CORSIA is the emissions offset system for international aviation, established by ICAO. Since March 29, 2022, the systems allows for HFLD (High Forest Low Deforestation) credits to be accounted against actual flight emissions. A group of internationally renown experts recently revealed this as an accounting trick in a joint Carbon Pulse article.
The Global Conservation Standard is so far the only international system that has provide the methodology for an ecosystem services payment for commercially unproductive protected areas for the last 12 years. The resulting (CCU) Conservation Credit Units will not offset any atmospheric damage by the buyer. However, they can be bundled to carbon emission offset credits, thereby increasing their market value.