ESG Economist - Carbon offsets: To Tree or Not to Tree?
UN Climate Change has a workstream on reducing emissions from deforestation and forest degradation in developing countries or REDD+. Countries have established the REDD+ framework to protect forests as part of the Paris Agreement [1]. Various companies have certified carbon standards programs to drive finance towards activities that reduce and remove emissions, improve livelihoods and protect nature. Moreover, there is a voluntary market that sells carbon credits to buyers that try to offset the carbon emissions that will take longer to reduce. One of the focus areas of these offsets is planting trees. On 26 June 2024, we published an ESG Economist – Could carbon sequestration technologies help to reach net zero? [2]. One of the biological carbon sequestration technologies is planting trees (afforestation and reforestation). Afforestation is converting long-time non-forested land into forest. Obviously, the intention of these initiatives is good. Generally, it is assumed that deforestation leads to higher global mean temperatures and that forestation will limit the increase of global mean temperature. But as often is the case we need to ask ourselves if all relevant aspects are considered and whether they have the desired effect. This report focuses on that question. We start with the possible climate effects of planting or removing trees. These effects are important to consider when certain carbon offset products are considered. Then we show the results of scientific research. We end with a conclusion.
The effects of human actions, such as deforestation (removing trees) or afforestation (planting trees), on climate are far more complex than one would initially assume.
The effects of deforestation can either be positive or negative depending on the prevalence of physical land surface effects or effects on the carbon cycle, and their respective magnitude
Hence, before committing to carbon offsets that promote afforestation, it is relevant to do an analysis on the consequences of afforestation
That is because the climate system is very complex, and the different parts interact with each other
Moreover, the changes to the climate system since the pre-industrial period are unprecedented in global history, such as the pace of increase of CO2 concentration in the atmosphere
The different climate impacts of planting or removing trees
The climate system consists of the elements atmosphere, vegetation, land surface, ocean and ice and they all interact with each other. There are several forces that impact the climate system. These can be for example changes in the earth’s orbit and changes in the Sun’s strength but also man-made changes such as cutting trees and burning fossil fuels. The climate system is a complex system. The complexity mainly comes from the non-linear response of any impacts on the climate. In this report we focus on the impact of planting or removing trees, which are a part of vegetation. Indeed, vegetation refers to all plants and trees collectively. Vegetation regions can be divided into five major types: forest, grassland, tundra, desert and ice sheets. A change in vegetation potentially can have an impact on climate. There are two climate responses from a change in vegetation that could impact the climate, which we will discuss below.
Effects of planting or removing trees on the carbon cycle
Planting or removing trees has an impact on the carbon cycle and the associated global atmospheric concentration, which is the concentration of CO2 in the atmosphere. Trees use CO2 from the atmosphere, water from the soil and energy from the Sun to produce glucose and oxygen. Next, trees convert the glucose back into energy and for this process oxygen is required again. Overall, trees remove more CO2 from the atmosphere to produce energy than they release to the atmosphere in the form of respiration. When more trees are planted, more CO2 is taken from the atmosphere. Understandably, removing trees has the opposite effect. Deforestation results in carbon being transferred from vegetation to the atmosphere and more carbon in the atmosphere results in higher global mean temperatures. This effect is well understood but there are more effects of planting trees. For instance, forestry can dry the surface soils, which leads to more oxygen exposure, and subsequently more CO2 emissions from soil respiration. This means that more carbon will move from the soil to the atmosphere. This effect is an interaction between the carbon cycle and the effects mentioned below.
Effects of planting or removing trees on the physical properties of the land surface
Next to the effects of planting or removing trees on the carbon cycle there are also changes in the physical properties of the land surface such as albedo, roughness and evapotranspiration, which we will explain below. Changes in the land surface properties will have an impact on the exchange of heat, moisture, and momentum between the surface and the atmosphere. We begin with explaining the impact of albedo. Albedo is the fraction of sunlight that is reflected by the land surface into space. Albedo varies between 0 and 1. Fresh snow has a high albedo of around 0.75-0.90. This means that a large fraction of the sunlight is reflected into space. In contrast, the ocean has a very low albedo, meaning that it absorbs a lot of heat from the sun. The average albedo of the planet earth (see chart below) is around 31% or 0.31. To increase or improve the earth’s average albedo, which will result in less absorption of heat from the sun, the earth needs more surfaces with higher albedo. The table below shows the albedo of various surfaces. If at higher latitudes with more snow during the winter coniferous forests are replaced by grasslands the albedo already improves as the table shows below. The albedo of various types of forest are lower - between 0.10-0.18 - (trapping more heat from the sun) than that of soil and grassland at 0.2. If then there is fresh snow on the soil or grassland the albedo is even increased to above 0.8. That is a very significant increase from 0.10-0.18 to 0.83. If the heat is not entering the climate system it doesn’t need to be mitigated. Forests covered in snow have lower albedos than grasslands or soil covered in snow.
Next to albedo there is surface roughness. Surface roughness refers to all irregularities on the Earth’s surface. Changes in roughness have an impact on the surface’s energy balance and the surface’s temperature. The surface’s energy balance determines the amount of energy that is available to evaporate surface water and to raise or lower the temperature of the surface. Moreover, there is also evapotranspiration. Evapotranspiration might sound complicated but it only is a combination of evaporation and transpiration. In fact, it is the combined process of water surface evaporation, soil moisture evaporation, and plant transpiration. Plant transpiration describes the process of plants extracting water from the soil through their roots and releasing it to the air via their leaves. Evapotranspiration increases with increasing temperature, increasing radiation, decreasing humidity, and increasing wind speed (see more ).
What does scientific research tell us?
Several scientific papers have focussed on the impact of changes in land surface on climate. A satellite-based study finds that deforestation triggers an annual mean local cooling at locations to the north of 50°N and triggers a warming further south due to reduction in evapotranspiration (Winkler et al., 2019, see more ).
Another study shows a strong local cooling over the Northern Hemisphere boreal latitudes which extends into most of the Northern Hemisphere mid-latitudes. A boreal ecosystem is an ecosystem with a subarctic climate located in the Northern Hemisphere approximately between 50° and 70° N latitude. Moreover, the tropics and subtropics show a strong local warming of the (deforested) tropical rain forests. Deforestation causes a local warming in the tropics and a cooling at boreal latitudes for all Earth System Models. For afforestation, a clear tropical cooling is consistent across Earth System Models (see more ). In the case of large-scale afforestation specifically, the non-local response could lead to global-scale unintended warming, particularly over the boreal and mid-latitude regions. The table below shows the results of this study.
Another scientific paper shows that deforestation strongly influences the local surface energy balance: the imposed changes in surface properties in the model (surface albedo, evapotranspirative efficiency and surface roughness) cause a surface warming for the local effects in most regions, except for the high northern latitudes where the local effects cause a surface cooling (see more ).
There have been only few scientific studies that have focussed on the combined effects (carbon cycle and land surface properties). One of these studies has split the different effects of deforestation such as albedo and carbon cycle on climate and then looked at the overall effect. It shows that physical land surface effects such as albedo result in a lower surface temperature for land, ocean and agricultural area. Most of the globe shows a net cooling. It also showed that changes in the carbon cycle result in a warming for most of the globe. Taken together the overall effect is that the warming effect is larger than the cooling effect (see ).
The outcomes of these studies depend on the climate models used and whether the model is more specialized on one of these effects. Different models could result in different outcomes. For example, one model could mainly focus on the albedo effect while another model focuses on the carbon cycle effect. Moreover, scientific research has a specific research question in mind. Often outcomes of different models are considered to get the full picture. But we need to keep in mind that today’s global warming is unprecedent in global history. Moreover, the different parts of the climate system react at different time horizons and in different ways. Therefore, a change occurring now in the atmosphere could take a very long time to be visible in the ocean.
Conclusion
Above we have shown that planting (or removing) trees can have different effects on the climate. The overall effect depends on where the trees are planted (or removed) and what climate model is used. There are several carbon offset products in financial markets that focus on planting trees as a compensation for emitting CO2. However, the results/consequences of planting trees, as mentioned previously, do not always result in a decrease in temperatures. Given how complex the dynamics are, it is important to understand them before committing to buying carbon offsets.
Moreover, the changes to the climate system since the pre-industrial period are unprecedented in global history, such as the pace of increase of CO2 concentration in the atmosphere.