Effect of land carbon accounting methods on the climate response to cumulative CO2 emissions

The proportionality between global temperature change and cumulative CO₂ emissions underpins our understanding of how climate will respond to future emissions, and what level of emissions reductions will be needed to stabilize global temperatures. Typically, fossil fuel and land-use CO₂ emissions are treated as equivalent drivers of this global temperature response, and emissions reductions from both sources are assumed to contribute similarly to mitigation targets. However, measuring land-use CO₂ emissions in the real world is complicated by the difficulty in separating direct emissions (those caused by deforestation and other human land-use activities) from indirect carbon fluxes caused by CO₂ fertilization and other land carbon responses to changing climate conditions. Consequently, an emission (or removal) of CO₂ from land use activities as measured and reported in national emissions inventories is not equivalent to a land-use emission as defined in modelling studies that have been used to quantify the climate response to cumulative fossil fuel and land-use CO₂ emissions. Here we assess the impact of these different land carbon accounting conventions on two key metrics of the climate response to cumulative CO₂ emissions: the Transient Climate Response to cumulative CO₂ Emissions (TCRE) and the Zero Emissions Commitment (ZEC). Using a spatially-explicit intermediate complexity Earth system model, we quantify these two metrics as a function of (1) fossil fuel CO₂ emissions only; (2) fossil fuel + direct land-use CO₂ emissions; and (3) fossil fuel + net land-use CO₂ fluxes including indirect land carbon sinks. We show that both the magnitude and time-dependence of the TCRE and ZEC metrics is sensitive to the inclusion and definition of land carbon emissions. This finding underscores the need for improved clarity and care in the application of scientific findings to real-world mitigation efforts related to land carbon emissions and removals.