International aviation and the Paris Agreement temperature goals

Lee, David (2018) International aviation and the Paris Agreement temperature goals. UNSPECIFIED. Department for Transport.

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Abstract

Long-term goals for carbon dioxide (CO2) emissions from total and international global aviation are considered here in the context of the Paris Agreement, which sets out a goal to hold increases in global mean surface temperature to well below 2°C above pre-industrial levels by 2100 and to pursue efforts to limit this increase to 1.5°C. In order to put this into practice, a scientifically-based ‘cumulative carbon budget’ approach is being taken, whereby the cumulative anthropogenic CO2 released scales with the global mean surface temperature response, as shown by the Intergovernmental Panel on Climate Change (IPCC) and others. For the Paris Agreement’s goals to be met, large reductions in global greenhouse gas emissions are required. International aviation emissions of CO2 (~65% of the current total from aviation) fall to the International Civil Aviation Organization (ICAO), whereas domestic emissions (~35% of the current total from aviation) come under states’ Nationally Determined Contributions (NDCs). ICAO has set a ‘Carbon Neutral Growth’ goal (CNG2020) for international aviation emissions of CO2 not to increase over 2020 levels, which it aims to achieve largely by offsetting. It is shown here that even if international aviation CO2 emissions are offset above 2020 levels, projected cumulative emissions of CO2 from international aviation between 2016 and 2050 will consume between ~3% and 10% of the cumulative global CO2 budget, as determined by background scenarios that limit global mean temperature increases to less than 2°C, or between ~4% and 15% for the sector as a whole. For scenarios closer to 1.5°C, aviation’s consumption of the CO2 budget would be a larger fraction. The Paris Agreement is a temperature-based target and therefore implies inclusion of all emissions that affect climate. Aviation has significant non-CO2 climate impacts from oxides of nitrogen (NOx), particle emissions, and effects on cloudiness that overall cause additional overall warming but these impacts are subject to greater scientific uncertainty than its CO2 impacts. Examples of CO2 emission-equivalents metrics indicate up to a doubling of aviation CO2 equivalent emissions to account for these non-CO2 effects. In order to limit temperatures to less than 2°C by 2100, global CO2 emissions will be required to go to zero around 2075; for 1.5°C scenarios, emissions will be required to go to zero around 2050. In both cases, further deeper emission cuts past the dates of zero emissions to ‘negative emissions’ are necessary – i.e. CO2 removal from the atmosphere. Therefore, for 1.5°C global scenarios, any continued emissions of CO2 from aviation using fossil fuels beyond around 2050 will be inconsistent with the Paris Agreement goals in the absence of extra measures, or alternatively, correspondingly increased negative emissions. Negative emissions technologies required to limit global mean surface temperatures are required as early as around 2035 but reliance on them to compensate for continued fossil fuel emissions from aviation would be risky. In light of this, revised goals for international aviation CO2 emissions over and above ICAO’s CNG2020 goal should be considered