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Aircraft fuselage low angle shot.
Photo: Flightlevel80/E+/Getty Images
INTERNATIONAL POLICY
8 August 2016 6:35

Analysis: Aviation could consume a quarter of 1.5C carbon budget by 2050

Carbon Brief Staff

Multiple Authors

08.08.16
International policyAnalysis: Aviation could consume a quarter of 1.5C carbon budget by 2050

The aviation industry faces huge challenges if it is to meet its own self-imposed climate change targets, according to a new UN report.

And even if it does meet all its targets, aviation will still have consumed 12% of the global carbon budget for 1.5C by 2050, Carbon Brief analysis shows. If it fails to reach this target, its share of this budget could rise to as much as 27%.

The sector has an aspirational goal to cap its emissions at 2020 levels, so that any growth after this year is achieved in a “carbon neutral” way.

This will not be easy. Airlines estimate that air travel will grow by an average of just under 5% per year up to 2034 — and the emissions from these extra air miles will be difficult to decarbonise.

Technology improvements

The International Civil Aviation Authority (ICAO), the UN body for flying, has set out the difficulties of cutting emissions in a growing sector in its 2016 Environmental Report, which is released every three years.

The UN report forecasts the greenhouse gas emissions of the aviation sector out to 2050, and looks at how they could be reduced. The following chart shows the extent to which improvements to the aviation sector itself could help to cut emissions.

Fig 5. Aircraft CO2 emissions from International Aviation, 2005-2050

Aircraft CO2 emissions from International Aviation, 2005-2050. Updated to include alternative fuels life cycle emissions reductions. Dashed line represents effect on net CO2 emissions association with the 100% replacement of jet fuel with alternatives by 2050. 100% replacement of alternative jet fuel would requite a complete shifting in aviation from petroleum refining to biofuel production and a substantial expansion of the agricultural sector, both of which would require substantial policy support. Illustrative case would require high availability of bioenergy feedstocks, the production of which is significantly incentivised by price or other policy mechanisms. Source:   ICAO 2016 Environmental Report

Under a business-as-usual projection, where the aviation industry grows by 5% per year, but there are no major changes to technology or infrastructure, total emissions over the period 2015-2050 reach approximately 56bn tonnes of CO2 (area under dark blue line).

The rate of emissions in 2050 has been revised downwards from ICAO’s previous environmental report three years ago, but it appears that they have also scaled down the potential for technological improvements to the industry.

Carbon Brief asked Dr Joeri Rogelj, one of the world’s leading experts on carbon budgets, to express the latest projections in terms of the internationally agreed climate targets adopted in Paris last year — to keep global temperature rise to “well below 2C”, with an additional aspirational target of 1.5C.

His calculations suggest aviation emissions between 2015 to 2050 will consume 27% of the remaining carbon budget to have a decent chance of keeping global temperature rise below 1.5C above preindustrial levels. The share of the remaining budget for 2C is smaller, at 7%, but still significant.

Infographic: Aviation's share of the 1.5C budget

Infographic: Aviation’s share of the 1.5C budget. By Rosamund Pearce for Carbon Brief.

This gives a new perspective to the oft-repeated claim that aviation is responsible for 2% of global emissions — a claim repeated in the ICAO report and one the sector has been stressing since the early 1990s. While it is true that aviation may be a small slice of a large pie at the moment, as other sectors seek to reduce their emissions in line with the carbon budgets, aviation will come to occupy an increasingly large share, if it continues to grow.

It’s worth noting that ICAO’s projections only go as far as 2050. Any emissions after mid-century will further shrink the remaining carbon budget.

Carbon dioxide is also not the only way aviation affects the climate. Aircraft emit other gases and aerosols that change the composition of the atmosphere. They also produce “contrails”, which affect the cloudiness of the sky and how much solar radiation reaches the surface of the Earth.

The extent to which these extra factors amplify the CO2 effect is still poorly understood and not captured in ICAO’s estimates of aviation’s impact on the climate. With new studies emerging in recent years, the report says a new scientific assessment of aviation’s impact on the climate is now needed. The “state of the science” section of the report says:

While CO2 is particularly understood, there are important uncertainties regarding some of the non-CO2 impacts and the underlying physical processes which require further investigation…Our understanding and confidence in aviation climate forcings would be enhanced by a new international scientific assessment.

Even the most optimistic scenario cannot hope to reverse the growth in CO2 from aeroplanes, ICAO’s data suggests. Dramatic improvements in aircraft technology (light blue shading) in addition to air traffic management and infrastructure (yellow shading) will only succeed in making the steep upwards curve of aviation emissions slightly less steep.

Assuming all these potential improvements materialise, total emissions from aviation by 2050 drop to approximately 41bn tonnes of CO2. Rogelj calculates that this means the industry’s share of the remaining carbon budget for 1.5C would fall to 20%.  

This means that there would still be a shortfall of 1,039 million tonnes of CO2 (MtCO2) in 2050 between what is technologically feasible and what is required for aviation to hit its emissions goal of pinning emissions at 2020 levels.

But even this self-imposed goal doesn’t go far enough, says Rogelj:

On the one hand, it is encouraging to see that the sector is thinking about ways to significantly limit its CO2 emissions. On the other hand, it is worrying to see no mention whatsoever of the idea that also in this sector net-zero CO2 emissions need to be achieved for stabilising climate change.

Biofuels

So how can this gap be filled? The scenario outlined above assumes that each kilogram of jet fuel burned generates 3.16kg of CO2.

But this can be reduced by adding biofuels into the mix, the report claims. It looks at how much can feasibly be added, and how far this takes growth of the aviation industry to “carbon neutrality” after 2020, assuming that technology and other emissions-saving measures have already advanced to the most optimistic levels.

It does this for both the short term and the long term — 2020 and 2050.

Its 2020 scenario for the volume of biofuel available depends upon the production plans of producers and states. Its 2050 projections are more speculative. They look at constraints to biofuel use, including various environmental and socio-economic factors, how much biomass could actually be produced, and how much fuel can be extracted from this.

Their calculations also take into account the fact that making biofuel is not a carbon-neutral process, and include all the emissions released during its production.

The graph below suggests that biofuels can, at a pinch, ensure that emissions do not exceed 2020 levels, and could potentially enable them to dip slightly below.

Figure 3 CO2 emissions trends from International Aviation, 2005 to 2050

CO2 emissions trends from International Aviation, 2005 to 2050. Dashed line in technology contribution sliver represents the “Low Aircraft Technology Scenario”. Note: results were modelled for 2005, 2006, 2010, 2020, 2025, 2030, and 2040, and then extrapolated to 2050. *Actual carbon neutral line is within this range. Source: ICAO 2016 Environmental Report

However, there are important caveats to ICAO’s analysis.

First, only the most optimistic end of the range of potential emissions reductions due to biofuels takes aviation into the realm of carbon neutral growth after 2020. This is in addition to already optimistic scenarios for technology improvements between now and 2050.

This implies that over the next three decades, both technological advancements and biofuel production have to go as well as they possibly can in order for aviation to reach its goals through these measures alone.

Even in this most optimistic vision, where technology and biofuel production advance enough to allow the aviation industry to meet its self-imposed target, the industry would still consume 12% of the remaining budget for 1.5C by the middle of the century. Rogelj says:

It is clear that low-carbon aviation is still in its infancy. But even the ultimate aspiration of the sector (carbon-neutral growth) is off the mark and out of tune with what is required to stabilise the climate system.

The upper end of the range indicates that conventional jet fuel is 100% replaced by biofuels. This itself has significant implications for policy and other areas that are hoping to reduce their emissions by switching to biofuels.

The report notes that “100% replacement of alternative jet fuel [biofuel] would require a complete shift in aviation from petroleum refining to biofuel production and a substantial expansion of the agriculture sector, both of which would require substantial policy support”.

This would be expensive. The report estimates that complete replacement by 2050 would require around 170 new large biorefineries to be built every year from 2020 to 2050, at a cost of $15bn to $60bn per year.

But this relies on progress happening straight away and in a linear fashion. If progress is initially slow, and then picks up after 2035, then 328 large biorefineries would have to be built each year from then on, at an approximate cost of $29bn to $115bn per year.

It also emphasises the other difficulties of this achievement, including the fact that it would limit the amount of bioenergy available for other uses. The report says:

Achieving this level of emissions reduction would also require the realization of the highest assumed increases in agricultural productivity, highest availability of land for feedstock cultivation, highest residue removal rates, highest conversion efficiency improvements, largest reductions in the GHG emissions of utilities, as well as a strong market or policy emphasis on bioenergy in general, and alternative aviation fuel in particular. This implies that a large share of the globally available bioenergy resource would be devoted to producing aviation fuel, as opposed to other uses.

The scaling up of biomass is already a controversial topic, due to concerns over land grabs and food security, for instance. The new idea of using bioenergy with carbon capture and storage (BECCS) to absorb carbon dioxide from the air to bring about “negative emissions” could place further pressure on the world’s available biomass resources.

The report also adds that carbon-neutral growth purely through the use of alternative fuels is unlikely to occur by 2021, as their production would need a period in which to be ramped up before they reach the levels required.

Market-based mechanisms

The report acknowledges that technological progress and biofuels are unlikely to deliver the scale of improvements necessary. To bolster aviation’s chances of hitting its carbon-neutral growth target, the sector also hopes to agree on a market-based measure that would see them paying to offset their emissions.

The industry would need to offset between 443m and 596m tonnes of CO2 in 2035, and this only includes international flights, which currently account for only 65% of aviation emissions. This would cost between $5.3bn and $23.9bn in 2035, depending on the assumed carbon price. The higher figure would diminish total revenues from international aviation by only 1.4% in 2035.

A separate paper, released today by the Campaign for Better Transport, suggests that the best way to reduce aviation emissions is simply to curtail growth, and that this could be done by adding a surcharge linked to the flight’s total emissions. Leo Murray, co-author of the report, told Carbon Brief:

The ICAO biofuels scenarios are clearly fanciful at best, while the proposed market-based mechanism is so weak as to bear almost no relationship to a 2C target, let alone 1.5C…Fundamentally, what all these industry-led analyses refuse to acknowledge or countenance is the very evident truth that the only reliable means by which to control aviation emissions is to manage — ie curtail — demand growth. That’s the point of our frequent flyer levy proposal; it’s an equitable method of doing that.

It is worth noting that only 5% of the world’s population have ever travelled in an aeroplane, according to transport campaigners Atmosfair.

The ICAO Council is expected to make a recommendation for how a market-based scheme could work in October, during the body’s 39th Assembly, which countries can then decide whether to adopt.

This decision will come to represent the official level of ambition of the aviation sector when it comes to climate change, since its emissions were not covered in the UN Paris Agreement. However, there have recently been reports that the aviation deal agreed could initially be voluntary for certain countries.

*Projections of international aviation emissions based on graphs from ICAO Environmental report 2016. Carbon Brief has requested the underlying data from ICAO, but has yet to receive this.

Main image: Aircraft fuselage. Photo: Flightlevel80/E+/Getty Images.
Sharelines from this story
  • Analysis: Aviation could consume a quarter of 1.5C carbon budget by 2050
  • GeoffBeacon

    It is almost two years since Carbon Brief calculated that “Six years worth of current emissions would blow the carbon budget for 1.5 degrees” (http://ow.ly/Cscn3031xQR ). Calculating the effects of air travel until 2050 implies a timescale that is far too optimistic as the remaining carbon budget for 1.5°C is now four years.

    I am trying to get to grips with the remaining carbon budget for 1.5°C and 2.°C (I have tried here http://ow.ly/MNVU3031xm8 ) so can I ask a few questions?

    1. Are these calculations for CO2 only – excluding other climate forcing agents?

    2. What is the current status of the “radiative forcing index“,, which some estimate roughly double the impact of air travel.

    3. There are climate feedbacks missing from the models that calculated the remaining budgets – e.g. permafrost emissions, forest fires and wetlands decomposition (http://ow.ly/6ykr3031xCj ). Have any kicked in yet? I note that measured emissions plateaued last year but CO2 concentrations accelerated. Is this a sign of feedback effects or does a measurement lag or the El Nino completely explain the difference?)

    • Robert McSweeney

      Hi Geoff, a few quick responses:

      1. The calculations are based on the threshold exceedance budgets (TEBs) in the latest IPCC report. Have a look at the explanation of TEBs in the second half of this article: https://www.carbonbrief.org/analysis-only-five-years-left-before-one-point-five-c-budget-is-blown

      2. See here for info on the RFI: http://www.co2offsetresearch.org/aviation/RFI.html

      3. Off the top of my head, I’m not aware of any research saying these feedbacks are in action (but that’s not say there isn’t any!). The Met Office has quite a nice summary of feedbacks here: http://www.metoffice.gov.uk/climate-change/guide/science/explained/feedbacks

      • GeoffBeacon

        Thanks Robert

        In reverse order as No 1 looks the most interesting.

        3. The Met Office summary leaves too many questions. (e.g. where are forest fires and wetland decomposition). Seems rather conservative stuff. DECC gave a better answer (http://www.brusselsblog.co.uk/carbon-budgets-a-straightforward-answer-from-decc/ ). I’m now a bit suspicious of the Met Office because it’s now sponsored by the Business department (http://www.brusselsblog.co.uk/will-the-new-government-department-sideline-new-climate-warnings/
        )

        2. I have seen that piece on RFI before and wasn’t too impressed. It says “RFI is an inappropriate metric to use for personal air travel emissions calculators”

        The question I looked to RFI to answer is “Does aircraft travel cause more global warming than it’s emissions of the Kyoto basket of greenhouse gasses?”. If not RFI, how should any extra effect accounted for?

        How is the paper by Borken-Kleefeld et al, “Specific Climate Impact of Passenger and Freight Transport“ now regarded. (http://pubs.acs.org/doi/pdf/10.1021/es9039693 ). It says

        “… the transport specific climate impact is lowest for rail and bus travel and highest for car and air travel. At long time horizons … the transport specific climate impact of car travel is larger than air travel on global average… Both are then about three times …

        higher than the impact from bus and rail travel. On short time scales however, the transport specific climate impact from aviation is strongly enhanced, while rail’s impact is reduced… Air travel’s specific climate impact becomes four times higher than the impact from car travel per passenger-kilometer at 5 years time horizon.”

        1. That Carbon Brief article is very helpful (https://www.carbonbrief.org/analysis-only-five-years-left-before-one-point-five-c-budget-is-blown ) but it has left me with one immediate question concerning how to estimate the number of years before a carbon budget is exhausted. It says

        “Scientists run the model until global temperature rise crosses a given threshold – say 1.5C. They then work out the cumulative CO2 in the atmosphere at that point – and this is the carbon budget.”

        Isn’t it true that a significant proportion of emissions are absorbed? If so what proportion of global emissions should be subtracted from the remaining carbon budget.

        • Robert McSweeney

          Hi again Geoff. A few more points…

          3. NB, the Met Office was already part of the Business department (or, strictly, the “Department for Business, Innovation and Skills”) before DECC was subsumed to create the new BEIS department. It’s been there since it was moved out of the Ministry of Defence in 2011 (http://www.metoffice.gov.uk/about-us/who/management). I don’t think the changes affect how trustworthy the Met Office is! One potential impact is on funding as the Met Office Hadley Centre received some of its funding from DECC (as well as Defra), so not sure what will happen with that now.

          2. Here’s some guidance on the RFI from the Aviation Environment Foundation: http://www.aef.org.uk/issues/climate/. They suggest (with caveats) using 1.9, which is also used by DECC for reporting greenhouse gas emissions. If you download DECC’s “Conversion factors 2016 – Full set (for advanced users) spreadsheet” (https://www.gov.uk/government/publications/greenhouse-gas-reporting-conversion-factors-2016), you’ll find details under the “Business Travel – Air” tab. For something a bit more in-depth, here’s a paper by Prof Keith Shine at the University of Reading on aviation, radiative forcing and climate change (though I haven’t read it, so not sure how much help it will be!): http://centaur.reading.ac.uk/40552/1/aea526_pub2_submitted.pdf

          1. Regarding carbon budgets, the climate models on which they’re based take into account the CO2 that the land, oceans, etc take up (the wording in my article could have been clearer). So the remaining budget is simply the cumulative CO2 emissions at the point that the temperature threshold is crossed, minus total emissions to date. To get the number of years left before the budget is used up, just divide the remaining budget by current annual emissions. This is what we did in our analysis in order to create our infographic (at the link I sent in earlier comment).

          • GeoffBeacon

            Thanks again Robert

            3. Although I did know the Met Office was sponsored by BIS, I only discover it last year.

            In departmental terms, the climate brief has passed from DEFRA, through DECC to this new business department. I read it like this:

            a) DEFRA worried about climate change too much. That was when Bob Watson was Chief Scientist.

            b) DECC took over the climate brief so that the energy lobby could keep climate policy under control. I saw DECC as the Department of big Energy and little Climate Change. (e.g. Guardian stuff here: http://ow.ly/WC6O3034IjU )

            c) Climate change was still too high profile for business interests so DECC is scrapped and wound into a new department which is even more business friendly. “Will the new government department sideline climate warnings?” (http://ow.ly/JR563034Kil ). I suppose that remains to be seen.

            However, I didn’t completely trust the Met Office before. There are brave outspoken people at the Met Office. (e.g. Julia Slingo. See ” The Met Office doubts the IPCC”, http://ow.ly/FtqR3034LIl ). However, other experiences have given me the sense that there is a tendency for the Met Office to do the government’s bidding.

            2. Thanks for that.

            3. Thanks for that too.

            I’m still not happy that there is no reasonable guess at what the feedbacks might do. (Actually some best guesses would be helpful to policy makers – or those of us trying to shout in their ears – even if the guesses aren’t very good)

            I’m also not happy that there is so little information that is widely known about how the things we do in our everyday life impact the carbon budgets.

            Carbon Brief and yourself are a great help. One of the few places that I can find information accessible enough for me to follow without days or weeks of effort and get explanations. Explanations which are succinct enough to use when I manage to track down a politician or real policy maker.

  • Ace Otana

    1.5C by 2050? What planet are you from? We’ll reach and pass 2C by 2020, and that’s optimistic! By 2030 I imagine we’ll be beyond 6C, and even that might be optimistic…

    • Robert McSweeney

      Hi Ace, thanks for your comment. The article is using the 1.5C budget as a way of illustrating how aviation emissions will become an increasingly large portion of our total emissions. It’s not commenting specifically on when the budget will run out – for that, see our latest analysis, which indicates that there are less than five years left: https://www.carbonbrief.org/analysis-only-five-years-left-before-one-point-five-c-budget-is-blown

      • Ace Otana

        Thanks for your response. Wasn’t expecting that.
        I must point out that we don’t have a carbon budget. 4C is already cooked in regardless of what we do. Which means we have an enormous carbon debt.
        We’re looking at reaching and passing 2C before 2020. We’re also looking at 6C to 10C by 2030. When you take feedbacks into account as well as global dimming which is suppressing around 2.5 and 3C of global warming, it’s clear there is no carbon budget.

        The moment we cut or stop emissions we lose global dimming, and global dimming is caused be sulphate particles that come from burning fossil fuels. The CO2 warms the planet by trapping heat, aerosols cool it by reducing the amount of solar energy reaching the surface of the earth. The moment you stop emissions, we’d lose those aerosols (global dimming) but global warming would continue, seeing as CO2 can remain in the atmosphere for centuries. Aerosols comes straight out of the sky shortly after being emitted. Once global dimming is gone, temps would rise sharply by about 3C. We’re looking at climate collapse around early to mid 2020’s…

  • C Horn

    The only graph is of growth in travel. I suspect their is a very high probability that air travel is going to decline by 2050. Very much sooner than I think people realize we are going to be running a carbon neutral civilization by design of man or consequence of nature. Consequence of nature maybe not be called a civilization could be more like extinction or the stone age.

  • Mark Roest

    There was zero discussion of the very large amount of R&D activity regarding battery-supplied, electric-motor-powered aircraft, on both sides of the Atlantic. I know that Airbus is involved, lots of light plane manufacturers are into it, and it’s possible that Boeing is also quietly hedging its bets. The Economist had a cover article on it a few months back, which was quite good. We are seeing huge improvements in aerodynamic efficiency from placing many small electric propellers along the front of the wings, and using them to direct the airflow over the wings. Some designs have most of the propellers fold up during cruising flight, to save power and minimize drag.

    As with Battery Electric Vehicles, the switch to electric aircraft is dependent on improvements in batteries. People write that today we have an effective flying time of about half an hour, in order to have 45 minutes emergency reserve. The performance that would at least support short-haul commuter aircraft is variously pegged at 400 to 600 Wh per kg. While this is not yet available, it is far closer than most people think.

    Professor Cui at Stanford University is a current star, and said in a public lecture recently that he is getting 300 Wh/kg, with a near-term target of 330 [or] 350 Wh/kg. He mentioned that some other work could lead to 700 Wh/kg mid-term.

    The battery start-up I am in is currently seeing 550 Watts/kg at the cell level. Our modeling points to 600 Wh/kg within a year or two, and continued progress from there, as we go up the learning curve, past 900 Wh/kg.

    The potential is great, and the possibilities are enormous.

  • Paul Barry

    I’m just trying to get my head around some of the numbers quoted above.

    You say
    “Under a business-as-usual projection, where the aviation industry grows by 5% per year, but there are no major changes to technology or infrastructure, total emissions over the period 2015-2050 reach approximately 56bn tonnes of CO2 (area under dark blue line).”

    Just looking at the first ICAO graph (above this text), it seems to suggest a BAU emissions growth from ~700 Mt in 2015 to ~2700 Mt in 2050
    With a little maths, this implies an exponential growth rate of 3.86 %. So either the 5% number is wrong or the graph is wrong.
    A growth rate of 5% would imply emissions of ~4030 Mt by 2050 (and cumulative emissions of ~67 Gt) – considerably higher than the graph indicates. If my calculations are correct – aviation emissions may be an even larger fraction of the budget than suggested here.

    But perhaps I’ve made an error. Is emissions growth the same as passenger number growth? I couldn’t find anything in the ICAO document to explain why I might be getting this discrepancy (They do mention growth-rates around about 5%)

    If anyone can clear this up I’d appreciate it.

    • RozPidcock

      Hi Paul, thanks for your comment.

      We took the 5% growth figure from the following paragraph about fuel burn on p16 of the report: “The trends forecast, which is for revenue tonne kilometres (RTK) and international aviation, shows a 20 year (2010-2030) compound average annual growth rate (CAGR) of 5.3 per cent.”

      There is also this on p168 about air traffic: “Despite significant developments in aircraft technology, and operational and infrastructural fields, the fuel efficiency improvements achieved by these means will likely not be enough to keep up with air traffic volume growth which is projected to continue at 4% to 5% annually in the coming decades.”

      Wrt the ICAO graph, we used 550Mt in 2015 and 2665 Mt in 2050 for our calculations, which returns a compound growth rate of about 4.6%.

      Even apart from this point, you’re right that aviation emissions are an even larger fraction of the budget than suggested in the ICAO figures, largely because they only account for international travel, which is ~ 65% of total air travel.

      • Paul Barry

        Many thanks for your reply. If its 550 Mt in 2015 then 4.6% is correct. I’m always wary of these growth forecasts. A half a percent can make a huge difference – it shows how easily they could overshoot their targets.
        Keep up the good work!


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