Impacts of land-use-environment interactions on sources and sinks of CO2

We are at a turning point in the history of land use: While the main purpose of land use over millennia had been food and fibre production, its huge side-effects on the Earth system became discernible. Though global land use, dominated by deforestation, was historically a driver of global warming, the potential to deploy certain land-use practices such as reforestation for climate change mitigation became evident and land-use an important part of climate policies. Understanding the interactions of land-use change and the Earth system under future climates is thus of paramount importance to ensure policy pathways are compatible with the Paris Agreement.

Historically, land-use change has profoundly depleted terrestrial carbon stocks, contributing roughly one third of historical anthropogenic CO₂ emissions. The entire land biosphere (including land-use change) has, however, acted as a major sink in recent decades, as it strongly responded to environmental changes such as rising atmospheric CO₂, which outweighed the land-use change emissions. These dual drivers – land-use changes and environmental changes – have motivated extensive efforts to quantify land–atmosphere carbon fluxes, leading to the parallel development of bookkeeping models and process-based models, which are now increasingly linked. However, once land-use change and environmental responses are considered jointly, carbon flux attribution becomes non-unique: land-use decisions and environmental change interact to generate synergistic fluxes that blur the distinction between “anthropogenic” and “natural” sources and sinks.

Here, we review the evolution and integration of land use in carbon-cycle modeling and synthesize the current understanding of land-use-environment interactions, focusing on their implications for global and national carbon budgets and future mitigation pathways. We show that synergistic effects – such as replaced and (re-)established sinks and sources – are not secondary details and discuss how recent advances have enabled a consistent treatment of these synergies in the Global Carbon Budget, while highlighting why this attribution remains, in part, a policy choice rather than a purely scientific one.

Finally, we argue that land-use–environment synergies will become increasingly consequential in the future, as land-based mitigation expands, carbon dioxide removal scales up, and climate impacts intensify. Robustly projecting the net land carbon balance will therefore require renewed attention to these interactions, supported by improved process understanding, modeling capabilities, and transparent accounting conventions. Recognizing and consistently treating land-use–environment synergies is essential for robust carbon budgeting and for assessing the effectiveness and risks of land-based climate mitigation in a rapidly changing climate.