Hidden costs of implementing afforestation as a climate mitigation strategy: A comprehensive assessment of direct and indirect impacts

Many people love forests for what they provide. But I want to emphasize that we must not blur the main point just because of the benefits and feelings associated with forests. When we discuss the purpose of ?global scale climate mitigation by planting trees?, we should focus on how forests affect global scale climate. Here?s how forests affect climate. The effects of forests on climate can be divided into two: biogeochemical effects and biogeophysical effects. Biogeochemical effects are the well known carbon dioxide (CO2) uptake during plant growth. Because CO2 is one of the major greenhouse gases, the more CO2 is removed from the atmosphere, the cooler the climate will be. Until we cut down the trees again, the majority of CO2 taken up by trees will be stored there. The lesser emphasized biogeophysical effects are the physical effects created by plants. The three major biogeophysical effects created by forests are albedo, evapotranspiration, and turbulence effects. Easiest way to understand albedo is to think about reflection. When the sun?s energy hits the land surface, some of it gets absorbed and some of it gets reflected back to space. If the surface is light, it reflects more energy and we call this high albedo. This is the same principle as our tendency to wear dark colored clothes in winter because they absorb more energy to keep us warmer and light colored clothes in summer because they don't absorb as much energy and keep us cooler. Trees have a deeper root system and leaf surface than grass. Deeper roots can pump more soil water to the canopy and larger leaves breath out, or transpire, more water. Larger leaf surface also increases evaporation of water from the leaf surface. These two combined are called evapotranspiration. Just as sweating cools our body, evapotranspiration cools the land surface. Turbulence effects are a bit more subtle. Rough surfaces tend to create more small eddies in the atmosphere and this has a cooling effect. Complex structure of forest canopy tends to cool the land surface more than homogeneous grasslands by creating some turbulence. In the tropics, the effects of forests on climate are easier to understand. When there is more forest area in the tropics, CO2 uptake increases very much (much cooling), evapotranspiration increases very much (much cooling), turbulence increases (cooling), but albedo effects in the tropics are too small to make much difference. In high latitudes, the effects of forests on climate are more difficult to understand. First of all, plants in high latitudes do not grow as fast as they do in the tropics. In addition, it snows in the winter. These are two very critical things to consider when thinking about the combined biogeochemical and biogeophysical effects of forests on climate. When there is more forest area in high latitudes, CO2 uptake increases but not very much (cooling), evapotranspiration increases (cooling), turbulence increases (cooling), but albedo decreases very much (much warming). This is because snow falls through the forest canopy, keeping the surface darker compared to snow covered grasslands. Even without snow, forest canopy in high latitudes are much darker than grasslands. In this posting, we are discussing the effects of forests on global climate. Since we are unable to make global scale observations, we need to depend on modeling studies to understand how forests affect global climate via combined biogeochemical and biogeophysical effects. Surprisingly, the state-of-the-art climate models have a consensus in this topic! First, tropical forests have an overall cooling effect on climate. Second, high latitude forests do not have an overall cooling effect on climate. The second part includes some uncertainty. Not every model agrees with each other; some models show high warming effects from high latitude forests and some models do not show noticeable effects at all. But the consensus is that high latitude forests do not have a global scale cooling effect on climate when biogeochemical and biogeophysical effects are combined. As we can see from climate modeling studies, the effects of tree planting on global climate can vary depending on where the trees are planted. So if we simply consider global scale climate mitigation by planting trees in new areas, it is more beneficial to plant them in the tropics and reconsider planting in high latitudes.

Our modeling results suggest that planting trees to mitigate climate has little to no effect on climate. On the other hand, there may be additional unaccounted for consequences on local environment such as changes in biodiversity and soil carbon accumulation. Potential environmental consequences we were not able to address directly but can be expected from afforestation may be increased forest damage by wind gusts, increased forest fires, and increased rain on snow events due to afforestation in Norway. We also observe that the opinion of the general public is quite negative towards afforestation. We have started a scientific blog based on our study and plan to disseminate towards better policy making beyond the project period.

The role of forests in mitigating and managing climate change has been recognized since the early policy discussions under the collective label of "Agriculture, Forestry and Other Land Use (AFOLU)". In Norway, extensive planting of trees in open landscapes has been suggested as an important policy measure (St. Meld. 21 2011-2012). Under the current Norwegian climate mitigation plan, afforestation of new areas is considered the 4th most viable method. The effects and merits of afforestation have been highly debated both in the scientific community and in the public. In parallel to afforestation, large areas of unused and abandoned semi-natural areas in Norway are now undergoing massive natural succession towards deciduous forest. The impact assessments, however, have not yet moved beyond simple back-of-the-envelope calculation of carbon binding capacity by aboveground biomass. HiddenCosts is based on the realization that the current policy for afforestation as a climate mitigation strategy is based on incomplete knowledge and needs more rigorous evaluation in the full range of direct and indirect effects and costs vs. the realistic alternative landscape management scenarios. It is of vital importance that the full costs and benefits of afforestation vs. these realistic alternative management scenarios are rigorously assessed. Such an assessment is also time-sensitive due to the ongoing pilot projects. We will apply a multidisciplinary approach by integrating Earth System and regional climate modeling (WP1), in situ observations of biodiversity, ecosystem structure, and carbon storage (WP2), and public valuation and ecosystem services analysis (WP3) to gain more holistic understanding of the effects (both costs and benefits) of afforestation, continued management, and natural succession in the open lowland landscapes of Norway. Information gained from WPs1-3 will then be synthesized (WP4) to communicate effectively with relevant stakeholders and the public.