system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Donating $5 to offset my carbon footprint during air travel - what exactly is that $5 doing?

Some airline websites will ask you to donate money to offset your carbon footprint - where is that money going and how is it offsetting my carbon footprint?
gpt
It depends on the airline, but most likely they are using your extra $5 to buy carbon offset credits. How these credits work, differs from country to country, but the gist is this:

A company performs some sort of work that is meant to reduce greenhouse gas emissions. This might be planting trees, generating carbon-neutral energy, promoting energy efficiency, etc. This work creates carbon offset credits, which are audited and verified by some sort of authority. The authority be be a government, an industry watchdog, a reputable non-profit organization, etc. These credits are then sold on a market to either companies or individiuals.

So basically, you're helping pay for work that has already been done to reduce greenhouse gas emissions so that the savings you've paid for will assumed match the emissions that are generated as a rule of your travel.