Perennial Biomass
Project Drawdown defines perennial biomass as: the use of perennial grasses and coppiced woody plants for bioenergy feedstock, instead of annual crops like corn. This solution replaces grazing or annual cropping.
Bioenergy from annual crops has a poor life cycle analysis in terms of climate impact, but some perennial bioenergy crops have modest potential. Perennial grasses and re-sprouting woody plants have naturally high productivity, need fewer inputs and water, and are not food crops; hence, many governments worldwide are choosing them as future energy farming systems. They have the advantage of sequestering modest amounts of soil carbon while producing bioenergy (energy impacts are accounted for in the biomass Energy Sector solution). Though not modeled here, they are also an ideal feedstock for clean cookstoves.
This study focuses on two types of perennial energy crops: herbaceous crops (in this case mostly giant grasses) and short rotation coppice, in which the aboveground biomass of re-sprouting woody crops is harvested mechanically on a 2-3 year rotation.
Bio-based energy cannot hope to replace fossil fuels. However, perennial biomass crops can sequester carbon while restoring degraded land. Their contribution to climate change may be most important in the next few decades, as clean energy gradually comes to dominate the energy sector. Though not modeled here, perennial biomass can be used as feedstock for many other uses, from paper and cardboard to insulation and bioplastics.
Methodology
Total Land Area [1]
The total land suitable for perennial biomass is 74 million hectares, representing degraded grassland areas. [2] This area is lower than many estimates, as Drawdown prioritizes food and reforestation over bioenergy. Current adoption [3] of perennial biomass is estimated at 0.2 million hectares, obtained by applying the national rates of four countries to all regions.
Adoption Scenarios [4]
Multiple future adoption projections were developed, based on the projected low (9.9 percent) and high (44.7 percent) growth rates from the United Kingdom. Since the total allocated land area for this solution is lower than many other estimates, six aggressive custom adoption scenarios were built, with some showing an early growth rate (50-80 percent of the allocated area by 2030). Preventing the clearing of forest area for perennial bioenergy cropping was also taken as one reason for the aggressive adoption of this solution in the degraded grassland area.
Impacts of increased adoption of perennial biomass 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 the adoption of perennial biomass on 57.8 million hectares of the allocated area by 2050.
- Drawdown Scenario: Scenario analysis shows the adoption of perennial biomass on 66.8 million hectares of the allocated area by 2050.
- Optimum Scenario: Scenario analysis shows the adoption of perennial biomass on 73.8 million hectares of the allocated area by 2050.
Sequestration Model
Sequestration rate is 1.2 tons of carbon per hectare per year, based on 17 data points from 4 sources. It is assumed that all sequestered carbon that is not harvested for energy production is re-emitted at the end of productive life, when fields are plowed up and re-planted. Perennial bioenergy crops were assumed to have a productive lifespan of 15 years based on meta-analysis of 3 data points (reporting averages) from 3 sources.
Financial Model
First cost is US$1,351.78 per hectare, [5] based on meta-analysis of 6 data points from 4 sources. It is assumed that first costs for the land use that perennial bioenergy crops are replacing have already been paid, as the land is already in production. Net profit margin is $495.28 per hectare per year, based on 20 data points from 7 sources. As this solution is implemented on degraded grassland, net profit per hectare of the conventional practice is not modeled.
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. Despite very broad climactic suitability, perennial bioenergy crops were given low priority so as to reduce impacts on food production.
The solution was relegated to degraded grasslands, where it was the sixth (and lowest) priority, as Drawdown rates food production and ecological restoration as higher priorities than energy.
Results
Total adoption in the Plausible Scenario is 57.9 million hectares in 2050, representing 78.2 percent of the total suitable land. Of this, 57.7 million hectares are adopted from 2020-2050. The impact of this scenario is 3.3 gigatons of carbon dioxide-equivalent emissions reduced by 2050. Net cost is US$77.9 billion. Net savings is US$541.9 billion.
Total adoption in the Drawdown Scenario is 66.8 million hectares in 2050, representing 90.2 percent of the total suitable land. Of this, 66.6 million hectares are adopted from 2020-2050. The impact of this scenario is 4.1 gigatons of carbon dioxide-equivalent by 2050.
Total adoption in the Optimum Scenario is 73.9 million hectares in 2050, representing 99.9 percent of the total suitable land. Of this, 73.7 million hectares are adopted from 2020-2050. The impact of this scenario is 4.9 gigatons of carbon dioxide-equivalent by 2050.
Discussion
Benchmarks
This solution is somewhat challenging to benchmark, as few projections are available. Leymus and Lal, 2005 project the biosequestration of 0.06 gigatons of carbon dioxide-equivalent per year by 2050 from perennial biomass. The Drawdown study calculates 0.04-0.15 gigatons of carbon dioxide-equivalent per year in 2050; thus, it is in alignment with Leymus and Lal.
Limitations
Many factors limit this study, as the industry is in its infancy. Current adoption, projected future adoption, and financials would all benefit from additional data points.
Conclusions
Perennial biomass may never be as central a solution as afforestation or multistrata agroforestry. Nonetheless, it offers a productive and carbon-sequestering use of degraded lands, farm borders, riparian edges, and other spaces. As wind, solar, and other energy sources come to meet civilization's energy needs, use of perennial biomass may shift to clean cookstoves and feedstock for paper, bioplastic, and other bio-based products.
[1] To learn more about the Total Land Area for the Land Use Sector, click the Sector Summary: Land Use link below.
[2] 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.
[3] 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.
[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.
