearly doubled since the 1960s, surpassing the global urbanization average of 55%.

This rapid expansion comes at a cost: as cities have traditionally expanded, they became more susceptible to the impacts of climate change. Concrete landscapes disconnect us from nature and exacerbate pollution, presenting significant environmental risks.

Yet, amidst these challenges lies a solution: by integrating green-blue infrastructure into a city’s fabric, we can combat climate change and create healthier, more sustainable urban environments.

Bringing nature back to our cities goes beyond mere aesthetics – it's about fostering good public health and transforming infrastructure industries for a better future.

Initiatives such as the Green Riyadh project in Saudi Arabia exemplify the potential of incorporating nature into city landscapes, offering benefits for both inhabitants and the planet.

This large-scale afforestation project is expected to result in the planting of 7.5 million trees across the capital of Saudi Arabia, with an ambient temperature reduction cross the city of 2°C and up to 15°C reduction where afforestation is most dense.

These estimates are supported by research which found that a 10% increase in vegetation cover can decrease ambient air temperatures by 2°C. This temperature decrease is expected to modestly reduce the energy consumed by artificial cooling systems.

In addition, the initiative is projected to enhance public health and improve air quality by reducing CO2 concentration by 3-6%, and reduce dust concentration. It will also increase the per capita green space to nearly 30 sqm, which is well above the World Health Organization's recommended 9 sqm, potentially boosting physical activity and enhancing mental health among the city’s residents.

But in such an arid environment, how will all this new greening survive and thrive? Firstly, the Green Riyadh plan includes the introduction of native plant species, that are compatible with the local environment. In addition, a water recycling and stormwater management project aims to increase daily volume of reused water from 90,000 cubic metres to approximately one million cubic metres. This will also help mitigate the impacts of flood events.

When considered with its potential to have long lasting effects also on biodiversity, greening the city will provide critical learnings for the entire region, while improving the lived experience of Riyadh’s residents.

Afforestation has long been recognised as a valuable technique, primarily for restoring wildlife habitats and providing forestry products. In recent years, it also gained prominence as a crucial tool in the fight against climate change. Its ability to sequester carbon and its cost-effectiveness make tree planting a frontline mitigation measure.

Urban afforestation is particularly noteworthy for addressing both environmental and societal needs. Recent research highlights the vast potential of the peri-urban forests, those located at the rural urban fringe, to host up to 240 billion trees globally and supercharge microclimate adaptation.

The cooling potential of urban parks in dry climates can provide long-term relief from the heat to nearby residents. Increasing tree coverage to 30% in neighbourhoods through parks and green corridors has been shown to lower surface temperatures by 7-10°C.

This simple measure has been associated with preventing a significant portion of premature deaths caused by heat stress, underscoring the life-saving potential of urban afforestation efforts.

In essence, afforestation emerges as the most cost-efficient method to enhance outdoor environmental conditions, offering tangible benefits for public health and well-being. As we strive to cool down our cities and mitigate the impacts of climate change, investing in urban afforestation is a clear and compelling solution.

Urban afforestation in arid regions offers more than just shade and beauty – it provides vital ecosystem services that contribute to overall environmental health.

As tree cover reaches a critical mass, it begins to regulate the water cycle, absorb air pollutants and remove carbon from the atmosphere, positively impacting macroclimatic conditions.

The success of urban tree planting relies on various factors, especially for their integration in the wider ecosystem. Careful consideration of biodiversity outcomes and ecosystem design factors is essential when planning and implementing urban afforestation programmes, ensuring the functionality of the intervention and that all the benefits can be fully realised.

In the battle against desertification, trees also play a crucial role by enriching the soil with nitrogen through their roots, fostering the growth of essential microbes and fungi that keep the soil healthy and fertile.

Innovative ecosystem design practices, such as the Miyawaki method, capitalise on symbiotic relationships between native species, reducing maintenance requirements and amplifying biodiversity and carbon sequestration potential, especially in water-scarce areas.

Moreover, increasing permeable surfaces can mitigate flooding risks, as demonstrated by the Sponge City concept pioneered in Chinese cities and applied across the world.

By redirecting water runoff to underground storage systems through nature-based solutions, cities like Baicheng, Qian’an, Jiann, and Xixian have already made significant strides towards water absorption targets.

Over time, these rainwater management systems will alleviate the burden on irrigation systems, benefiting public parks and afforested areas alike.

A precise understanding of the environmental, social and economic impact of large-scale afforestation projects, like the Green Riyadh initiative, is crucial for a transparent accounting of its benefits. Only then cities can certify their progress in achieving net zero and nature-positive goals and targets.

The new Nature Positive: Guidelines for the Transition in Cities report by the World Economic Forum, in collaboration with Oliver Wyman, lays out the key steps for cities to reap the benefits of a nature-positive transition.

According to these guidelines, cities can become pioneering agents of change by:

Nature-positive cities will be those that inspire a global urban transformation that heeds nature to protect cities and their inhabitants, and build a better world for all.

This article is part of a series of publications from the Nature-Positive Cities initiative, by the World Economic Forum, in collaboration with Oliver Wyman.