Land ecosystems offer an effective nature-based solution to climate change mitigation by absorbing approximately 30% of anthropically emitted carbon. This absorption is primarily based on constraints from atmospheric and oceanic measurements while quantification from direct studies of the land carbon cycle itself displays great uncertainty. The latter hinders prediction of the future fate of the land carbon sink. Here, we show a unified framework for quantifying land carbon sequestration. The framework unifies all carbon cycle processes under a theory of dynamic disequilibrium and all models with one matrix equation to represent a nonautonomous compartmental system. While carbon input and residence (transit) time have been used to quantify carbon storage capacity at dynamic equilibrium, we introduce the third quantity, carbon storage potential that integrates fluxes and timescales, to define dynamic disequilibrium of land carbon cycle under global change. The carbon storage potential is presently not directly measurable. Its estimation requires the integration of multiple data sets with models. We urge the research community to convert the existing models and develop new models in the matrix form before we can fully identify sources of uncertainty in model predictions, evaluate relative values of various data sets, and improve the accuracy of quantification of land carbon sequestration.