Green paradox: planting trees will cool a megacity unless it’s dry

The recent study highlighting the 'Green Paradox' – where planting trees can paradoxically worsen urban warming in dry climates – presents a critical re-evaluation of conventional urban greening strategies. For decades, the conventional wisdom has been that increasing green cover, particularly tree planting, is a universally beneficial solution for mitigating the urban heat island (UHI) effect and combating climate change. This belief stems from the understanding that trees provide shade, reducing direct solar radiation, and cool the surrounding environment through evapotranspiration, a process where water is released into the atmosphere from leaves.

The urban heat island effect is a phenomenon where urban areas experience higher temperatures than surrounding rural areas due to the absorption of solar radiation by dark surfaces like asphalt and concrete, reduced vegetation, and heat generated by human activities. With global temperatures rising and an increasing frequency of heatwaves, especially in rapidly urbanizing regions, cities worldwide have embarked on ambitious tree-planting drives, often as a cornerstone of their climate adaptation plans. Programs like 'Million Trees' initiatives are common globally, aiming to enhance urban aesthetics, improve air quality, and, crucially, cool down cities.

However, this new study, which analyzed data from 761 cities across various climatic zones, challenges this simplistic view. It found that while trees generally cool cities in temperate and humid regions, the effect can be counterproductive in dry climates. In arid and semi-arid regions, water availability is limited, leading to reduced evapotranspiration from trees. When evapotranspiration is low, the primary cooling mechanism of trees is diminished. Instead, the darker canopy of trees can absorb more solar radiation than lighter-colored artificial surfaces or dry soil, converting it into sensible heat, which directly warms the surrounding air. This mechanism, combined with reduced wind flow within dense tree canopies, can exacerbate local temperatures, creating a 'green paradox'.

Key stakeholders involved in this discourse include urban planners, municipal corporations, environmental ministries (like India's Ministry of Environment, Forest and Climate Change - MoEFCC), local communities, climate scientists, and non-governmental organizations (NGOs) involved in environmental conservation. Urban planners are at the forefront, responsible for designing sustainable cities. Municipal bodies implement greening projects and manage urban infrastructure. Environmental ministries formulate policies and provide funding for such initiatives. Climate scientists provide the empirical data and research necessary for informed decision-making, while local communities are both beneficiaries and participants in these efforts.

For India, these findings are profoundly significant. India is home to a vast range of climatic zones, including extensive arid and semi-arid regions, particularly in states like Rajasthan, Gujarat, parts of Maharashtra, Andhra Pradesh, and Karnataka. Many of India's megacities and rapidly growing urban centers are situated in or near these dry areas. India is also experiencing rapid urbanization, with its urban population projected to reach nearly 600 million by 2030. This rapid growth, coupled with increasing heatwave frequency and intensity, makes understanding context-specific climate solutions critical. If urban greening efforts in cities like Jaipur, Ahmedabad, or Hyderabad are implemented without considering the 'green paradox', they could inadvertently worsen the heat stress on their populations, leading to increased health risks, higher energy consumption for cooling, and strain on already scarce water resources.

Historically, India has a strong tradition of tree planting, often linked to cultural and religious practices. Post-independence, afforestation drives gained momentum with programs like the Van Mahotsav, initiated in 1950. While these initiatives have largely been positive, the new study calls for a scientific reassessment of species selection and planting strategies in specific climatic zones. India's commitment to environmental protection is enshrined in its Constitution. Article 48A, a Directive Principle of State Policy, mandates that "The State shall endeavour to protect and improve the environment and to safeguard the forests and wild life of the country." Furthermore, Article 51A(g) lists it as a Fundamental Duty of every citizen "to protect and improve the natural environment including forests, lakes, rivers and wild life, and to have compassion for living creatures." The Environment (Protection) Act, 1986, provides the legal framework for environmental protection and improvement. Policy initiatives like the National Action Plan on Climate Change (NAPCC) and its various missions (e.g., National Mission for a Green India, National Water Mission) and the Smart Cities Mission all emphasize sustainable urban development and climate resilience. The 'green paradox' necessitates a nuanced approach within these existing frameworks.

Looking ahead, the future implications are clear: India needs to adopt highly localized and climate-resilient urban planning strategies. This involves thorough climate assessments before implementing large-scale greening projects. It means selecting native, drought-resistant tree species that are well-adapted to local conditions and require minimal water. Furthermore, integrating other cooling strategies, such as using reflective materials for buildings (cool roofs), enhancing urban ventilation, and developing efficient water management systems for urban green spaces, will be crucial. Research and development into urban microclimates and the ecological impacts of different tree species in various Indian climatic zones must be prioritized. This study underscores the need for a scientific, adaptive, and integrated approach to sustainable urban development, moving beyond one-size-fits-all solutions to truly mitigate climate change impacts and enhance urban livability.