[berkes]

If you fly from Rome to Barcelona, you could "offset" the CO2; often directly when buying that ticket.

This works in two ways: 1. they "plant trees" and/or 2. they buy && hold or destroy certificates.

Certificates are limited regulated and deflationary. E.g. the EU buys X certificates from the market every year and destroys them and grants less certs each year. Every company that emits "significant" CO2 needs to have, buy or be granted certs to do so. The setup makes those certs more expensive, so every year, there's a tradeoff: do I buy certs, or do I invest in lowering my emission.

Apps such as Fairshares allow public to buy (pieces of) such certs.