Divergence in land CO₂ flux estimates: new research offers more consistent accounting

Land-based mitigation is crucial to achieve the Paris Agreement goal of reaching climate neutrality by balancing anthropogenic greenhouse gas emissions (GHG) with the CO₂ removals of carbon sinks.

There is a difference in the accounting methods used by countries – who use national GHG inventories based on agreed IPCC methodologies – and global scientific models (called Integrated Assessment Models). This results in a gap which complicates the task of assessing collective progress under the Paris Agreement’s periodic assessment known as the Global Stocktake.

A new paper published in Nature, led by IIASA and co-authored by the JRC, makes further steps towards consistent accounting of land use CO₂ fluxes: it aims to ensure accurate assessment of progress without “comparing apples and oranges".

The study, which uses the OSCAR climate model, aligns global pathways with countries’ greenhouse inventories, revealing that emission benchmarks become more challenging when using countries’ definition of anthropogenic sinks.

As such, achievement of net-zero emissions must happen faster, up to five years before previously estimated target dates (about 2050 and 2070 for 1.5˚C and 2˚C scenarios respectively). The remaining global carbon budget compatible with the Paris Agreement's goal needs to be reduced.

The study warns against depending too heavily on land CO₂ sinks for national climate targets, as they may become a net source of emissions by the end of this century. This stems from the analysis that the CO₂ fertilisation effect that drives the current land sink is expected to decline in low-warming scenarios, and negative impacts of climate change may increase.

Recommendations from this study include:

• ensuring transparency in the expected land sector contribution to national climate targets
• clarifying countries’ deforestation pledges
• establishing an "operational translation system" for better understanding between scientific and practitioner communities
• incorporating the alignment proposed here in future IPCC reports.

Considering the imperative need for drastic emissions cuts in this decade to stay within the Paris Agreement limits and keep temperature rise well below 2˚C, this study emphasises the importance of measuring progress in a like-for-like manner. It provides critical information for the forthcoming COP28, where the Global Stocktake of the Paris Agreement will assess collective climate progress, and for the next round of countries’ climate pledges, expected within two years.

National inventories vs global models

A study led by the JRC, published back in 2021, highlighted for the first time a large discrepancy in estimates of land use CO₂ fluxes between the global models that provide the emission benchmarks to reach the Paris Agreement’s goal and the national greenhouse inventories.

This discrepancy, which amounts to more than all annual emissions from USA, results mainly from differences in defining anthropogenic forest CO₂ sinks and areas of managed land. In national GHG inventories, the land use CO₂ flux includes the response of land to human-caused environmental changes, such as CO₂ fertilisation on broadly-defined managed land. In contrast, global models treat this response as part of the non-anthropogenic sink and adopt a narrower approach to identifying managed land.

Consistent with the IPCC guidelines for national GHG inventories, managed land includes land that fulfills social, ecological and economic functions. For global models, this definition is narrower, and consequently the carbon sink capacity lower.

The difference in definitions has significant implications for assessing collective climate progress, determining the remaining carbon budget, and, more broadly, instilling confidence in land use estimates under the Paris Agreement.

All roads to Paris pass through forests. This study helps not to get lost among the trees.

Related links

Aligning climate scenarios to emissions inventories shifts global benchmarks

Critical adjustment of land mitigation pathways for assessing countries’ climate progress