# Corpsia--A Feasible Second Best Solution?
## 2 Institutional and Technical Background
### Scope of CORSIA and Eligible Offsets  
In October 2016, CORSIA was agreed at the 39 th ICAO Assembly after decades of difficult international negotiations (Assembly Resolution A39-3). CORSIA aims at supporting ICAO's Carbon Neutral Growth (CNG) goal from 2021 onwards. CORSIA is a baseline-and-credit scheme. The baseline above which emissions have to be compensated or abated was initially defined as the average total CO\({}_{2}\) emissions of all international flights under the scheme in the years 2019 and 2020, respectively. To reflect the COVID-19-related decline in 2020, this was changed in the meantime to 2019 only. CORSIA is the first global scheme for the limitation of aviation CO\({}_{2}\) emissions. However, criticism has been raised concerning CORSIA's environmental ambitiousness, which we discuss below. CORSIA will capture an increasing number of participating states over time. In the medium term, roughly 90 percent of civil international air traffic will be included. CORSIA is a baseline-and-credit scheme, requiring offsets for all CO\({}_{2}\) emissions exceeding the 2019 emission baseline. Of course, a number of exemptions apply. For instance, CORSIA only applies to CO\({}_{2}\) emissions from international flights, as ICAO is the UN Agency for international civil air transport while domestic operations are fully under responsibility of the individual states. Other exemptions refer to emissions from small aircraft with an MTOM below 5.7 tons, small emitters emitting less than 10,000 tons of CO\({}_{2}\) annually, and humanitarian, medical, and firefighting flights. As ICAO is responsible for civil operations only, military and governmental flights are completely excluded. However, monitoring, reporting, and verification (MRV) of CO\({}_{2}\) emissions under CORSIA is mandatory for all ICAO contracting states irrespective of any voluntary or mandatory participation. MRV obligations started in 2019. CORSIA consists of three phases: Pilot Phase (from 2021-2023), Phase 1 (2024-2026), and Phase 2 (2027-2035). Whereas the Pilot Phase and Phase 1 are voluntary, participation in Phase 2 is mandatory for all states with airlines accounting for more than 0.5 of global international RTK in the year 2018. Only least-developed countries, land-locked developing countries, and small island nations are exempted from this obligation. Of course, voluntary participation is possible for any ICAO contracting state. Offset requirements are defined by the routes flown. Only routes between participating states (\"CORSIA States\") are subject to these offsetting requirements. Figure 1 summarizes CORSIA's main characteristics.  
In March 2019, the ICAO Council adopted Emission Unit Eligibility Criteria (EUCs) which specify the requirements for carbon offset credits to be eligible under CORSIA. Carbon-offsetting programs have to fulfil these requirements and need to be approved by the ICAO Council as eligible programs. Currently, six offset program standards are eligible: Clean Development Mechanism, Gold Standard, VERRA, American Carbon Registry (ACR), Climate Action Reserve (CAR), and China Greenhouse Gas Voluntary Emission Reduction Program (GHG). Among these programs, offsets are eligible from projects that started their first crediting period from 1 January 2016 and in respect of emissions reductions that occurred through 31 December 2020. Certain program specific offsets have been declared as non-eligible [11].