Biomass is often considered a climate-neutral fuel because the carbon emitted during combustion is the same carbon that was absorbed from the atmosphere during the biomass's growth. As a result, biomass may appear carbon neutral at first glance. In reality, the issue is not that simple, as several other factors must be considered, including temporal aspects, land use changes, and different types of biomass including residues.
We have made this case before, including in Jannick Schmidt's presentation to the parliamentary hearing on biomass. The logic runs in three steps.
When we call biomass climate-neutral, we assume that the carbon released when we burn it is the same carbon the forest absorbed while growing. Over a long enough time horizon, the carbon balances.
The problem lies in the time gap between emission and reabsorption. When a tree is harvested and burned, the CO₂ is released into the atmosphere immediately, whereas the forest takes several decades to reabsorb an equivalent amount of carbon. During this period, the additional CO₂ in the atmosphere contributes to global warming.
This timing is precisely what matters. The most rapid increase in global temperatures is occurring now, meaning that any additional acceleration of global warming creates further challenges as both economy and ecosystems, including plant and animal species, struggle to adapt to rapidly changing climatic conditions. Consequently, a CO₂ emission released today and reabsorbed only decades later can never be considered impact-neutral, as the intervening period of elevated atmospheric carbon contributes to warming and its associated effects.
On top of that, harvesting, transport, replanting and the land itself all carry emissions of their own. So even in the best case, biomass is lower impact. From a life-cycle perspective, biomass can never be truly climate-neutral.
Biomass can be divided into two overall categories: Biomass with higher climate impact and biomass with lower climate impact.
High-impact biomass production requires land. Consider the addition of one hectare of energy crops: that land must be sourced from somewhere, and it is often already in use for purposes such as food production. When introducing energy crops, where other crops would else have been grown, displaces these other crops, and they will reappear elsewhere to satisfy food or other demand. Meeting this demand may require the conversion of forests or other natural ecosystems into productive land, a process which according to IPCC corresponds to approximately 11% of global GHG emissions. Alternatively, production may be intensified on existing farmland to increase yields, which can also generate additional emissions through greater use of fertilisers. These effects are known as indirect land-use changes (iLUC), and their impacts often occur beyond national borders compared to where the bionmass is used or produced. Land is a resource, and its climate implications cannot be ignored.
Low-impact biomass is a different story. This refers to residues, which are not fully utilised, and which would have been left for decay if not used for energy purposes. This kind of biomass does not require additional land use and therefore it is not associated with iLUC.
However, there is still an impact in terms of the timing of the CO2 when burning the biomass compared to the counterfactual, i.e. if the residues were left for decay, which may take several years. Here the impact from using the residues is preponed CO2 emissions compared to the CO₂ release with the slower process of natural decomposition. The impact from this type of biomass is much lower compared to biomass that requires land.
It should be noted that low-impact biomass is a scarce and very limited resource – keep on reading below.
It is important to emphasize that to have a "low-impact" biomass, it requires that the alternative use of it would be natural decomposition. Most forest residues and residual streams from other industries, such as bark and sawdust from sawmills, are already fully utilised today. When diverting an already fully utilised residue for biomass purposes, it will taken away from current users, who have to source the material from elsewhere: production of high-impact biomass elsewhere.
This is the part that usually gets missed, and it matters most. Even if you decide to use only "waste" as fuel, such as forest residues or sawdust from sawmills, you most often end up driving demand for the high impact biomass.
The reason is that many biomass residues are already fully utilised by other industries and the amount of sawdust available does not depend on demand for sawdust but rather the demand for sawn wood.
So, when an energy company buys a pile of sawdust that someone else was already using, this other consumer must source their biomass from alternative sources. On the margin, this can only be met by biomass which is able to increase their production to match demand, which means a return to the high-impact biomass that causes indirect land use changes. In other words, your biomass residue purchase quietly pushes another user towards high-impact biomass. This is leakage, and it means that "we only use waste or residues" is not a guarantee on its own.
Thus, understanding counterfactuals and market dynamics is key to ensure that biomass remains low impact. To avoid these leakage effects, one must identify sources of biomass residues that are not fully utilised. These represent a resource with great potential for the energy sector, since they can be used to replace fossil fuels while at the same time avoiding pushing another consumer towards high impact biomass.
Read an example of potential assessment of biofuel feedstocks here.
From a life-cycle perspective, biomass is not a single, uniform category with a single climate impact. Its overall effect depends entirely on its origin and on what its use displaces in the broader economy.
The most accurate conclusion is that biomass cannot be considered climate-neutral, it exists on a spectrum ranging from high to low impact, and that the availability of low-impact biomass is limited. Residual biomass streams may have the potential to be lower impact than dedicated energy crops, but this advantage only holds if robust safeguards ensuring that the alternative use will be natural decomposition are in place to prevent indirect increases in demand for higher-impact biomass elsewhere.
A proper understanding of the complexity of global supply chains is essential for assessing the real consequences of energy policy decisions. Given the urgency of climate change, there is little room for policy choices that fail to account for these system-wide effects.
Carbon opportunity cost (COC) and indirect land use changes (iLUC) are two different approaches to calculate GHG emissions related to the use of land, e.g. by crops for food and biofuels. I.e. they are used for the same purposes, yet they are fundamentally different and lead to different calculated results. In this blog, I have tried to briefly describe the two approaches, to make a one-by-one comparison for the most important features, and finally to provide my remarks and recommendations.
At 2-0 LCA we have more than 15 years of experience in calculating land use change emissions using different approaches, hereunder iLUC, dLUC, sLUC, and COC. In this blog, I take a deep dive into the comparison of COC and iLUC, as the release of the GHG Protocol Land Sector and Removals Standard requires that the issue of "land use and leakage" shall be addressed, where COC shall be used if there is a high risk of "land carbon leakage". This has led to many questions on what the difference is between COC and iLUC.
Carbon opportunity cost (COC) refers to the CO2 sequestration that would have occurred if the occupied land had been left with native ecosystems instead of the current land cover. COC for 1 ha*year of a specific land use is calculated as the difference in carbon stock (difference between 1 and 2 below) annualised over an amortisation period of e.g. 100 years and converted to CO2 by multiplying with 44/12. The carbon stock of the two situations is described below:
Indirect land use changes (iLUC) account for upstream life cycle consequences of the use of land (also sometimes referred to as land leakage). Different models exist, e.g. see De Rosa et al. (2016), while here I only go into details with the model described in Schmidt et al. (2015).
iLUC refers to the impacts (not necessarily only GHG emissions) that are caused by the occupation of land, e.g. 1 ha*year. When occupying 1 ha*year of land in a certain location, this means that other crops cannot be cultivated on that plot of land, and consequently, these displaced crops will have to be produced in another way and/or somewhere else. This can either be on new land obtained from land conversion somewhere else, or by intensification of existing cropland, e.g. by additional fertiliser application. In addition to the above indirect effects, the actual studied activity that occupies land may store a different amount of carbon than the displaced crop, e.g. when cultivating apple trees on cropland, then the apple trees store more carbon that the crops they displace. This is referred to as direct land use changes. The identification of the indirect land conversion and intensification is made via a market for arable land, that has inputs of land conversion and intensification by country – identified via recent trends of the two based on FAOSTAT timeseries for all crops in all countries.
The total impact includes the sum of:
Direct land use changes
Indirect land use change (market for land affected)
Note that the area of indirectly affected land is calculated based on a weighting using potential net primary production (NPP0).
For the indirect effects, the proportion between land use change and intensification is calculated based on time-series of harvested crops, yields, and area of cropland based on FAOSTAT for all crops and all countries.
The emissions in (1) and (2) above are "gross" emissions, where it has not yet been considered that the conversion for the first year of cultivation of a crop is handed over to the next crops after a year, so that these crops can be cultivated without land conversions. As the occupied land for the crop under study is released for other crops after the occupation in one year, the land can be "recycled" to grow other crops. This means that the effect from occupying 1 ha in 1 year is that the emissions from (1) and (2) above are only preponed by one year, i.e. a bit simplified, if there was no demand for the crop under study, the land conversion effects would have happened one year later (because there currently is a general trend that cropland expands). The effect from changing (preponing) the timing of CO2 emissions can be calculated using the standard GWP100 formula as presented in the supplementary materials of the IPCC Assessment reports, see Schmidt et al. (2015).
In the table below, key features of the two approaches are compared.
| Feature | COC | iLUC |
| Counterfactual: this is essential as this represents the situation without land occupation | Natural vegetation on the actual occupied piece of land after 100 years of no disturbance. | The effect from land use is dLUC + iLUC + intensification. The counterfactual for these three elements are:
|
| Geographical scope | Applicable for any land cover at location at any detailed level of granularity. | Applicable for any land cover at location at any detailed level of granularity. |
| Function and functional unit of land | Most often COC are calculated directly per unit of final products, i.e. there is not a well-defined function and a comparable unit of the supply of land. | The functional unit refers to the potential productivity of land and is measured as 1 ha*year global average land. The use of land in a specific location is linked to the functional unit via relative difference in NPP0 of the affected land plot and the global average NPP0 of the affected market for land, i.e. most often arable land. This means that land use in the wet tropics (with high NPP0) induce more iLUC than in the colder temperate ecozones. |
| Driver of impact | Size of land occupation (ha*year) and potential sequestration through vegetation on the affected plot of land. | Size of land occupation (ha*year), affected market for land, and NPP0 of affected land relative the global average. |
| Market for land | No market for land, and no indirect effects are considered. | A global market for each of the following three market segments is considered: arable land, forest land, and grassland. The markets refer to the marginal use of the land. |
| Temporal aspects | Emissions from 1 year of occupation are estimated from expected carbon stocks over a decided timeframe, e.g. 30-100 years, from the time of occupation and decided years onwards (also assuming that a similar occupation activity as the studied one continues during this time). The decision on this amortization period is arbitrary and has large influence on results. | Emissions from 1 year of occupation are estimated from predicted caused land conversion, where the net effect is induced land conversion at start of occupation and avoided land conversion at end of occupation. |
| GHG metrics | CO2 emissions are calculated based on carbon stock changes and by using 100 years amortization. | GHG emissions include both CO2 (from land conversion) and N2O (from intensification). As described under "Temporal aspects", CO2 emissions from land conversion are induced at beginning of occupation and avoided at end of conversion. The GWP100 of this preponement of CO2 is calculated using time dependent GWP100. |
| Impact | Only CO2 emissions. No good overviews have been identified. Through, based on the supplementary information in Searchinger et al. (2018), the emissions from 1 ha*year occupation have been estimated as typically ranging from ~3-20 t CO2e depending on location and type of land/counterfactual. | The iLUC model is a general life cycle inventory model, which can be linked to any impact category. However, most relevant ones are global warming, nature occupation/biodiversity, and respiratory inorganics (caused by NH3 emissions). The GWP100 from 1 ha*year occupation of global average arable land is ~1.8 t CO2e, data per country is available here: https://2-0-lca.com/wp-content/uploads/iLUC-GHG-summary-per-country-and-land-market_OPEN_20230630-2.xlsx |
The counterfactual of COC is highly hypothetical and makes predictions about what will happen on the land up to 100 years into the future. It can be observed in statistics that >>99% of all global cropland is replanted with new crops every year, indicating that the assumed counterfactual of renaturalisation in COC is not realistic. On the other hand, iLUC models more correctly assume that local use of land will divert cultivation of other crops to somewhere else, and also addressing the fact that intensification is affected. In fact, more than 75% of the global annual increase in crop production is achieved through intensification, which is a key driver of increase in fertiliser demand. So, this must not be ignored.
COC only considers land use effects on the studied plot of land. This means that no land leakage is assumed. Therefore, COC can be said to be a kind of "direct land use change model". This is in conflict with the concept of life cycle assessment, which requires that all affected up- and downstream effects must be included. On the other hand, iLUC models explicitly consider that fact that changing the occupation of a specific land plot will displace another land use, i.e. other crops must be cultivated elsewhere. When identifying this effect, iLUC models assume that there is a market for land (or alternatively that land can be indirectly affected via markets for crops). Therefore, iLUC models do not introduce a cut-off at direct effects for land use changes, as is the case for COC.
Based on the comparison of carbon opportunity cost (COC) and indirect land use change (iLUC) models, I would recommend using iLUC models, as they overcome several serious limitations of COC. iLUC models:
Blaustein-Rejto D, Soltis N, Blomqvist L (2023). Carbon opportunity cost increases carbon footprint advantage of grain-finished beef. PLoS ONE 18(12): e0295035. https://doi.org/10.1371/journal.pone.0295035
De Rosa M, Knudsen M T, Hermansen J E (2016). A comparison of Land Use Change models: challenges and future developments. Journal of Cleaner Production, Volume 113, 1 February 2016, Pages 183-193. https://doi.org/10.1016/j.jclepro.2015.11.097
Schmidt J, Weidema B P, Brandão M (2015). A framework for modelling indirect land use changes in life cycle assessment. Journal of Cleaner Production 99:230-238. https://doi.org/10.1016/j.jclepro.2015.03.013
Searchinger TD, Wirsenius S, Beringer T, Dumas P (2018). Assessing the efficiency of changes in land use for mitigating climate change. Nature; 564(7735): 249–53. https://doi.org/10.1038/s41586-018-0757-z
