This paper explores the sensitivity of energy system decarbonisation pathways to the role of afforestation and reduced energy demands as a means to lessen reliance on carbon dioxide removal.
Large uncertainties surround the viability of carbon dioxide removal
The stringency of climate targets set out in the Paris Agreement has placed strong emphasis on the role of carbon dioxide removal (CDR) over this century. However, there are large uncertainties around the technical and economic viability and the sustainability of large-scale CDR options. These uncertainties have prompted further consideration of the role of bioenergy in decarbonisation pathways and the potential land-use trade-offs between energy crops and afforestation. The interest in afforestation is motivated by its potential as an alternative to large-scale bioenergy with carboncapture and storage (BECCS), with its arguably lower risk supply chains, and multiple co-benefits. Furthermore, doubt over the viability of large-scale CDR has prompted a renewed examination of the extent to which their need can be offset by lowering energy demands.
Testing the impacts of reduced demands and afforestation
A global optimisation model (TIAM-UCL) was used to examine decarbonisation pathways for the global energy system. Based on core assumptions, where energy demands follow business as usual trends and degraded land is used for energy crops, the model was unable to find a solution for a 1.5°C target. Over the period 2020-2100, the carbon budget of GtCO2 is exceeded by 332 GtCO2.
Scenarios where also run to examine how the least-cost decarbonisation pathway changes if i) energy demands are significantly reduced, or ii) degraded land is used for large-scale afforestation instead of energy crops. Each option on its own reduced the CO2 budget exceedance but both were required to allow the model to meet the 1.5°C target.
Impacts on the energy system
Under the 2°C target, afforestation reduced the reliance on BECCS by 60%. Under the 1.5°C target, the system still used all of the biomass available, as the target is so ambitious. When the energy demands were lower, the effect of afforestation on biomass use was dependent on the climate target. Under the 2°C target, less biomass was used across all economic sectors, whereas under the stringent 1.5°C target, all the available wood and crop biomass was exploited, but its use shifted away from the production of liquid fuels towards use in power generation.
Lowering energy service demands had a larger effect on the energy mix than large-scale afforestation. This is because demands are lowered differently across the sectors according to their economic drivers. However, afforestation had a bigger impact on the marginal cost of climate change mitigation, as it substantially decreases the scale and pace of change required by the energy system, especially in the 2°C case.
Given its key role, afforestation should be considered more in deep decarbonisation scenarios, as should lower demand scenarios.
Lowering energy demand and introducing large-scale afforestation both present significant challenges and opportunities. Further work should focus on factors affecting the carbon sequestration potential of afforestation, along with an interdisciplinary research agenda on the scope for large scale energy demand reduction. Research on the social, technical and economic factors that affect the potential for converting abandoned agricultural land to energy crops or new forest would be beneficial. An interdisciplinary research agenda is needed that brings together techno-economic modelling and qualitative scenario development with research on the social change that could lead to large reductions in energy demand