ABSTRACTThe concept of “blue carbon” is, in this study, critically evaluated with respect to its definitions, measuring approaches, and time scales. Blue carbon deposited in ocean sediments can only counteract anthropogenic greenhouse gas (GHG) emissions if stored on a long‐term basis. The focus here is on the coastal blue carbon ecosystems (BCEs), mangrove forests, saltmarshes, and seagrass meadows due to their high primary production and large carbon stocks. Blue carbon sequestration in BCEs is typically estimated using either: 1. sediment carbon inventories combined with accretion rates or 2. carbon mass balance between input to and output from the sediment. The inventory approach is compromised by a lack of accurate accretion estimates over extended time periods. Hence, short‐term sedimentation assays cannot be reliably extrapolated to long timescales. The use of long‐term tracers like 210Pb, on the other hand, is invalid in most BCEs due to sediment mobility by bioturbation and other physical disturbances. While the mass balance approach provides reasonable short‐term (months) estimates, it often fails when extrapolated over longer time periods (> 100 years) due to climatic variations. Furthermore, many published budgets based on mass balance do not include all relevant carbon sources and sinks. Simulations of long‐term decomposition of mangrove, saltmarsh (Spartina sp.), and eelgrass (Zostera sp.) litter using a 3‐G exponential model indicate that current estimates of carbon sequestration based on the inventory and mass balance approaches are 3–18 times too high. Most published estimates of carbon sequestration in BCEs must therefore be considered overestimates. The climate mitigation potential of blue carbon in BCEs is also challenged by excess emissions of the GHG methane (CH4) and nitrous oxide (N2O) from biogenic structures in mangrove forests and saltmarsh sediments. Thus, in many cases, carbon sequestration into BCE sediments cannot keep pace with the simultaneous GHG emissions in CO2 equivalents.