Researchers in Canada have used a comprehensive Earth system model to evaluate the climate impact of varying levels of afforestation, in different parts of the world. They looked at scenarios in which the world’s current cropland was gradually converted to forest, over the next 50 years. The scientists used a computer simulation to determine the cropland areas in which forests could feasibly become established so that an artificial supply of water or nutrients would not be required.
The focus here was on evaluating how the biogeophysical (e.g. changes to the brightness of the land surface, known as the albedo) and biogeochemical effects (e.g. carbon sequestration) of afforestation would ‘add up’ to give an overall climate impact.
Taking carbon from the atmosphere is a process that results in a climatic cooling, and is the basis for suggesting afforestation as a strategy to mitigate climate change. However, the fact that trees are generally darker than most other land coverings means that forested parts of the Earth’s surface reflect away less incoming solar radiation, giving forests a warming effect. On a global scale, the cooling effect of carbon sequestration dominates and, in this work, afforestation of all the climatically viable cropland gave a global temperature reduction of 0.45oC by the end of this century.
This is an order of magnitude smaller than the likely warming induced by anthropogenic emissions over the same time period. Regionally however, it is well established that the magnitude of the individual effects varies significantly giving forests an overall climate impact that is very much location dependent.
In this study, the “temperature benefit” per unit of afforestation in tropical regions was around three times greater than that in northern-temperate or boreal regions. This is because carbon is sequestered at a faster rate, and other biogeophysical processes (e.g. increased transfer of moisture from the surface to the atmosphere, known as evapotranspiration) enhance the cooling, in the tropics.
But could there be more to it?
Although this work considered the effect of afforestation on atmospheric carbon dioxide concentration, there are other biogeochemical processes not yet captured by current global climate models that could influence the impact of any afforestation scheme. These include the effects that trees have on local atmospheric chemistry and potentially the clouds above them; until these are fully understood it is somewhat difficult to attribute a “temperature benefit” of a specific magnitude to a given afforestation scenario.
…so, should we be planting trees or not?
The title of the original article, that temperature benefits from afforestation are only small, perhaps detracts from the important conclusion that, based on this study at least, afforestation in all locations does give a “temperature benefit”, despite the varying contributions of warming and cooling effects. These particular simulations suggest that planting trees in tropical regions is considerably more climatically beneficial than doing so elsewhere, reinforcing the need to maintain and improve existing efforts to reduce deforestation and forest degradation in these areas.
Whilst the “temperature benefits” observed in this study are small compared to the level of warming that is likely to occur over the next century, it is perhaps premature to dismiss the role of afforestation until a more complete understanding of the climatic impacts of forests is achieved.
© Cat Scott | www.bitsofscience.org