The rate of transient warming is determined by a number of factors, notably the radiative forcing from increasing CO₂ concentrations and the net radiative feedback. Uncertainty in transient warming comes from both the uncertainty in each factor and from the warming’s sensitivity to uncertainty in each factor. An energy balance model is used to untangle these two components of uncertainty in transient warming, which is shown to be most sensitive to uncertainty in the forcing and not to uncertainty in radiative feedbacks. Additionally, uncertainty in the efficacy of ocean heat uptake is more important than uncertainty in the rate of ocean heat uptake. Three further implications are: (1) transient warming is highly sensitive to uncertainty in emissions; (2) caution is warranted when extrapolating future warming trends from short‐lived climate perturbations; and (3) climate models tuned using the historical record are highly sensitive to assumptions made about the historical forcing. This paywalled article appeared on the Geophysical Research Letters website at