Afforestation, reforestation and forest management

Although the practise of planting of trees and managing forests already exists and is clearly technologically feasible (The IPCC AR6 WG3, 2022, and The State of Carbon Dioxide Removal report, 2023, assign it a TRL of 8 to 9), research is still being done into specific elements. This measure could potentially even include selecting for highest potential albedo effect (see bio-geoengineering). There is moreover still significant uncertainty about the overall climate effect of afforestation, especially in the high latitudes (Vogt et al, 2022).

The CDR potential for a scaling up of AR and forest management is large. However, physically, it must be ensured that it does not compete with other requirements, like food and water security, and that it is not detrimental to climate goals, such as local biophysical effects (e.g., albedo reduction might outweigh the effect of carbon sequestration; ​​Pielke et al., 2011).

It must also be ensured that the planted trees are climate resilient and can resist likely increased extreme temperatures. There are very accessible sites where forests could cheaply and effectively be restored or planted at scale (IPCC AR6 WG3). Because a/reforestation in tropical regions comes with significant biophysical climate benefits, these regions would be especially suited for AR. However, Windisch et al (2022) note that such projects would likely clash with food production.

If countries realize their pledges regarding forests to the Paris Agreement, forests could become a net carbon sink by 2030 (Grassi et al. 2017), and play an increasingly important goal in GHG mitigation.///One issue with AR and forest management is related to its permanence as a CDR measure. Although trees could capture large amounts of carbon effectively, ineffective strategies, forest fires, droughts, and other causes could see massive re-release of CO2 (Fuss et al 2018; Chiquier et al, 2022). Melnikova et al (2022) found that afforestation therefore could take up more carbon than another CDR method BECCS (See BEECS) in the short term (20/30 years), but that BECCS could be more effective in the long run.

The Vogt et al. (2022) study on the role of AR in the Nordic region clearly states that 'The potential for re-/afforestation in the Nordic countries is limited compared to other European countries, as a large proportion of their land is already forested, with relatively limited areas suitable for agriculture.' They moreover conclude that both the CDR effect and the net climate effect of AR in the Nordic region remains uncertain and is still debated in scientific studies. Although many assessments of AR neglect biophysical effects and mostly focus on CDR estimates (Breil et al. 2023).

Dashti et al. (2023) study strongly highlighted the importance of the biophysical effects of land cover change, and note this is particularly strong in higher latitudes. In fact, whereas AR generally has an extra cooling effect on lower latitude regions, it risks reducing albedo in Northern regions and negating all climate positive effects (Bright et al, 2017; Windisch, 2022).

Alongside being a CDR measure, AR and forest management improvements are also used in the Arctic and Northern regions to increase biomass production to replace fossil fuels (see BECCS). Melnikova et al (2022) study found that BECCS would likely be a more effective CDR method at high latitudes. As growing forests for both functions could lead to different ecosystem and climate effects, these aims have to be weighed against each other (Vogt et al, 2022). 

Due to warming temperatures, land cover changes in northern and arctic regions are already changing rapidly. This must be taken into consideration when planting trees on a large scale, as it might for example be a threat to previously largely open spaces or low-growth ecosystems (Halldórsson et al., 2008). This effect is for example visible in Iceland, where incentives to reforest the island have been observed to have a major impact on ground nesting birds (Pálsdóttir et al, 2022).

Estimates of total sequestration would partially depend on carbon prices, and Austin et al. 2020 suggested that 1.6 GtCO2 yr–1 could be sequestered globally through AR for an annual cost of USD130 billion if prices were at USD100 tCO2 –1. Roe et al’s (2021) literature review showed a median AR CDR rate of 475 MtCO2 yr−1 in 2050. The IPCC AR6 Wg3 (2022) gives 'medium confidence that the global technical mitigation potential of afforestation and reforestation activities by 2050 is 3.9 (0.5–10.1) GtCO2 yr–1', while Fuss et al. (2018) give a lower global estimate of 0.5–3.6 GtCO2 year-1 by 2050.

With regards to forest management, Ameray et al. (2021) note that there is currently low understanding of ‘how forest management strategies affect the net removal of greenhouse gasses and contribute to climate change mitigation’, and Roebroek et al (2023) even show that cessation of management strategies and allowing natural forest development can have positive climate effects.

For AR, Fuss et al (2018) give a potential cost range from a very low 5 to 53 US$ per ton of CO2 removed (Fuss et al 2018), while the IPCC AR6 Wg3 (2022) estimates a cost of 0-240 USD per tonne. Costs would likely highly depend on geographical area and project. Potential costs could be reduced through payment for ecosystem schemes like REDD+, or other carbon offset schemes, which could draw in money from polluters to offset their emissions.

The large-scale planting of trees can have significant beneficial environmental effects if done right, but, done incorrectly, it can be highly detrimental for local ecosystems and biodiversity (IPCC AR6 Wg3, 2022). Although these effects will likely be not as pronounced as those of other CDR measures, in the Northern regions effects on biodiversity could be significant (Vogt et al, 2022. Pálsdóttir et al (2022), for example, found a major impact of reforestation practices in Iceland on ground nesting birds, and Mooney and Lee (2022) attribute an increase of "rain-on-snow" events that poses major difficulties to local ecosystems and communities to afforestation.

The large-scale planting of trees can have significant benefits, but potentially also some detrimental effects depending on the execution of the project in question (IPCC AR6 Wg3, 2022). Afforestation could importantly provide additional sources of income and restore soils and biodiversity. Although it is generally noted that afforestation could also compete for land and drive up food prices (Kreidenweis et al, 2016), this is likely to be minimal in the Arctic and Northern regions. ///In the sub-polar and Arctic, afforestation could have detrimental social effects on communities that rely on open landscapes (Vogt et al, 2022) and lead to an increase of "rain-on-snow" events, which pose major difficulties to local ecosystems and communities (Mooney and Lee 2022).

If found to be undesirable, trees could be removed again.

It must be ensured that the planted trees are climate resistant as increasing heat extremes could increase forest fires significantly in the future (see forest fire management).

Planting trees on national territory is unproblematic, although care must be taken this is done in a just way, and does not infringe on water and food security.

Afforestation is generally a popular and acceptable CDR method and often framed as a “natural” measure (Walller et al, 2023). With regards to this, the IPCC AR6 Wg3 notes that ‘The sometimes sole attention on afforestation and reforestation – suggesting it may solve the climate problem to large extent, in combination with the very high estimates of potentials – have led to polarization in the debate, resulting in criticism to these measures or an emphasis on nature restoration only’ (2022, p781).

There are major international AR campaigns, like the One Trillion Trees project by the World Economic Forum (https://www.1t.org/) and the The Bonn Challenge (bonnchallenge.org), which aims to restore 350 million hectares by 2030. There is also significant commercial interest through measures such as REDD+ and carbon offsets, and many companies are now investing in tree planting to offset their emissions. 

The State of Carbon Dioxide Removal report (2023) finds that the relative number of scientific studies on AR has significantly decreased over the last years, in favour of other CDR measures like soil carbon sequestration and especially biochar.