Fossil fuel resources, decarbonization, and economic growth drive the feasibility of Paris climate targets

Understanding how reducing carbon dioxide (CO2) emissions impacts climate risks requires probabilistic projections of the baseline (“business-as-usual”) emissions. Previous studies deriving these baseline projections have broken important new ground, but are largely silent on two key questions: (i) What are the effects of deep uncertainties surrounding key assumptions such as remaining fossil fuel resources? (ii) Which uncertainties are the key drivers of the projected emissions and global warming? Here we calibrate a simple integrated assessment model using century-scale observations to project global emissions and committed temperature anomalies over the 21st century. We show that the projected emissions are highly sensitive to assumptions about available fossil fuel resources and decarbonization rates. We find that even under an optimistic, low-fossil fuel resources scenario, the median committed warming just by emitted CO2 in 2100 exceeds the 1.5 degree Celsius Paris Agreement target. Across the analyzed scenarios, the probability of exceeding the 2 degree Celsius target just from emitted CO2 ranges from 24% to 39%. The climate-system uncertainties and decarbonization rates are the key factors directly driving this uncertainty. Several economic growth parameters explain a large share of variability though their interactions.