Does Planting Trees Slow Global Warming? The Evidence

It’s long been thought that trees, which remove CO₂ from the atmosphere and can live much longer than humans, exert a cooling influence on the planet. But a close look at the evidence reveals that the opposite could be true – that planting more trees may actually have a warming effect.

This is the tentative conclusion reached by a senior scientist at NASA, in evaluating the results of a 2019 study to estimate Earth’s forest restoration potential. It’s the same conclusion that the IPCC (Intergovernmental Panel on Climate Change) came to in a comprehensive 2018 report on climate change and land degradation. Both the 2019 study and IPCC report were based on various forest models.

The IPCC’s findings are summarized in the following figure, which shows how much the global surface temperature is altered by large-scale forestation (crosses) or deforestation (circles) in three different climatic regions: boreal (subarctic), temperate and tropical; the figure also shows how much deforestation affects regional temperatures.

Trees impact the temperature through either biophysical or biogeochemical effects. The principal biophysical effect is changes in albedo, which measures the reflectivity of incoming sunlight. Darker surfaces such as tree leaves have lower albedo and reflect the sun less than lighter surfaces such as snow and ice with higher albedo. Planting more trees lowers albedo, reducing reflection but increasing absorption of solar heat, resulting in global warming.

The second main biophysical effect is changes in evapotranspiration, which is the release of moisture from plant and tree leaves and the surrounding soil. Forestation boosts evapotranspiration, pumping more water vapor into the atmosphere and causing global cooling that competes with the warming effect from reduced albedo.

These competing biophysical effects of forestation are accompanied by a major geochemical effect, namely the removal of CO₂ from the atmosphere by photosynthesis. In photosynthesis, plants and trees take in CO₂ and water, as well as absorbing sunlight, producing energy for growth and releasing oxygen. Lowering the level of the greenhouse gas CO₂ in the atmosphere results in the cooling traditionally associated with planting trees.

The upshot of all these effects, plus other minor contributions, is demonstrated in the figure above. For all three climatic zones, the net global biophysical outcome of large-scale forestation (blue crosses) – primarily from albedo and evapotranspiration changes – is warming.

Additional biophysical data can be inferred from the results for deforestation (small blue circles), simply reversing the sign of the temperature change to show forestation. Doing this indicates global warming again for forestation in boreal and temperate zones, and perhaps slight cooling in the tropics, with regional effects (large blue circles) being more pronounced. There is strong evidence, therefore, from the IPCC report that widespread tree planting results in net global warming from biophysical sources.

The only region for which there is biogeochemical data (red crosses) for forestation – signifying the influence of CO₂ – is the temperate zone, in which forestation results in cooling as expected. Additionally, because deforestation (red dots) results in biogeochemical warming in all three zones, it can be inferred that forestation in all three zones, including the temperate zone, causes cooling.

Which type of process dominates, following tree planting – biophysical or biogeochemical? A careful examination of the figure suggests that biophysical effects prevail in boreal and temperate regions, but biogeochemical effects may have the upper hand in tropical regions. This implies that large-scale planting of trees in boreal and temperate regions will cause further global warming. However, two recent studies (see here and here) of local reforestation have found evidence for a cooling effect in temperate regions.

But even in the tropics, where roughly half of the earth’s forests have been cleared in the past, it’s far from certain that the net result of extensive reforestation will be global cooling. Among other factors that come into play are atmospheric turbulence, rainfall, desertification and the particular type of tree planted.

Apart from these concerns, another issue in restoring lost forests is whether ecosystems in reforested areas will revert to their previous condition and have the same ability as before to sequester CO₂. Says NASA’s Sassan Saatchi, “Once connectivity [to the climate] is lost, it becomes much more difficult for a reforested area to have its species range and diversity, and the same efficiency to absorb atmospheric carbon.”

So, while planting more trees may provide more shade for us humans in a warming world, the environmental benefits are not at all clear.

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