Buying an offset at checkout produces a receipt and a number. Understanding what has actually been purchased requires following the chain from that payment to a physical change in the atmosphere.
The unit is a tonne, produced by a project
An offset credit represents one tonne of carbon dioxide either kept out of the atmosphere or removed from it by a specific project at a specific place.
Projects fall into broad families: renewable energy replacing fossil generation, protection of forest that would otherwise be cleared, tree planting, and methane capture from landfill or agriculture.
A verifier assesses the project against a methodology and issues credits, which are then sold, transferred and eventually retired so they cannot be resold.
Additionality is the central test
A credit is only meaningful if the emissions reduction would not have occurred without the offset revenue. This is the additionality requirement, and it is the hardest to prove.
The proof involves a counterfactual: what would have happened otherwise. That baseline cannot be observed, only estimated, and the estimate is made by parties with an interest in the outcome.
Where a project was already commercially viable, the credits it issues represent no change at all, and the buyer's emissions remain entirely unoffset.
Permanence and leakage undermine forest credits
Carbon stored in trees is permanent only while the trees stand. Fire, disease, drought or a change of ownership can release it decades later.
Schemes address this with buffer pools holding back a share of credits as insurance, though the reserve has to be sized against risks that are increasing.
Leakage is the second problem. Protecting one forest area can push logging or clearing into the next valley, which moves the emissions rather than preventing them.
Double counting is an accounting failure
If the host country counts a reduction toward its own national target and a traveller also claims it, the same tonne has been used twice.
Corresponding adjustment mechanisms exist to prevent this by requiring the host country to remove the tonne from its own account when the credit is exported.
Voluntary market credits do not always carry such an adjustment, which is why the same project can produce credits of quite different quality.
Where offsets sit in the order of operations
Offsetting cannot substitute for a flight not taken, because a claimed reduction elsewhere carries uncertainty that an avoided emission does not.
The defensible sequence is to reduce first, shift mode where possible, and offset only the remainder that cannot be avoided.
Removal credits, which physically take carbon out of the air and store it durably, sit on firmer ground than avoidance credits, and their prices reflect the difference.