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Climate impacts from aviation 2020 Carbon Neutral Goal: stabilized emissions but increasing radiative forcing and temperatures

Click here for the full report.

The International Civil Aviation Organization (ICAO) has set itself a goal to:

to strive to achieve a collective medium term global aspirational goal of keeping the global net carbon emissions from international aviation from 2020 at the same level…”

How might the emissions reductions required to stabilize emissions at 2020 levels be achieved?  What does achieving this goal mean in terms of climate impacts? If international aviation emissions are stabilized at 2020 levels and don’t grow further, will the climate impact also stabilize?

Building on two previous reports, which examined whether projected aviation emissions will meet such goals and quantified the ‘climate impacts’ of various proposed mitigation measures (using the accepted science metric ‘radiative forcing’), this new report answers these questions above.

How would aviation CO2 emissions be stabilized at 2020 levels? ICAO is still discussing how this might be achieved, given that aviation CO2 emissions are projected to grow strongly to 2050 under a business as usual scenario. It is envisaged that a basket of measures including technological and operational improvements, alternative lower-carbon intensity fuels, and market-based measures (such as emissions trading and offsetting) will be needed to achieve the ‘2020 Carbon Neutral Growth’ emissions stabilization from 2020 to 2050. ICAO only has a mandate for international aviation emissions of CO2, some 62% of the total, yet consideration is still required of what may happen to global ‘domestic’ emissions of aviation CO2.

We formulate 3 scenarios of ‘2020 Carbon Neutral Growth’ (CNG2020) and analyse them for their comparative impacts on climate through radiative forcing, and temperature response using sophisticated calculations and nearly 2,000 model simulations.

  • Scenario 1 includes stabilization of international aviation CO2 emissions from 2020 to 2050 (CNG2020), and business as usual growth of domestic emissions (‘S2’ BAU technological and operational improvements)
  • Scenario 2 includes stabilization of international aviation CO2 emissions from 2020 to 2050 (CNG2020), and growth of domestic emissions applying ‘maximum feasible reductions’ from technological and operational improvements plus speculative levels of biofuels (‘S5’ MFR + biofuels)
  • Scenario 3 is a world in which both global international and domestic emissions of aviation CO2 are stabilized at 2020 levels through to 2050.

The resulting reductions in radiative forcing from aviation CO2 over the S2 business as usual baseline for international and total emissions are shown in the animations below.

Figure 1. Effect of CNG2020 international mitigation on CO2 RF to 2050 attributable to international aviation (upper panel), and total aviation (lower panel). The central aviation growth scenario with the RCP8.5 background scenario has been used, for the purposes of illustration.


What the simulations show is that, as might be expected from ‘textbook’ climate science, even if all the aviation CO2 emissions stabilize, the radiative forcing and temperature responses never stabilize but inexorably increase as shown below. This is because CO2 emissions accumulate faster than they can be removed, since although 30% of CO2 emissions are removed in a few decades, a very large fraction of CO2 emissions (50%) are not removed for several centuries, and the remaining 20% remain for millennia. This is the reason why under overall climate (temperature) stabilization scenarios, global CO2 emissions must be reduced dramatically.

Figure 2.Total global G-CNG2020 emissions (i.e. international and domestic at 2020 levels), and the absolute CO2 radiative forcings from total global aviation in mW m-2 for scenario 3. The central aviation growth scenario with the RCP3-PD background scenario has been used, for the purposes of illustration.

Moreover, comparing the radiative forcing impacts from CNG2020 international emissions with a previous assessment of mitigation measures from technological and operational improvements, and use of biofuels, we show that these measures do not appear to provide enough CO2 mitigation by 2020, which means other policy measures must be introduced, such as emissions trading or quality offsetting, in order to at least stabilize aviation CO2 emissions by 2020. Such market-based mechanisms have the advantage of being able to reduce emissions quickly, an important aspect when considering ‘climate impact” as highlighted by our previous report.

Click here to download hi-res figures from the report (as a single zip file).

(Airplane image on the Projects’ page courtesy of Creative Commons)