The fight against climate change has placed urban trees and forests at the heart of mitigation and adaptation strategies, to the point that tree planting is often perceived as a simple, visible, and widely accepted solution for reducing greenhouse gas emissions. In many countries, urban greening programs are expanding, with tree-lined streets, boulevards, and public spaces, while large-scale reforestation and forest restoration initiatives are prominently featured in Nationally Determined Contributions (NDCs) under the United Nations Framework Convention on Climate Change (UNFCCC).
This situation raises a central question: does planting trees along streets genuinely contribute to reducing greenhouse gas emissions, or are forests essential to achieve a meaningful climate impact? The answer is neither binary nor ideological, but rather grounded in a scientific analysis of carbon sequestration mechanisms, orders of magnitude, and international experience.
Trees, whether urban or forest-based, capture atmospheric carbon dioxide through photosynthesis and store it as carbon in their aboveground biomass, root systems, and indirectly in soils. This fundamental principle is emphasized by the Intergovernmental Panel on Climate Change, which identifies terrestrial ecosystems as major carbon sinks capable of absorbing approximately 30% of annual anthropogenic CO₂ emissions worldwide [1]. However, sequestration capacity depends strongly on ecological context, vegetation density, available surface area, and the duration of carbon storage.
In urban environments, trees planted along streets, in parks, and in public squares do contribute to carbon sequestration, but in quantitatively limited ways. According to estimates by the Intergovernmental Panel on Climate Change (IPCC) and the Food and Agriculture Organization (FAO), a mature urban tree absorbs on average between 10 and 40 kg of CO₂ per year, depending on species, age, and growing conditions [2]. This absorption remains modest when compared with overall urban emissions, which are dominated by transport, buildings, and industry. Nevertheless, the primary climate value of urban trees lies in their indirect effects.
By lowering local temperatures through shading and evapotranspiration, trees mitigate the urban heat island effect, resulting in reduced demand for air conditioning and, consequently, lower emissions associated with electricity generation. Studies conducted in the United States and Europe indicate that urban vegetation can reduce building energy consumption by 5 to 20% during heatwave periods [3].
Urban trees also play an important role in climate change adaptation by improving thermal comfort, air quality, and population resilience to heatwaves, which are among the deadliest climate-related hazards. From this perspective, their contribution to greenhouse gas reduction must be assessed systemically, incorporating avoided emissions and broader social and health co-benefits. The United Nations Environment Programme (UNEP) has emphasized that the climate value of urban trees is often underestimated when assessments focus solely on their direct carbon sequestration capacity [4].
By contrast, natural forests and large-scale forest plantations constitute carbon sinks of an entirely different magnitude. One hectare of temperate or Mediterranean forest can store between 100 and 300 tonnes of carbon in biomass and soils and absorb between 2 and 10 tonnes of CO₂ annually, depending on forest type and development stage [5]. Forest soils, often overlooked in public debates, can represent a carbon reservoir equal to or even greater than that of aboveground biomass. The destruction or degradation of forests releases this stored carbon into the atmosphere, which explains why deforestation accounts for approximately 10 to 15% of global greenhouse gas emissions [1].
International benchmarks clearly illustrate this difference in scale. China’s massive reforestation program, implemented since the 1990s and often referred to as the “Great Green Wall,” has restored tens of millions of hectares of degraded land. According to the FAO, these efforts have significantly increased China’s national carbon sink, offsetting part of the emissions associated with the country’s rapid industrialization [6]. Similarly, Brazil, despite ongoing challenges related to deforestation in the Amazon, remains one of the countries with the highest forest carbon sequestration potential worldwide, and its forest policies have a direct impact on the global carbon balance.
At the urban scale, cities such as New York, London, and Paris have implemented ambitious tree-planting and urban forest strategies. New York’s “MillionTreesNYC” program, which aimed to plant one million trees over ten years, provides a well-documented example. Evaluations show that while the direct impact on CO₂ emissions remains limited relative to the city’s total emissions, the indirect benefits in terms of energy savings, public health, and climate resilience are significant [7]. Paris, through its Climate Plan and urban greening strategy, also relies on urban trees to cope with heatwaves, while acknowledging that carbon neutrality cannot be achieved without structural actions in the energy and transport sectors [8].
Another relevant benchmark can be found in Nordic countries such as Sweden and Finland, where sustainable forest management has long been integrated into climate strategies. These countries combine extensive forest cover, responsible timber harvesting, and low-carbon energy policies. Forests play a dual role there: acting as carbon sinks and providing biomaterials that can substitute for fossil-based materials and energy sources, thereby further reducing emissions [9].
In arid and semi-arid regions, the issue takes on a different dimension. Water constraints limit tree growth and forest density, requiring adapted approaches. The African Great Green Wall initiative, aimed at restoring degraded landscapes across the Sahel, demonstrates that reforestation and agroforestry can contribute to carbon sequestration while strengthening food security and the resilience of local communities [10].
In Algeria, the Green Dam project represents an emblematic reforestation policy designed to combat desertification, stabilize soils, and create a long-term carbon sink. In this context, urban trees, while valuable for improving urban comfort, cannot replace large-scale ecological restoration efforts.
Comparative analysis therefore shows that urban trees and forests do not serve the same climate objectives, even though both contribute to addressing climate change. Urban trees are primarily tools for adaptation and indirect emission reduction, while forests constitute central levers for long-term mitigation. Confusing these two scales may lead to unbalanced climate policies, where highly visible but quantitatively limited actions are overemphasized at the expense of more structural strategies.
International organizations stress the need for an integrated approach. The IPCC recalls that nature-based solutions, including reforestation, ecosystem restoration, and urban greening, cannot on their own offset current greenhouse gas emissions, but they are an essential complement to drastic reductions in fossil fuel emissions [1]. FAO and UNEP further emphasize that plantation quality, species diversity, long-term management, and community involvement are critical conditions for ensuring genuine climate benefits [2,4].
Bottom Line
Planting trees along streets does contribute to reducing greenhouse gas emissions, but in an indirect and limited manner, whereas forests remain indispensable for achieving significant climate impacts at national and global scales. The challenge is therefore not to choose between urban trees and forests, but to integrate them into a coherent, hierarchical strategy adapted to ecological and socio-economic contexts. In a world facing climate emergency, trees must not be treated as mere green symbols, but as components of climate policies grounded in science, scale, and sustainability.
References
[1] IPCC, Climate Change 2022: Mitigation of Climate Change. Contribution of Working Group III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, UK and New York, USA, 2022.
[2] Food and Agriculture Organization of the United Nations (FAO), Forests and Climate Change: Working with forests to slow climate change and adapt to its impacts, FAO Forestry Paper, Rome, 2020.
[3] Akbari, H., Pomerantz, M., Taha, H., Cool surfaces and shade trees to reduce energy use and improve air quality in urban areas, Energy and Buildings, 33 (2001) 1–10.
[4] United Nations Environment Programme (UNEP), Nature-based Solutions for Climate Change, UNEP, Nairobi, 2021.
[5] Pan, Y., Birdsey, R.A., Fang, J., Houghton, R., Kauppi, P.E., Kurz, W.A., et al., A large and persistent carbon sink in the world’s forests, Science, 333 (2011) 988–993.
[6] Food and Agriculture Organization of the United Nations (FAO), Global Forest Resources Assessment 2020, FAO, Rome, 2020.
[7] Nowak, D.J., Greenfield, E.J., Hoehn, R.E., Lapoint, E., Carbon storage and sequestration by trees in urban and community areas of the United States, Environmental Pollution, 178 (2013) 229–236.
[8] Ville de Paris, Plan Climat Air Énergie Territorial de Paris, Mairie de Paris, Paris, 2022.
[9] European Environment Agency (EEA), Forests, carbon sinks and climate neutrality in Europe, EEA Report No 13/2021, Copenhagen, 2021.
[10] World Bank, The Great Green Wall: Implementation Status and Way Ahead to 2030, World Bank Group, Washington DC, 2020.
