Climate tipping indicators for improved environmental sustainability assessment of bio-based materials

Introduction of bio-based materials represents one of the key strategies in the transition to a bio-based economy, as these materials are thought to contribute to climate change mitigation through reduction of fossil-related GHG emissions. However, currently recommended metrics used in life cycle impact assessment (LCIA), i.e. the global warming potential (GWP) and the global temperature change potential (GTP), do not account for the potential of GHG emissions to trigger dramatic and potentially irreversible changes in the climate system (so called, climate tipping points).

Climate tipping points are crossed when the global temperature reaches specific thresholds. Earlier efforts to include climate tipping in LCIA are limited to the climate tipping potential (CTP) metric, which considers only the loss of Arctic sea ice as tipping point. Yet, several other tipping points are foreseen to be crossed within this century.

Examples include the collapse of the ocean circulation in the Atlantic or the irreversible melting of the Greenland ice sheet. This makes climate tipping particularly relevant to consider for bio-based materials, as they are often designed for biodegradability and may contribute to crossing multiple climate tipping points.

The objectives of this PhD work were (1) to develop a methodology to account for multiple climate tipping points for assessing the climate performance of products in life cycle assessment (LCA), and (2) to apply this methodology to selected case studies on bio-based materials. This resulted in the development of new characterization factors, the multiple climate tipping points potentials (MCTP), at both midpoint and endpoint level.

Substance-specific MCTP, expressing the impact per unit of emitted substance, were calculated for the three major anthropogenic GHGs: carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). The MCTP at midpoint level was built taking the earlier climate tipping potential (CTP) of Jørgensen et al. (2014) as starting point.

The CTP depends on the proximity of an emission to the tipping point (Arctic summer sea ice loss) and on the share of carrying capacity up to this tipping point that is taken up by emissions. Their framework was expanded and adapted to include 12 additional projected climate tipping points, which were selected based on a set of criteria defined to ensure feasibility of modelling.

Each tipping point was then modelled considering the potential effects that crossing a tipping point has on speeding up the next tipping points, while accounting for uncertainties in the temperature thresholds triggering tipping. The resulting midpoint MCTP gives a measure of the potential contribution of a product’s life cycle GHG emissions to crossing multiple climate tipping points.

The MCTP depends on the emission year and it assigns larger impacts to those emissions occurring right before when a tipping point is expected. Given this dynamic character, MCTPs are provided as sets of year-specific factors. The method, however, is not without limitations. Particularly, the calculation of year-specific MCTP factors depends on knowledge about future expected tipping points and development of the background atmospheric GHGs concentration, which are uncertain.

Building on the midpoint MCTP methodology, the framework was further developed by translating the contribution to tipping (midpoint level impacts) into potential temperature increase and then into potential loss of species following the temperature increase. The resulting MCTP at endpoint level therefore expresses the damage to ecosystems quality in terms of potential loss of terrestrial species resulting from the contribution of GHG emissions to cross climatic tipping points.

To improve comparability with other indicators for species loss used in LCA, the endpoint MCTP factors were expressed as either local loss of species, i.e. loss from delimited areas that can be reverted through recolonization from other areas, or global loss of species, i.e. irreversible extinction across the world.

Being generally proportional to the MCTP at midpoint, the resulting endpoint MCTP attributes a larger potential species loss to emissions with higher contribution to crossing tipping points, given that crossing could intensify warming and exacerbate species loss. It follows that the main advantage of the endpoint MCTP is to express impacts in terms of damage to terrestrial species.

This, however, warrants further harmonization efforts to make them fully comparable with endpoint indicators representing other impact categories. To demonstrate the potential added value of MCTPs in environmental sustainability assessment of bio-based materials, the developed MCTPs at midpoint level were applied to three ‘cradle-to grave’ case studies on: (i) engineered biochar obtained from biomass residues, (ii) engineered hydrochar obtained from green waste and (iii) polyhydroxyalkanoate (PHA) bio-plastic produced from molasses.

The cases on hydrochar and PHA showed that MCTPs can bring additional insights compared to GWP and GTP metrics. While with GWP the climate performance improved with increasing stability of the bio-based material, due to larger benefits from carbon storage, MCTP showed the opposite trend because when degradation is slow a substantial share of emissions occurs in proximity to tipping points, where climate tipping impacts are the largest.

In the case of biochar, MCTP did not lead to additional insights, showing that the performance of very stable materials like biochar depends more on total amount of emissions released over a certain timeframe rather than on emission timing and proximity to tipping points. Thus, the use of MCTP is more relevant for assessing bio-based materials that slowly degrade over time but achieve at least 95% degradation in about 70 years.

This is also expected to be the case for the endpoint MCTPs. In conclusion, this PhD work contributed to the development of a more robust LCIA methodology accounting for climate tipping impacts of products’ life cycle GHG emissions. Climate tipping represents a new impact category in the context of climate change impacts and thus the developed MCTP factors should be considered complementary, and not a substitute, to the recommended GWP and GTP.

Their use for assessment of the climate tipping impacts of selected bio-based materials can add new insights on their environmental sustainability performance depending on the stability of the bio-based material. A practical implication of the new MCTP factors is the requirement for emission inventories to be provided in time-differentiated format, where relevant and where necessary.