Anthromes, CO2 and Terrestrial Carbon – Session 5: Implications of land use change uncertainties for the CO2-driven natural land sink
Authors: Erb Karl-Heinz , Obermeier Wolfgang , Grassi Giacomo , Yu Zhen , Walker Anthony 
Published: March 2023

10.10 - 11.20: Title TBC, Karl-Heinz Erb

11.20 - 11.40: Uncertainties in modelled CO 2 fluxes from land use and land use change, Wolfgang Obermeier

11.40 - 12.00: Translating land-use fluxes for 2001-2020 from global models to national inventories, Giacomo Grassi (presenting remotely)

12.00 - 12.20: Simulating China's carbon dynamics using improved LUCC data: from source to sink, Zhen Yu (presenting remotely)

12.20 - 12.40: How anthrome carbon cycling influences our understanding of the CO2-fertilzation carbon sink, Anthony Walker

Scrutinizing the role of land management for the carbon state of ecosystems

Improving our understanding of the complex dynamics of the Earth system is crucial for forging strategies to cope with the current climate and biodiversity crisis. Still, large uncertainties relate to the ability to map and quantify land management and its impacts on ecosystem functioning. A particular knowledge gaps relates to the human-induced changes in terrestrial ecosystems that do not result in land-cover conversions. In this presentation, I present empirical approaches that allow to isolate and quantify the impact of land use on the global ecosystem carbon state. One such approach is the Human Appropriation of Net Primary Production, which systematically compares the current carbon fluxes in vegetation with those that would prevail in the absence of land use, i.e. those of the potential vegetation. This approach, focussing on fluxes, can be expanded to the study of carbon biomass stocks and turnover rates in vegetation. From empirical time-series analyses, the presentation draws conclusions on quantitative relations, current trends and possible futures, as well as on still prevailing knowledge gaps and uncertainties related to land use in the global carbon cycle.

Uncertainties in modelled CO2 fluxes from land use and land use change

Land use and land-use changes (LULUCs) have greatly influenced the global carbon cycle, primarily through emissions from the conversion of natural forests to agricultural land. Additionally, LULUCs have diminished the potential natural land sink, as the cleared primary forests could have sequestered carbon due to favorable environmental changes (mainly via so-called CO2 -fertilization). As in the Global Carbon Project’s global carbon budget, CO2 fluxes from LULUCs can be estimated using models, such as dynamic global vegetation models (DGVMs) and bookkeeping models (BMs). We assess differences in these modelling approaches, from the global to the country-level, particularly focusing on the assumed environmental conditions. BMs do not consider environmental changes as they rely on observed, fixed carbon stocks (similar to inventory-based approaches). In contrast, transient LULUC flux estimates from DGVMs accumulate the loss of additional sink capacity, accounting for C stock changes that would have occurred in the absence of LULUCs that destroyed the natural land sink. We find that the loss of additional sink capacity accounts for  40% of recent fluxes from LULUCs on the global scale. Additionally, LULUC flux estimates are highly uncertain in some countries, strongly dependent on the assumptions of the environmental conditions and the land use forcing data.

Translating land-use fluxes for 2001-2020 from global models to national inventories

A striking discrepancy exists in estimates of global anthropogenic land-use CO2 fluxes between the national greenhouse gas inventories (NGHGIs) and the IPCC assessment reports, based on bookkeeping models (BMs). This discrepancy is thought to be mainly due to differences in how anthropogenic forest sinks are defined. Building on previous studies, we implement an approach that adds the CO2 sink due to environmental change from countries’ managed forest area (estimated by 16 Dynamic Global Vegetation Models, DGVMs) to the original land-use flux from three BMs. Our results confirm a large difference (6.5 GtCO2 yr-1) in global land use CO2 fluxes between the BMs, estimating a source of 4.8 GtCO2 yr-1 for the period 2001-2020, and NGHGIs, which estimate a sink of -1.7 GtCO2 yr-1. Most of the gap is found on forest land. By adding the DGVMs’ sink in managed forests to BM estimates, the gap between BMs and NGHGIs becomes substantially smaller both globally and in most regions and countries. By offering a blueprint for operationalizing future comparisons between BMs and NGHGIs, and identifying areas to be further investigated, this study represents an important step forward for increasing transparency and confidence in land-use CO2 flux estimates at the country level.

Simulating China's carbon dynamics using improved LUCC data: from source to sink

Carbon budget accounting relies heavily on Food and Agriculture Organization (FAO) land-use data reported by governments. Here we develop a new land-use and cover-change database for China, finding that differing historical survey methods biased China’s reported data causing large errors in FAO databases. Land ecosystem model simulations driven with the new data reveal a strong carbon sink of 8.9±0.8 Pg carbon from 1980 to 2019 in China, which was not captured in FAO-data-based estimations due to biased land-use and cover-change signals. The land-use and cover-change in China, characterized by a rapid forest expansion from 1980 to 2019, contributed to nearly 44% of the national terrestrial carbon sink. In contrast, climate changes (22.3%), increasing nitrogen deposition (12.9%), and rising carbon dioxide (8.1%) are less important contributors. This indicates that previous studies have greatly underestimated the impact of land-use and cover-change on the terrestrial carbon balance of China. This study underlines the importance of reliable land-use and cover-change databases in global carbon budget accounting.

How anthrome carbon cycling influences our understanding of the CO2-fertilzation carbon sink

The global CO2-driven carbon sink is highly uncertain and is quantified using an ensemble of carbon cycle models. Confidence in this estimate arises from its close fit with the residual of the global carbon budget. However, the global carbon budget relies on estimates of the other major components of the carbon cycle, including the land use and land cover change flux that is quantified using book-keeping models. Combined, these two major fluxes represent a redistribution of carbon across the land-surface rather than substantial net changes in global land carbon. Furthermore, both of these fluxes are highly uncertain and it is possible that models are structurally biased towards predicting a strong CO2 response. In this presentation I will show some global and regional results related to how assumptions and uncertainty in estimates of the land use and land cover change fluxes could have significant implications for our understanding of the global CO2-driven carbon sink.