Transient tracers are a group of (chemical) compounds that can be used in the ocean to quantify ventilation, transit time distribution and transport time-scales. These compounds are all conservative in sea-water, or have well-defined decay-functions, and a well-established source function over time at the ocean surface. Measurement of transient tracers in the interior ocean thus provides information on the time-scales since the ocean was ventilated, i.e. in contact with the atmosphere. Knowledge of the transit time distribution (TTD) of a water-mass allows for inference of the concentrations or fates of other transient compounds, such as anthropogenic carbon or nitrous oxide. Commonly measured transient tracers are the chlorofluorocarbons (CFCs) 11 and 12, although in the past also CFC-113 and CCl4 have been measured. More recently also the related compound sulphur hexafluoride (SF6) is regularly measured since it provides information on ventilation of the fast ventilated parts of the ocean. 

Knowledge about ventilation is key to quantify the transport of (climate) perturbations from the surface to interior ocean. For instance, the ocean is storing about 95% of anthropogenic global warming (heat content) and about 30% of anthropogenic carbon. A system of global sustained observations of interior ocean transient tracer observations is an important component of the global climate observing system. 

Here I will present some basic concepts of transient tracers, and put that in a global context with focus on societal benefits.