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To limit global warming and achieve the long-term goals of the Paris Agreement, we must change the way we use land while reducing emissions from deforestation, as well as enhancing the current capacity of CO2 sinks. Terrestrial ecosystems, mainly forests, absorb nearly one third of the total CO2 emissions caused by human activities. While new technologies to remove CO2 from the atmosphere will be eventually needed, forests are, at present, the most important CO2 sink that humans can manage.
In the just-released IPCC Special Report “Global Warming of 1.5°C” are illustrated the different mitigation strategies that can achieve the net emissions reductions that would be required to follow a pathway that limit global warming to 1.5°C with no or limited overshoot. A breakdown of the global net anthropogenic CO2 emissions into the contributions in terms of CO2 emissions from fossil fuel and industry, agriculture, forestry and other land use (AFOLU), and bioenergy with carbon capture and storage (BECCS) is shown, but the report states also that “AFOLU estimates reported […] are not necessarily comparable with countries’ estimates”.

This is because unlike emissions from fossil fuel use and industry, emissions and sinks from land-based activities are notoriously difficult to measure and verify.
At present, there is a discrepancy of about of 4 gigatonnes of CO2 per year (GtCO2 /y, that is 10% of total human-caused CO2 emissions over the reference period) in global anthropogenic net land-use emissions between global models (assessed in the last IPCC assessment report AR5) and national GHG inventories (reported to UNFCCC). The global modelling community and national governments currently apply in fact different methods to estimate and report land-based greenhouse gas (GHG) emissions. This makes it difficult to track progress towards the goals of the Paris Agreement.
How to reconcile conceptual differences in anthropogenic forest sink estimation between models and GHG inventories?

A new study recently published on Nature Climate Change (among the authors, CMCC researcher Lucia Perugini) explores the reasons for these differences and outlines ways to reconcile them. More in detail, the study quantifies the differences in independent estimates of land-related GHG uptakes and emissions (known collectively as “fluxes”) between those reported by countries in their GHG inventories and those produced by the global modelling community.
As highlighted in the study, estimates of emissions of land-based activities from global models and country reports are different, because they have different approaches to estimating what “anthropogenic” fluxes are. In other words, the reasons of this discrepancy lie in the differences in estimating what part of a forest CO2 sink is “anthropogenic”, or human caused. Having different purposes and objectives, the scientific modelling community supporting the IPCC assessment reports and the IPCC guidelines for national GHG inventories (used by countries for their emission reporting) have in fact developed different approaches to distinguish what is “anthropogenic” from what is not in relation to GHG sources and sinks from land. Both approaches are valid in their own specific contexts, but they are not comparable.

Globally about 80% of the aforementioned discrepancy is related to conceptual differences in anthropogenic forest sink estimates, in both developed and developing countries. In particular, country GHG inventories often include estimates from large areas of managed forests and the impact of indirect effects (environmental changes, such as CO2 fertilisation, nitrogen deposition and climate change). Global models, in contrast, estimate the anthropogenic land flux by considering fewer management activities on a smaller managed forest area, and include most of the indirect effects on extant forests in the residual (non-anthropogenic) land response.
This gap, the authors suggest, can be largely reconciled when the impact of environmental change (indirect effects) on managed lands (fur further details, see fig. 5 of the paper on Nature Climate Change, ndr) is added to the direct effects from IPCC AR5. An even greater reconciliation occurs when differences in managed land area between GHG inventories and global models are taken into account.

In order to facilitate a more robust global stocktake, the study underlines some concrete recommendations. Countries could provide more transparent and complete information on what is included in their GHG inventories (e.g., maps, harvested area and cycle, forest age and if – and how – indirect and natural effects are included). In parallel, the global modelling community should design model experiments and outputs to increase their comparability with country inventories and thus more policy relevant.

The team of authors was led by researcher Giacomo Grassi – Grassi – Joint Research Centre, European Commission, Ispra, Italy.
For further information, read the integral version of the paper: Grassi G., et al. (2018) Reconciling global model estimates and country reporting of anthropogenic forest CO2 sinks, Nature Climate Change, doi:10.1038/s41558-018-0283-x

Read also the post on Carbon Brief: “Guest post: Credible tracking of land-use emissions under the Paris Agreement” and the article: “Tracking land-based CO2 emissions under the Paris Agreement”.