Planetary limits to BECCS negative emissions (D2.a)

The acknowledged threat of many damaging aspects of climate change over the next century and beyond has prompted research into ways to reduce and reverse the recent increase in the amount of carbon in the atmosphere. Many climate model projections that prevent more than 2°C of climate change rely on negative emissions, particularly from BioEnergy with Carbon Capture and Storage (BECCS). The concept of BECCS is to grow bioenergy crops to produce energy and sequester carbon. The crop takes in carbon dioxide from the atmosphere through photosynthesis. When the bioenergy crop is combusted for energy, the resultant carbon emissions are then captured and stored. This therefore results in negative emissions – removal of carbon from the atmosphere. The potential for negative emissions is dependent on three aspects: the amount of carbon taken in by the bioenergy crop; the efficiency with which that biocrop carbon is captured and stored; and amount of carbon emissions from the land use change (LUC) necessary to grow the bioenergy crop. Additionally, there is a need to consider how large-scale deployment might influence other carbon sinks and the physical coupling of the atmosphere with the land surface. These complexities are often omitted from studies.

This work takes the approach by considering the supply side of first generation annual energy crops with a greater focus than earlier work on the climate drivers associated with biofuel production – i.e. what is the maximum potential for negative emissions from BECCS and what are the greatest planetary limitations to the provision of BECCS? Do future changes in climate offset or enhance the BECCS potential? Here we present a new quantification of the amounts of BECCS available from existing agricultural land or forest areas. We account for the net carbon effects of BECCS, considering the carbon lost from the land use changes as well as the gains from annual energy crops, and the non-carbon effects of BECCS on climate. This perspective can help give a fuller answer to scepticism about the potential of BECCS to provide significant amounts of negative emissions. As in many scenarios from integrated assessment models (AR5 WG3) it is assumed here that CCS is readily available. This will be considered separately elsewhere in AVOID 2.

Key Findings

1. Most but not all IPCC WG3 emission scenarios stabilising climate at low levels, such as 2°C, require large scale deployment of BECCS.
2. Land-use emissions embedded in BECCS scenarios can be large and need to be included in emission pathways. In one scenario, the expected BECCS associated temperature offset of 1.34°C by the end of the century is reduced to 0.15°C due to land-use emissions.
3. Gross negative emissions from BECCS are unlikely to exceed 640GtC (2346 GtCO2) over the 21st century. When land-use change is included the net maximum contribution is 130 GtC (476 GtCO2). These numbers are derived from the HadGEM2-ES Earth System Model and include the impact of climate on yields. Gross values account for yield only, whilst net values include carbon losses from associated deforestation. The highest gross emissions come from a scenario assuming rapid expansion of bioenergy crops from 2020 in the highly productive tropics covering 18% of the land surface in 2100. The highest net emission comes from a scenario assuming available abandoned agricultural land is put into production reaching a maximum of 5% of land cover in 2100.
4. In the scenario with the least mitigation benefit an additional 100 GtC (366 GtCO2) is emitted to the atmosphere due to the combined effect of emissions from clearing forests in regions with marginal yields vulnerable to climate change. In this scenario, the land use emissions exceed the potential carbon sequestration from BECCS over the 21st Century.
5. The contribution of negative emissions from BECCS is unlikely to exceed cooling of 0.7°C by 2100, given technological and social constraints. The biophysical effect of deforestation on the decadal-centennial timescale dominates the cooling from negative emissions. More realistic values are around 0.25°C from redeploying agricultural land to bioenergy.
6. In the scenarios considered here, yields increase under climate change, with important regional variation. We find no global threshold under which BECCS production decreases.
7. The median gross BECCS requirement compatible with a 2°C climate target is 166 GtC (608 GtCO2) within the IPCC scenario database. The highest net estimate presented here is 130GtC (476 GtCO2) implying that deploying enough BECCS within land-use constraints may be highly challenging.
8. In the absence of limits on CCS the largest constraint on BECCS, as found in this study, is the amount of land allocated to bioenergy crops and the rate of deployment. Competition for land for food production is a key uncertainty. Other major constraints on the amount of BECCS achievable are the harvest and sequestration efficiencies.
9. The most productive areas globally are the tropics. Expansion of bioenergy crop into those areas has a higher marginal gain.
10. Biophysical cooling associated with deforestation for biofuels may have benefits to stabilising climate at low levels. Biophysical effects may also include reduced precipitation and other non-temperature linked negative impacts. Other modelling studies have also found warming associated with tropical deforestation.
11. Uncertainties are large, particularly with regard to the impacts of climate on crop productivity with literature values spanning a possible increase to decrease.