Great hope is being placed in the ability of forest ecosystems to contribute to greenhouse gas (GHG) emission reduction targets to limit global warming. Many nations plan to rely on forest-based climate mitigation activities to create additional and long-term carbon sequestration. Here, we take a critical look at the state of the policy and ecology surrounding forest-based natural climate solutions (NCS), with a focus on temperate forests of the United States (US). We first provide a high-level overview of carbon accounting, including key concepts used in the monitoring, reporting and verification of forest-based NCS. Second, we provide a high-level overview of forest carbon dynamics, including pools and fluxes, and drivers of their change. We then identify gaps in the current systems of GHG accounting, and between current ambitions and basic forest ecology. Improved use of data in models provides a path forward to better assessment and anticipation of forest-based climate mitigation. We illustrate this with the creation of a climate-sensitive forestry model, using tree-ring time series data. This climate-sensitive forest simulator will improve planning of site-level climate mitigation activities in the US by providing more realistic expectations of the carbon sequestration potential of forests undergoing climate change. Our review highlights the sobering complexity and uncertainty surrounding forest carbon dynamics, along with the need to improve carbon accounting. If we are to expect forests to play the significant emissions reduction role that is currently planned, we should view immediate emissions reductions as critical to preserve the climate mitigation capacity of forest ecosystems.