Climate impacts from aviation 2020 Carbon Neutral Goal: stabilized emissions but increasing radiative forcing and temperatures
A new, and rather technical, paper has been produced by CATE, the Centre for Aviation Transport and the Environment, at Manchester Metropolitan University. They have looked at the climate impacts of future scenarios of aviation emissions. The ICAO process that is attempting to deal with aviation emissions only deals with international aviation, ie between countries. Not domestic aviation, which is flying within a country, and of which there is a great deal in countries like the US and China. CATE has produced various scenarios which show that even if the global aviation industry managed to achieve its stated aim of “Carbon Neutral Growth” after 2020, for domestic and international flights, there would be a continuing increase in both radiative forcing (ie. the difference of radiant energy received by the earth and energy radiated back to space) and global temperatures from the aviation emissions. This is because although 30% of CO2 emissions are removed in a few decades, some 50% is not removed for several centuries, and about 20% remain for millennia. This is why under overall climate (temperature) stabilisation scenarios, global CO2 emissions must be reduced dramatically.
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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.
26.9.2013 (CATE -MMU) CATE = Centre for Aviation Transport and the Environment, at Manchester Metropolitan University
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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’) (weblink to RF report), this new report answers these questions above. [ Radiative forcing is defined as the difference of radiant energy received by the earth and energy radiated back to space.]
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.
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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.
Click here to download hi-res figures from the report (as a single zip file).
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 CO2emissions 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).
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The report’s conclusion states:
Conclusions
• As the ICAO 2020 Carbon Neutral Growth goal applies to international aviation,
scenarios of global domestic emissions need to be considered in order to
calculate total climate impacts in terms of radiative forcing and temperature
response for total aviation. Such scenarios have been formulated as a range
between domestic ‘business as usual’ emissions, and ‘maximum feasible
reductions’ emissions from technology and operational improvements, coupled
with “speculative” levels of biofuels. These combined scenarios of CNG2020
international emissions and S2 or S5 plus biofuel domestic scenarios, result in
reductions in radiative forcing over the baseline ‘business as usual’ scenario of
~21.5% (range 16.3 to 25.4%) or ~26% (range 20.5 to 30.5%) by 2050.
• A combined domestic and international aviation Carbon Neutral Growth
scenario, i.e. stabilized domestic plus international emissions from 2020 to 2050
does not result in stabilized CO2 radiative forcing; rather, as expected, the
radiative forcing never stabilizes but constantly increases, as CO2 accumulates
faster in the atmosphere than it is removed. This is also clearly illustrated in a
hypothetical simulation of constant 2020 emissions to 2500, in which the
radiative forcing increases by nearly a factor of 7 over this timescale from the
constant emission rate.
• Considering the potential achievement of the ICAO 2020 Carbon Neutral Growth
Goal on international aviation alone, this would result in reductions of aviation
CO2 radiative forcing of ~21% (overall range 16 to 25.3%, depending upon
growth scenario) by 2050 over a baseline ‘business-as-usual’ scenario. The way
in which the CNG2020 goal might be achieved is not prescribed, but by
comparison with other previous work with scenarios of mitigation, it is apparent
that in order to achieve the CNG2020 goal, a substantial fraction of the emissions
reductions would need to come from MBMs such as emission trading or carbon
offsetting in addition to the necessary reductions from technology and
operational improvements and low-carbon alternative fuels to fossil-fuel
aviation kerosene.
• It is noted that whilst a constant stabilized emission rate from 2020 would be a
remarkable achievement for the aviation sector, the overall requirements across
all sectors, together, to stabilize global mean surface temperatures to an increase
of 2 °C or less, over pre-industrial values by 2100 requires large reductions in
global CO2 emissions after peaking around 2020. In such a case, the IATA goal of
halving 2005 emissions by 2050 is more in line with keeping climate impacts
from aviation within the 2 °C goal, than aviation emissions stabilization from
2020
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