1.5°C Rating Methodology
The CAT rates international aviation’s targeted emissions level in 2030 under its carbon neutral growth goal as ‘Critically insufficient’. We rate IATA’s target of net zero carbon by 2050 as ‘Highly insufficient’. The ‘Critically insufficient’ and ‘Highly insufficient’ ratings indicate that, if all other sectors were to follow the same approach, ICAO’s and IATA’s targets are consistent with warming of 4°C and greater than 4°C, respectively.
The rating methodology applied for international aviation and shipping policies is different from the CAT nationally determined contribution (NDC) rating system.
To rate international aviation and marine sectors we start with the observation that to achieve the Paris Agreement’s temperature goal, global CO2 emissions need to reach net-zero around mid-century (IPCC, 2018). If the aviation sector decarbonises later than mid-century, other sectors would need to reach net-zero carbon emissions sooner, and/or deployment of corresponding negative emission technologies will be needed.
To reflect that different sectors should share the decarbonisation burden, we applied the principle that aviation should decarbonise at the same average rate as emissions from energy and industrial processes, thereby taking a comparable share of emissions efforts. This implies that international aviation (together with all emissions from energy and industrial processes) should reach zero CO2 emissions by around 2060.
For this assessment, we have taken the global CO2 emission pathways for energy and industrial provided in Integrated Assessment Models (IAMs) and assessed in the IPCCSR1.5. We filtered these pathways to exclude those that exceed sustainability limits for Bioenergy with Carbon Capture and Storage (BECCS) and Agriculture, Forestry and Other Land Use (AFOLU), as defined in the IPCC SR1.5. This gave us 19 scenarios that are consistent with the Paris Agreement and that we define as “CAT Filtered Pathways” (Yanguas Parra et al., 2019). This is the same filtering approach that we used for developing the fair share ratings used in individual country assessments.
We have also extracted pathways that lead to a temperature increase between 1.5˚C–2˚C; between 2˚C–3 ˚C; between 3˚C–4˚C; and >4˚C from the IPCC IAM Database, so we have five sets of pathways that correspond to one temperature category each.
From each pathway set, we took the median emission levels between 2000 and 2100, with ten-year intervals. Using these median levels, we then calculated the annual growth rate for each temperature pathway and applied the growth rates to IEA’s estimate of international aviation emissions in 2015 (IEA, 2019). This approach differs slightly to that adopted for international shipping, where the growth rate was applied to the historical average for the period of 2007-2017 and is due to the fact that emissions in that sector have fluctuated greatly, whereas emissions from international aviation have increased steadily since 1990.
This exercise supplied us with five temperature pathways, leading to global temperature increases ranging from 1.5˚C to over 4˚C compared to pre-industrial levels. We have assigned a rating to each of these temperature categories: 1.5˚C Paris Agreement compatible, 2˚C compatible, correspond to those temperatures, while the Insufficient, Highly insufficient and Critically insufficient ratings correspond to warming of between 2–3˚C; between 3–4˚C; and >4˚C, respectively.
While we use the same terms as for the individual country assessments, the definition of the ‘insufficient’ rating is different. For countries, these equates to the top of their fair share range, while for international aviation and shipping this rate equates to a temperature range.
The 1.5˚C pathway reflects the maximum level of international aviation emissions for the sector to be rated as ‘Paris compatible’.
Note, we used the IEA’s estimate of 2017 international aviation emissions to develop the temperature pathways for our assessment in 2020 (IEA, 2019). To align the aviation assessment with the country assessments, we now base these pathways on the IEA’s 2015 emissions estimate. This change in the base year moved the temperature bars slightly upwards, but this has no implications for our rating of ICAO’s carbon neutral growth target.