How is Net Zero represented in the model?
How do you represent carbon stored in biomass?
How do you deal with land-use and land-use change (including afforestation and reforestation)?
Do you account for carbon sequestration in products?
What are Negative Emissions Technologies (NETs)?
What NETs do you include in ZERCalc and why?
How do you project future availability of ZERS?
How does the model work?
The model uses linear algebra to estimate the implicit resource demands of net-zero proposals in terms of three common metrics, non-emitting electricity, biomass, and carbon storage; these are referred to as ZERs (Zero Emissions Resources).
The method uses a matrix of “process recipes” to transform a physical demand for goods and services into increasingly more “fundamental” resources. The model is based on the maths of conventional Inventory Analysis, as used for Life-Cycle Analysis (LCA), and described by Heijungs & Sun (2002). Full details of the methodology behind the calculator will be given in our upcoming paper.
Net Zero: what and why?
Net Zero (emissions) means that the quantity of emissions into the atmosphere are balanced by removals out of it. The IPCC have concluded that net zero carbon dioxide (CO2) emissions are needed by around 2050 to limit average global warming to 1.5 °C, and that we must achieve this goal if we want to avoid extremely high risks for both natural and human systems (IPCC, 2018). Although CO2 emissions must reach net-zero by around 2050, some residual emissions of non-CO2 greenhouse gas (GHG) emissions at the same date may be compatible with limiting warming to 1.5°C provided there were “deep reductions” in emissions of those gases (IPCC, 2018). Around 90% of global emissions are now represented in some form of national net-zero target (Net Zero Tracker, 2023).
How is Net Zero represented in the model?
Although mid-century net-zero non-CO2 emissions are not necessarily required to limit warming, we have chosen to include methane (CH4) and nitrous oxide (N2O) in our net zero target for the following reasons:
- Including non-CO2 GHGs gives us a practical approach to quantify the requirement for “deep reductions” in the most significant non-CO2 greenhouse gases.
- Estimated dates for net zero GHG assume rapid short-term emissions reductions with immediate downward trajectories (within five years) which are not reflected in current global policies (Climate Action Tracker, 2023). If near term reductions slower than expected, the date for net zero GHG needs to be sooner for the same level of mitigation (IPCC AR6 Ch3 Cross Chapter Box 3).
- Although there are 1.5°C-compatible pathways which reach net-zero GHG emissions around 2070-2100, the lowest 5th percentile pathways reach net-zero GHG by 2050 (according to IPCC AR6 Ch3 Cross-Chapter Box 3). If we want a low-risk climate mitigation pathway, we need net-zero GHG by 2050.
In a future version of the model we could make a distinction between different GHGs to refine this assessment.
We compensate for residual methane and nitrous oxide emissions with CO2 sequestration, using GWP over 100 years (consistent with the metric used by IPCC pathways).
How do you represent carbon stored in biomass?
We assume that the carbon in emissions from biomass (from respiration, combustion and decomposition) is balanced by carbon sequestration in growing biomass within the same year, with no net changes in carbon stocks, and no net land-use emissions. This means that carbon dioxide absorbed in photosynthesis, and emitted in respiration or combustion, does not need to be explicitly accounted for, consistent with IPCC 2006 Guidelines (Eggleston et al., 2006).
How do you deal with land-use and land-use change (including afforestation and reforestation)?
Given high uncertainty in deforestation estimates is large (up to 50% according to van der Werf et al. (2009)) and that any storage in natural ecosystems is subject to reversal (see Section below on NETs), we do not explicitly account for land-use change in the model. Instead, we have assumed that almost all emissions from land-use change (deforestation and forest degradation) stops by 2050 but there is no permanent net benefit from reforestation or other land-use, giving us net zero emissions in 2050 from land use, land use change and forestry. It is also worth noting that trees could take decades after planting to start to recoup the initial carbon costs associated with forestry.
Do you account for carbon sequestration in products?
Carbon sequestration in products (for example use of timber in construction) is not accounted for since the current model framework is not able to account for delays in carbon emissions and sequestration related to tree growth and harvest. Given there are only decades until 2050, and timber would take decades to grow, it is unrealistic that this would be a significant carbon sink.
What are Negative Emissions Technologies (NETs)?
The term Negative Emissions Technologies (NETs) describes technologies (and sometimes practices and approaches) which extract Carbon Dioxide (CO2) from the atmosphere and store them over a long duration (ideally permanently). The term Carbon Dioxide Removal (CDR) is effectively synonymous with NETs. Carbon may be extracted by photosynthesis in plants, or by technologies such as Direct Air Capture (DAC). Storage reservoirs may be biomass (trees, plants and seaweeds), soils and sediments, the oceans, or geological formations.
What NETs do you include in ZERCalc and why?
Only Carbon Capture and Storage are counted as NETs in the model. This is because all other forms of Negative Emissions are either too immature to estimate a realistic future availability (Enhanced Weathering), or rely on impermanent sequestration with significant risks of reversal (an unexpected early release of carbon). Natural storage reservoirs, oceans and terrestrial systems, are subject to many possible reversal mechanisms including drought, harvest, wildfire, disease, or pests (Babiker et al., 2022) which may become less predictable with increasing levels of climate change.
Although impermanent carbon removals can ‘buy time’ for developing more permanent forms of storage and for adaption, they cannot mitigate against future heating.
This approach is consistent with Principle 3 of the Oxford Offsetting Principles which encourages a transition from short-lived to long-lived storage (Allen et al., 2020) and analysis by CarbonPlan.com (Chay et al., 2022) who found that only DAC and biomass carbon removal and storage had a high confidence level; only removal via those technologies was deemed possible to determine with confidence.
See also How do you deal with land-use and land-use change (including afforestation and reforestation)
What are ZERs?
The following definitions are used to account for the Zero Emissions Resources (ZERs) within the model:
- Biomass is accounted as the dry weight of organic plant matter, used to supply services to humans, with the exclusion of pasture for livestock rearing.
- NEE is defined as gross electricity generation, where gross operational emissions are zero. Nuclear and renewable power is therefore included in this definition although biomass-fuelled, BECCS and other generation with CCS are not.
- Carbon Storage is accounted in terms of the mass of carbon dioxide gas placed in long-term storage. Carbon dioxide which is captured and then released for other uses is not included (i.e. CCUS is not included in this value).
How do you project future availability of ZERS?
The trajectories are based on a review of available data and literature for each ZER, considering historical trends in deployment alongside future enablers, barriers and challenges. The aim is to represent a plausible rate of growth with cautious optimism. Take a look at https://zercalc.web.app/supplyRisks for more details.
References
Allen, M., Axelsson, K., Caldecott, B., Hale, T., Hepburn, C., Hickey, C., Mitchell-Larson, E., Malhi, Y., Otto, F., Seddon, N., & Smith, S. (2020). The Oxford Principles for Net Zero Aligned Carbon Offsetting. University of Oxford, September, 15. https://www.smithschool.ox.ac.uk/publications/reports/Oxford-Offsetting-...
Babiker, M., Berndes, G., Blok, K., Cohen, B., Cowie, A., Geden, O., Ginzburg, V., Leip, A., Smith, P., & Sugiyama, M. (2022). Cross-sectoral Perspectives. In Climate Change 2022 - Mitigation of Climate Change (pp. 295–408). Cambridge University Press. https://doi.org/10.1017/9781009157926.005
Chay, F., Klitzke, J., Hausfather, Z., Martin, K., Freeman, J., & Cullenward, D. (2022). Verification Confidence Levels for carbon dioxide removal. CarbonPlan. https://carbonplan.org/research/cdr-verification-explainer
Climate Action Tracker. (2023). Warming Projections Global Update (Issue December). https://climateactiontracker.org/publications/no-change-to-warming-as-fo...
Eggleston, H. S., Buendia, L., Miwa, K., Ngara, T., & Tanabe, K. (2006). 2006 IPCC guidelines for national greenhouse gas inventories. https://www.ipcc-nggip.iges.or.jp/public/2006gl
Heijungs, R., & Sun, S. (2002). The computational structure of life cycle assessment. The International Journal of Life Cycle Assessment, 7(5), 314–314. https://doi.org/10.1007/BF02978899
IPCC. (2018). Summary for Policymakers. In Global Warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change,. Cambridge University Press. https://doi.org/10.1017/9781009157940.001.
Net Zero Tracker. (2023). Net Zero Stocktake 2023: NewClimate Institute, Oxford Net Zero, Energy and Climate Intelligence Unit and Data-Driven EnviroLab.
van der Werf, G. R., Morton, D. C., DeFries, R. S., Olivier, J. G. J., Kasibhatla, P. S., Jackson, R. B., Collatz, G. J., & Randerson, J. T. (2009). CO2 emissions from forest loss. Nature Geoscience, 2(11), 737–738. https://doi.org/10.1038/ngeo671