How Do Trees Influence Climate at Local, Regional and Global Scales?
Trees and forests influence climate through several interacting mechanisms: shade, evapotranspiration, exchanges of water with the atmosphere, carbon storage and changes in albedo. Their effects differ at local, regional and global scales, and also depend on climate, tree species and forest type.
In this article:
Temperature regulation and local cooling
First, trees arbres ont un effet climatiseur. This comes from physical and biological processes that redirect part of the incoming solar energy instead of simply storing it as sensible heat. One important mechanism is evapotranspiration. Under favourable conditions, a mature tree may transpire tens to hundreds of litres of water per day. Moving this water from vegetation to the atmosphere consumes energy and can cool the surrounding air, but the magnitude varies strongly with species, soil, water availability and weather. Simple comparisons with a fixed number of air-conditioning units should therefore be treated cautiously.
Direct shade also changes the heat balance of ground and built surfaces by intercepting solar radiation. Trees can therefore help reduce urban heat islands, especially when canopy design, water availability and urban form allow their cooling effects to operate efficiently.
Moisture recycling and hydrological extremes
Trees and forests also affect atmospheric moisture by recycling moisture through evapotranspiration and potentially influencing downwind rainfall. Forests do not single-handedly create atmospheric rivers in the meteorological sense, but evapotranspiration returns water vapour to the atmosphere and can contribute to moisture transport over long distances.
Forests also release biological particles and volatile organic compounds — including some terpenes — which can contribute to aerosol formation and, under certain conditions, influence cloud formation and condensation nuclei. Forest canopies intercept part of rainfall, while forest soils can promote infiltration through their structure, roots and biological activity. But trees also use water and return it to the atmosphere through transpiration. In dry regions, afforestation can therefore sometimes reduce downstream water availability. Hydrological outcomes depend on context.
Depending on climate and watershed, forests can buffer some hydrological extremes, for example by slowing some runoff, protecting soils and altering infiltration. They do not eliminate floods or droughts, however, and water effects can be contrasting, especially in dry climates. They may reduce some risks associated with runoff and erosion.
Carbon sinks and greenhouse gases
Forests also influence global climate through the carbon cycle. They make a major contribution to the terrestrial carbon sink, which absorbs a substantial share of anthropogenic CO₂ emissions. It would nevertheless be too simplistic to say that forests alone consistently absorb one third of human emissions: figures of that order generally refer to the land sink as a whole and vary by year and accounting method. As a rough order of magnitude, one cubic metre of wood can represent several hundred kilograms of atmospheric CO₂ stored in biomass, sometimes approaching a tonne depending on wood density and moisture. It is not a universal conversion.
This involves longer-term carbon storage, with residence times ranging from years to centuries depending on the carbon pool. Carbon can be stored in living biomass, roots and forest soils, as well as in harvested wood products such as furniture or construction timber. How long it remains stored depends on decomposition, combustion, product lifetime and recycling.
It is also essential to mention photosynthesis, the process by which plants use light energy to convert water and carbon dioxide into organic molecules. Carbon from CO₂ becomes incorporated into sugars and then into compounds such as cellulose and lignin that build wood.
Protecting trees does more than preserve landscapes: it also protects the ecological processes that support life.
Climate influence across multiple scales
Temperature buffering, evapotranspiration and shade operate directly at the local scale. Tree-planting and shade projects can therefore support local heat adaptation, including in very hot cities such as Bakel in Senegal. The Bakel Écocitoyenneté project aims to reduce urban heat-island effects by planting shade trees in and around schools, where heat exposure can make learning conditions difficult.
At regional and continental scales, forests can influence moisture circulation and some weather patterns. Moisture recycled through evapotranspiration can contribute to downwind rainfall. The idea of a “biotic pump” pulling moist ocean air inland remains a debated scientific hypothesis rather than a universally established mechanism. Landscape-restoration projects nevertheless illustrate the importance of restoring soils and vegetation, as in Africa’s Great Green Wall initiative. Launched in 2007, the initiative is no longer understood as a simple continuous band of trees stretching 8,000 kilometres, but as a mosaic of land and ecosystem restoration across the Sahel, from Senegal towards the Horn of Africa.
At the global scale, carbon storage matters greatly, but other mechanisms can offset part of forests’ cooling influence. Deforestation releases part of stored carbon and reduces future uptake. Albedo also matters: dark forests absorb more solar radiation than light surfaces, which can create a warming influence, particularly at high latitudes. The final climate balance therefore depends on biome and scale.
Going further...
In today’s climate challenge, trees are valuable allies, but their role goes far beyond carbon uptake. They cool cities, influence water cycles, protect soils and affect climate processes over much larger regions.
Learning about trees also means learning how to protect the ecosystems we all depend on. This is one of the intentions behind my approach to forest therapy: creating a more conscious relationship with forests while remaining curious about the science that helps us understand how they function.
If you would like to deepen your knowledge of trees, forests and their interactions with people and the environment, my spiritual forest therapy training programme explores forest ecology, human relationships with nature and practices for reconnecting with the Living World in a sensitive and informed way.
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