Terrestrial ecosystems annually fix about 120 ± 7 Pg of carbon through photosynthesis, with forests being responsible for a large portion of this flux. 
After photosynthesis, this new carbon has different fates depending on allocation to plant parts and transfers to forest floor and soils. The time it takes carbon to pass through an ecosystem, since photosynthetic fixation until its final release as CO₂, is defined as the transit time of carbon. The transit time of carbon can be characterized by continuous probability distributions that indicate where carbon is allocated to, for how long it stays in a certain ecosystem compartment, to what other compartments it is transferred to, and for how long carbon is stored in organic forms before its return back to the atmosphere. 
In this presentation, I will show why the transit time distribution of carbon is an appropriate metric to answer the question of where does the carbon go? I will present estimates of the transit time distribution of carbon for two tropical forests and for the entire terrestrial biosphere, which indicate that on average carbon only stays stored in ecosystems for about one decade, and with about half of the fixed carbon respired in half a year. I will show a new approach to quantify the mean transit time of carbon based on measurements of radiocarbon in plant parts, soils, and respired CO₂; and will show results from a global carbon model that suggests that the transit time of carbon of the terrestrial biosphere is becoming younger, with faster dynamics in tropical and temperate forests, and emissions of older carbon from boreal and arctic regions.