Biomass storage prevents or slows the decay of biomass by maintaining dry, cold, or anoxic conditions. In cases where decay is not fully prevented, leakage can be directly monitored with sensors in or around the storage site, but the potential for future leakage will likely be modeled. Modeling of future leakage must consider expected decomposition under optimal storage conditions, as well as suboptimal conditions (e.g. wetter, warmer, or more oxic than optimal), and should be informed by direct measurements. Long-tail risks like fire, pests, human disturbance, or physical changes to the storage site (e.g. erosion or settlement) must be estimated. GWP assumptions used to incorporate the leakage of non-CO₂ gasses into the calculation of net carbon removal should be transparently disclosed. Our uncertainty evaluation reflects the wide range of storage mechanisms being explored and the uncertainty associated with storing carbon for 1000+ years. Those interested in specific storage mechanisms, particular storage sites, or storage on longer timescales may find it appropriate to adjust these uncertainties.