Afforestation
Project Drawdown defines afforestation as: the cultivation of trees for timber or other biomass uses on degraded land. Climate mitigation is achieved through biosequestration in soils, biomass, and timber. This practice replaces annual cropping on active cropland, and other uses on degraded grassland, cropland, and forest. Afforestation also aims to reduce emissions from deforestation by providing an alternative source of timber, though this impact is not modeled here.
Afforestation has been widely promoted as a land-based mitigation strategy due in part to its high sequestration rates. Drawdown's afforestation scenarios are more modest than many. This is because other tree-focused solutions with high sequestration rates are given higher priority, including tree intercropping, silvopasture, multistrata agroforestry, tropical tree staples, tropical forests, and temperate forests. Nonetheless, afforestation is of critical importance for mitigation, building material, and restoration of degraded lands.
Methodology
Total Land Area [1]
The total area allocated for afforestation is 437 million hectares, and is comprised of degraded grassland and forest. Current adoption [2] is estimated at 287 million hectares, based on data from the Food and Agriculture Organization (FAO, 2015). [3]
Adoption Scenarios [4]
Projected adoption of afforestation is based on historic country-level growth rates from FAO (2015), aggregated by region. Future adoption of afforestation in different regions was projected using five custom adoption scenarios, based on the regional linear growth rate, regional growth rate reported maximum for a given time period, and maximum reported growth rate for a given region (e.g. Asia). Some of these scenarios also assume early adoption, i.e. 70 percent of the total by 2030.
Impacts of increased adoption of afforestation from 2020-2050 were generated based on three growth scenarios, which were assessed in comparison to a Reference Scenario where the solution’s market share was fixed at the current levels.
- Plausible Scenario: Scenario analysis shows afforestation adoption on 369.5 million hectares of degraded land in the Plausible Scenario.
- Drawdown Scenario: The most aggressive adoption scenario yields adoption of afforestation on 436.5 million hectares (100 percent) of the allocated degraded land area. This scenario was built on the assumption that afforestation will not only restore the degraded land area, but will also prevent any future clearing of forests.
- Optimum Scenario: Same as the Drawdown Scenario.
Emissions, Sequestration, and Yield Model
The sequestration rate is 4.7 tons of carbon per hectare per year, based on 12 data points from 8 sources. This is in line with Intergovernmental Panel on Climate Change (IPCC) estimates (Watson, 2007). It is assumed that all sequestered carbon is re-emitted at harvest, except for carbon stored in timber. Average timber yield per hectare per year was based on meta-analysis. An average lifespan of 15 years was considered for an afforestation plantation, based on the literature review.
Financial Model
First cost of afforestation is US$286.71 per hectare, [5] based on meta-analysis of 26 data points from 9 sources. It is assumed that first costs for the land use that afforestation is replacing have already been paid, as the land is already in production. Net profit margin is US$271.03 per hectare per year, based on 9 data points from 4 sources. This compares to US$407.46 for the conventional practice of annual cropping.
Integration [6]
Drawdown's Agro-Ecological Zone model allocates current and projected adoption of solutions to the planet’s forest, grassland, rainfed cropland, and irrigated cropland areas. Adoption of afforestation was constrained by Drawdown's higher prioritization of food production and forest restoration. In degraded forest, afforestation was the second-highest priority, and in degraded grassland the fourth-highest.
Results
Total adoption in the Plausible Scenario is 369.5 million hectares in 2050, representing 84.5 percent of the total available land. Of this, 82.5 million hectares are adopted from 2020-2050. The sequestration impact of this scenario is 18.1 gigatons of carbon dioxide-equivalent greenhouse gases by 2050. Net cost is US$29.4 billion. Net savings is US $392.3 billion.
Total adoption in the Drawdown Scenario is 436.5 million hectares in 2050, representing 99.9 percent of the total available land. Of this, 149.5 million hectares are adopted from 2020-2050. The impact of this scenario is 41.6 gigatons of carbon dioxide-equivalent by 2050.
Total adoption in the Optimum Scenario is 436.5 million hectares in 2050, representing 99.9 percent of the total available land. Of this, 149.5 million hectares are adopted from 2020-2050. The impact of this scenario is 41.6 gigatons of carbon dioxide-equivalent by 2050.
Discussion
Benchmarks
The IPCC provides a benchmark of 4.0 gigatons of carbon dioxide-equivalent per year in 2030 from afforestation, given a price of $100 per ton of carbon dioxide (Metz, 2007). The Drawdown model shows 0.3 gigatons of carbon dioxide-equivalent per year in 2030 in the Plausible Scenario, and 0.6 and 0.8 in the Drawdown and Optimal Scenarios, respectively. These impact are substantially lower, as this study has much lower adoption than most studies due both to higher prioritization of other land uses and to not modeling a price on carbon dioxide. When combined with the bamboo and tropical staple trees solutions, however, emissions reduction in 2030 increased to 1.0, 1.9, and 2.5 gigatons of carbon dioxide-equivalent per year in 2030 in the three Scenarios, respectively.
Limitations
The study has several limitations. One limitation is the use of a single sequestration rate across all climates. The current version of the model does not account for albedo impacts at temperate and boreal latitudes. This will be addressed in future versions. It would also be desirable to model the impacts of timber replacing carbon and steel in construction, as these materials are emissions-intensive.
Conclusions
Afforestation is already practiced on a wide scale, and represents an important high-carbon land use. It produces products of critical importance, and can help reduce pressure on intact forests. Though not a "silver bullet," it is an essential component of land-based mitigation efforts.
[1] To learn more about the Total Land Area for the Land Use Sector, click the Sector Summary: Land Use link below.
[2] Current adoption is defined as the amount of functional demand supplied by the solution in the base year of study. This study uses 2014 as the base year due to the availability of global adoption data for all Project Drawdown solutions evaluated.
[3] Determining the total available land for a solution is a two-part process. The technical potential is based on the suitability of climate, soils, and slopes, and on degraded or non-degraded status. In the second stage, land is allocated using the Drawdown Agro-Ecological Zone model, based on priorities for each class of land. The total land allocated for each solution is capped at the solution’s maximum adoption in the Optimum Scenario. Thus, in most cases the total available land is less than the technical potential.
[4] To learn more about Project Drawdown’s three growth scenarios, click the Scenarios link below. For information on Land Use Sector-specific scenarios, click the Sector Summary: Land Use link.
[5] All monetary values are presented in US2014$.
[6] For more on Project Drawdown’s Land Use integration model, click the Sector Summary: Land Use link below.
