Forest restoration and tree plantation is a well-established strategy for mitigation of climate change. However, use of this approach in arctic worsens warming and is counterproductive to climate change mitigation. This is because tree coverage reduces albedo (or reflection of sunlight) and increases surface darkness which results in net warming (because trees absorb more heat from the sun than snow). Further, tree planting activities also disturb the carbon pool of arctic soil which store more carbon than all plants on Earth. Therefore, climate change mitigation approach need not necessarily be carbon focused. Climate change is about Earth’s energy balance (net of solar energy staying in atmosphere and solar energy leaving atmosphere). Amount of greenhouse gases determine how much heat is retained in Earth’s atmosphere. In arctic regions, at high latitudes, albedo effect (i.e., reflection of sunlight back into space without being converted into heat) is more important (than greenhouse effect due to atmospheric carbon storage) for the total energy balance. Hence, the overall goal of slowing down climate change requires a holistic approach.
Plants and animals continuously release carbon dioxide (CO2) in atmosphere through respiration. Some natural events like wildfires and volcanic eruptions also release CO2 in the atmosphere. A balance in atmospheric CO2 is maintained by the regular carbon sequestration by the green plants in presence of sunlight through photosynthesis. However, human activities since 18th century, particularly extraction and burning of fossil fuels such as coal, petroleum oil, and natural gas, have raised concentration of atmospheric CO2.
Interestingly, an increase in concentration of CO2 in the atmosphere is known to show carbon fertilization effect (i.e., green plants photosynthesize more in response to more CO2 in the atmosphere). A good part of the current terrestrial carbon sink is attributed to this increased global photosynthesis in response to rising CO2. During 1982-2020, global photosynthesis increased by about 12% in response to a 17% increase in global carbon dioxide concentrations in the atmosphere from 360 ppm to 420 ppm1,2.
Clearly, increased global photosynthesis is unable to sequestrate all anthropogenic carbon emissions since industrialization began. As a result, the atmospheric carbon dioxide (CO2) has effectively increased by about 50% in the last two centuries to 422 ppm (in September 2024)3 which is 150% of its value in 1750. Since carbon dioxide (CO2) is an important greenhouse gas, this significant overall increase in atmospheric CO2 has contributed to global warming and climate change.
Climate change manifests in the form of melting polar ice and glaciers, warming oceans, rising sea levels, flooding, catastrophic storms, frequent and intense drought, water scarcity, heat waves, severe fires, and other adverse conditions. It has severe consequences on people’s lives and livelihoods hence the imperative of mitigation. Therefore, to limit global warming and temperature rise to 1.5°C by the end of this century, the UN Climate Change Conference has recognized that global greenhouse gas emissions need to be cut 43% by 2030 and has called parties to transition away from fossil fuels to reach net zero emissions by 2050.
In addition to reduction in carbon emission, climate action can also be supported by removal of carbon from the atmosphere. Any enhancement in capture of atmospheric carbon would be helpful.
Marine photosynthesis by phytoplankton, kelp, and algal planktons in oceans is responsible for about half of the carbon capture. It is suggested that microalgal biotechnology could contribute to carbon capture through photosynthesis. Reversing deforestation by tree plantation and restoration of forest land can be very helpful climate mitigation. One study found that enhancing global forest cover could make significant contributions. It showed that global tree canopy capacity under the current climate is 4.4 billion hectares which means an extra 0.9 billion hectares of canopy cover (equivalent to 25% increase in forested area) could be created after excluding existing cover. This extra canopy cover if created would sequestrate and store about 205 gigatonnes of carbon which amounts to about 25% of the current atmospheric carbon pool. Global forest restoration is an imperative also because uninterrupted climate change would result in reduction of about 223 million hectares of forest cover (mostly in tropical areas) and loss of associated biodiversity by 20504,5.
Tree plantation in arctic region
Arctic region refers to northerly part of the Earth above the 66° 33′N latitude within the artic circle. Much of this region (about 60%) is occupied by sea ice covered arctic ocean. The artic landmass is situated around the southern margins of artic ocean which support tundra or the northern boreal forest.
Boreal forests (or taiga) are situated south of the Arctic Circle and are characterized by coniferous forests consisting mostly of pines, spruces, and larches. It has long, cold winters and short, wet summers. There is predominance of cold-tolerant, cone-bearing, evergreen, coniferous trees (pines, spruces, and firs) that retain their needle-shaped leaves year-round. Compared to temperate forests and tropical wet forests, boreal forests have lower primary productivity, have fewer plant species diversity and lack layered forest structure. On the other hand, the arctic tundra is situated north of the boreal forests in the Artic regions of the northern hemisphere, where the subsoil is permanently frozen. This region is much colder with the average winter and summer temperatures in the range of -34°C and 3°C – 12°C respectively. The subsoil is permanently frozen (permafrost) hence roots of the plants cannot penetrate deep into soil and plants are low to the ground. Tundra has very low primary productivity, low species diversity and short growing season of 10 weeks when plants grow rapidly in response to long daylight.
The tree growth in arctic regions is affected by permafrost because subsurface frozen water restricts deep root growth. Most of tundra has continuous permafrost while boreal forests exist in areas with little or no permafrost. However, the arctic permafrost is not unaffected.
As the arctic climate warms (which is happening twice as fast as global average), the resultant melting and loss of permafrost would enhance survival of early tree seedling. Presence of shrub canopy was found to be positively associated with further survival and growth of seedlings into trees. The composition of species and functioning of ecosystems in the region is undergoing rapid change. As the climate warms and permafrost degrades, vegetation may shift from tree-less arctic to tree-dominated in future6.
Would vegetation shift to tree-dominated arctic landscape reduce atmospheric CO2 through enhanced photosynthesis and help climate change mitigation? Could arctic region be considered for afforestation to remove atmospheric CO2. In both situations, the arctic permafrost should thaw or degrade first to allow growth of trees. However, thawing of permafrost releases methane in the atmosphere which is a powerful greenhouse gas and contributes to further warming. Methane release from permafrost also contributes to massive wildfires in the region.
As for the strategy of removal of atmospheric CO2 through photosynthesis by afforestation or tree planation in artic region and consequent mitigation of warming and climate change, the researchers7 found this approach to be unsuitable for the region and to be counterproductive to climate change mitigation. This is because tree coverage reduces albedo (or reflection of sunlight) and increases surface darkness which results in net warming because trees absorb more heat from the sun than snow. Further, tree planting activities also disturb the carbon pool of arctic soil which store more carbon than all plants on Earth.
Therefore, climate change mitigation approach need not necessarily be carbon focused. Climate change is about Earth’s energy balance (net of solar energy staying in atmosphere and solar energy leaving atmosphere). Greenhouse gases determine how much heat is retained in Earth’s atmosphere. In arctic regions at high latitudes, albedo effect (i.e., reflection of sunlight back into space without being converted into heat) is more important (than atmospheric carbon storage) for the total energy balance. Hence, the overall goal of slowing down climate change requires a holistic approach.
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References:
- Keenan, T.F., et al. A constraint on historic growth in global photosynthesis due to rising CO2. Nat. Clim. Chang. 13, 1376–1381 (2023). DOI: https://doi.org/10.1038/s41558-023-01867-2
- Berkeley Lab. News – Plants Buy Us Time to Slow Climate Change – But Not Enough to Stop It. Available at https://newscenter.lbl.gov/2021/12/08/plants-buy-us-time-to-slow-climate-change-but-not-enough-to-stop-it/
- NASA. Carbon Dioxide. Available at https://climate.nasa.gov/vital-signs/carbon-dioxide/
- Bastin, Jean-Francois et al 2019. The global tree restoration potential. Science. 5 July 2019. Vol 365, Issue 6448 pp. 76-79. DOI: https://doi.org/10.1126/science.aax0848
- Chazdon R., and Brancalion P., 2019. Restoring forests as a means to many ends. Science. 5 Jul 2019 Vol 365, Issue 6448 pp. 24-25. DOI: https://doi.org/10.1126/science.aax9539
- Limpens, J., Fijen, T.P.M., Keizer, I. et al. Shrubs and Degraded Permafrost Pave the Way for Tree Establishment in Subarctic Peatlands. Ecosystems 24, 370–383 (2021). https://doi.org/10.1007/s10021-020-00523-6
- Kristensen, J.Å., Barbero-Palacios, L., Barrio, I.C. et al. Tree planting is no climate solution at northern high latitudes. Nat. Geosci. 17, 1087–1092 (2024). https://doi.org/10.1038/s41561-024-01573-4
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