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Climate benefits of proposed carbon dioxide mitigation strategies for international shipping and aviation

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While individual countries work to achieve and strengthen their nationally determined contributions (NDCs) to the Paris Agreement, the growing emissions from two economic sectors remain largely outside most countries' NDCs: international shipping and international aviation. Reducing emissions from these sectors is particularly challenging because the adoption of any policies and targets requires the agreement of a large number of countries. However, the International Maritime Organization (IMO) and the International Civil Aviation Organization (ICAO) have recently announced strategies to reduce carbon dioxide (CO2) emissions from their respective sectors. Here we provide information on the climate benefits of these proposed measures, along with related potential measures. Given that the global average temperature has already risen 1 ∘C above preindustrial levels, there is only 1.0 or 0.5 ∘C of additional “allowable warming” left to stabilize below the 2 or 1.5 ∘C thresholds, respectively. We find that if no actions are taken, CO2 emissions from international shipping and aviation may contribute roughly equally to an additional combined 0.12 ∘C to global temperature rise by end of century – which is 12 % and 24 % of the allowable warming we have left to stay below the 2 or 1.5 ∘C thresholds (1.0 and 0.5 ∘C), respectively. However, stringent mitigation measures may avoid over 85 % of this projected future warming from the CO2 emissions from each sector. Quantifying the climate benefits of proposed mitigation pathways is critical as international organizations work to develop and meet long-term targets.
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Atmos. Chem. Phys., 19, 14949–14965, 2019
https://doi.org/10.5194/acp-19-14949-2019
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
Climate benefits of proposed carbon dioxide mitigation strategies
for international shipping and aviation
Catherine C. Ivanovich, Ilissa B. Ocko, Pedro Piris-Cabezas, and Annie Petsonk
Environmental Defense Fund, 1875 Connecticut Ave NW, Washington, D.C. 20009, USA
Correspondence: Ilissa B. Ocko (iocko@edf.org)
Received: 6 February 2019 – Discussion started: 9 April 2019
Revised: 9 August 2019 – Accepted: 11 October 2019 – Published: 11 December 2019
Abstract. While individual countries work to achieve and
strengthen their nationally determined contributions (NDCs)
to the Paris Agreement, the growing emissions from two eco-
nomic sectors remain largely outside most countries’ NDCs:
international shipping and international aviation. Reducing
emissions from these sectors is particularly challenging be-
cause the adoption of any policies and targets requires the
agreement of a large number of countries. However, the In-
ternational Maritime Organization (IMO) and the Interna-
tional Civil Aviation Organization (ICAO) have recently an-
nounced strategies to reduce carbon dioxide (CO2) emis-
sions from their respective sectors. Here we provide infor-
mation on the climate benefits of these proposed measures,
along with related potential measures. Given that the global
average temperature has already risen 1 C above preindus-
trial levels, there is only 1.0 or 0.5 C of additional “allow-
able warming” left to stabilize below the 2 or 1.5 C thresh-
olds, respectively. We find that if no actions are taken, CO2
emissions from international shipping and aviation may con-
tribute roughly equally to an additional combined 0.12 C to
global temperature rise by end of century – which is 12 %
and 24 % of the allowable warming we have left to stay be-
low the 2 or 1.5 C thresholds (1.0 and 0.5 C), respectively.
However, stringent mitigation measures may avoid over 85%
of this projected future warming from the CO2emissions
from each sector. Quantifying the climate benefits of pro-
posed mitigation pathways is critical as international orga-
nizations work to develop and meet long-term targets.
1 Introduction
There are clear benefits to limiting global average temper-
ature rise to 1.5 C above preindustrial levels (Intergovern-
mental Panel on Climate Change, 2018). However, in or-
der to achieve this, carbon dioxide (CO2) emissions likely
need to reach net zero around mid-century (Intergovernmen-
tal Panel on Climate Change, 2018). This would require un-
precedented changes to energy systems, land use, transporta-
tion, infrastructure, and industry worldwide.
Two sectors for which establishing carbon dioxide miti-
gation policy is particularly complex are international avi-
ation and shipping. The Conference of the Parties to the
United Nations Framework Convention on Climate Change
(UNFCCC) in the late 1990s urged that emissions reduc-
tions from these sectors be pursued through the UN’s Interna-
tional Civil Aviation Organization (ICAO, established 1944)
and International Maritime Organization (IMO, established
1948), respectively (United Nations Framework Convention
on Climate Change, 1997). While the existence of these UN
bodies unites global perspectives for regulation development,
this arrangement also requires the agreement of a large num-
ber of countries for the adoption of any new policies and tar-
gets, a feat much more difficult than if only one or several
countries were involved.
While current emissions from international aviation and
shipping account for around 4 % of global energy-related
CO2emissions (IMO, 2014; ICAO, 2019a; International En-
ergy Agency, 2018), emissions from each sector are fore-
casted to increase anywhere from 200 % to 400 % (Lee,
2018) and 50 % to 250% (IMO, 2014) by mid-century, re-
spectively, in the absence of effective policy.
Therefore, to support the objectives of the Paris Agree-
ment adopted in 2015, the ICAO and IMO have recently an-
Published by Copernicus Publications on behalf of the European Geosciences Union.
14950 C. C. Ivanovich et al.: Climate benefits of proposed CO2mitigation strategies
nounced strategies to reduce carbon dioxide emissions from
international aviation and shipping, respectively. As part of a
“basket of measures” to address aviation emissions, includ-
ing a CO2efficiency standard for aircraft, the ICAO adopted
a resolution in 2016 to establish a Carbon Offsetting and Re-
duction Scheme for International Aviation (CORSIA). COR-
SIA requires states to ensure airlines limit their net emissions
of carbon dioxide to 2020 levels and allows airlines the flex-
ibility to achieve those reductions directly through improved
technologies and operations by reducing emissions outside
the sector and by using fuels that have lower emissions on
a life-cycle basis. Efforts are now underway to implement
CORSIA, while ensuring that these emissions reductions are
not double-counted (once by Paris Parties where the reduc-
tions occur and again by airlines in CORSIA). A long-term
goal for international aviation CO2emissions has been in
development since 2008, but the ICAO has yet to formally
adopt such a target.
On the other hand, the IMO announced in 2018 a mini-
mum ambition long-term target of cutting international ship-
ping emissions by at least 50 % by 2050 compared to 2008,
followed by rapid full decarbonization (IMO, 2018). This
long-term target was preceded by various policy options fa-
cilitating the reduction of carbon dioxide emissions from the
shipping sector. These policies include the Energy Efficiency
Design Index (EEDI), which requires increasingly stringent
minimum energy efficiency levels for new ships, and the Ship
Energy Efficiency Management Plan (SEEMP), which pro-
vides an approach for monitoring the energy efficiency of
current fleets in use.
While both of these measures – CORSIA and the IMO
target – will reduce carbon dioxide emissions from interna-
tional aviation and shipping, respectively, it is important to
analyze the impact that these measures will have on global
warming. This information is further important as the ICAO
2019 Assembly considers the next steps and the IMO revises
and reviews its long-term target in 2023 and 2028.
Several studies have previously quantified the current and
future climate impacts of the transport sector, including avia-
tion and shipping. Many studies aggregate the climate im-
pacts of each sector through the use of CO2equivalence
(CO2e) (Lee et al., 2010; Lee, 2018; Eyring et al., 2010; Azar
and Johansson, 2012). While the use of this simple metric at-
tempts to describe the global warming intensity associated
with the emission of multiple greenhouse gases, it does not
account for continuous emissions or convey warming im-
pacts over time (Ocko et al., 2017). Studies that do inves-
tigate the climate impacts of these sectors over time often
consider the effects of a single emissions pulse or sustained
present-day emissions (Fuglestvedt et al., 2009; Berntsen and
Fuglestvedt, 2008; Unger et al., 2010).
There are a few studies that have modeled the contribu-
tion to warming from future aviation and shipping emis-
sions pathways. One of the earliest estimates was published
in the IPCC special report on aviation and its impact on
the global atmosphere in 1999 (Intergovernmental Panel on
Climate Change, 1999); the report estimated aviation’s ex-
pected business-as-usual (BAU) contribution to warming in
the year 2050 at 0.05 to 0.09 C. Skeie et al. (2009) also an-
alyzed future warming impacts from shipping and aviation
and included prospective technological improvements in ad-
dition to BAU projections. The authors estimated that avia-
tion’s contribution to warming in 2100 will range from 0.11
to 0.28 C, while the shipping sector’s contribution will range
from 0.01 to 0.25 C, depending on future trends in global
economic development. The cooling impact of the shipping
sector by mid-century was estimated by Lund et al. (2012)
at 0.02 to 0.04 C, depending on the assumed emissions
scenario. Huszar et al. (2013) estimated that the CO2emis-
sions from global aviation would produce 0.1 C of warming
by the end of the century, and an additional 0.1C of warm-
ing would stem from non-CO2impacts. More recent esti-
mates from Terrenoire et al. (2019) project that CO2emis-
sions from the global aviation sector will be responsible for
up to 0.1 C by the end of the century in the absence of miti-
gation action.
Here we build on these previous analyses by providing in-
formation on the climate benefits over time of all proposed
and prospective mitigation strategies to date in terms of ex-
pected avoided warming compared to BAU projections. We
focus our analysis on international emissions as opposed
to total emissions (international and domestic). We avoid
simple metrics, which do not account for continuous emis-
sions or convey warming impacts over time, by employing a
reduced-complexity climate model. We also account for all
climate pollutant emissions from aviation and shipping. We
consider the proposed target for international shipping paired
with current mitigation policies and more stringent potential
revisions, along with various pathways for international avi-
ation that include current CORSIA targets, an extension of
CORSIA, and similar targets as those agreed upon by the
shipping industry.
Action by both sectors simultaneously is essential because
the economics of transport are intertwined. Moreover, the cli-
mate impacts of international aviation and shipping are inex-
tricably linked; the success of each industry in its efforts to
limit greenhouse gas emissions could drive down the costs of
climate solutions and open up new clean fuel supply chains.
2 Methods
2.1 Business-as-usual emissions from international
bunkers
We account for present-day and future emissions of both
CO2and non-CO2pollutants from international shipping and
aviation in our BAU baseline scenarios. Projected emissions
over time can be found in Fig. 1.
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C. C. Ivanovich et al.: Climate benefits of proposed CO2mitigation strategies 14951
Figure 1. Projected future emissions from international shipping and aviation. Shipping CO2emissions data from the Third IMO Greenhouse
Gas Study (IMO, 2014) and the Update of Maritime Greenhouse Gas Emission Projections (Hoen et al., 2017). Aviation CO2emissions data
from Present and Future Trends in Aircraft Noise and Emissions (ICAO, 2019a). Both datasets end in 2050; shipping data are linearly
extrapolated through the year 2100, and aviation data utilize the described growth extrapolation after 2040 through the year 2100. Aviation
black carbon and NOxemissions as well as shipping black carbon, CH4, NOx, SO2(adjusted based on IMO’s recently adopted sulfur fuel
regulation), organic carbon, and CO extracted from the RCP database for the RCP8.5 scenario. Aviation SO2and CO are linearly extrapolated
from the EDGAR dataset, and aviation organic carbon emissions are derived from their relationship with black carbon emissions.
2.1.1 International shipping
International shipping CO2emissions data for the years 2007
to 2030 are taken from the Third IMO Greenhouse Gas Study
(IMO, 2014) and for the years 2030 to 2050 are taken from
the Update of Maritime Greenhouse Gas Emission Projec-
tions (Hoen et al., 2017). Following the 2015 to 2050 growth
trend for international shipping, CO2emissions are linearly
extrapolated through the year 2100. These projections in-
clude estimated CO2emissions reductions associated with
the implementation of EEDI and SEEMP. Because estima-
tions of how these programs will impact the non-CO2emis-
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14952 C. C. Ivanovich et al.: Climate benefits of proposed CO2mitigation strategies
sions from international shipping are uncertain, they are not
included in the baseline emissions profiles for the sector.
For non-CO2emissions, BAU projections for shipping
are taken from the Representative Concentration Pathways
database (RCP database) using the scenario that most closely
represents BAU – RCP8.5 (Riahi et al., 2007). Data are avail-
able for the historical and projected shipping emissions of
methane (CH4), nitrogen oxides (NOx), carbon monoxide
(CO), sulfur dioxide (SO2), black carbon, and organic car-
bon. These projections include progressive reductions of sul-
fur dioxide emissions associated with the amendments to
MARPOL Annex VI, which leads to a 0.5 % sulfur diox-
ide emissions cap in 2020. However, this simplified approach
does not consider current emission control areas (ECAs)
around the United States, Canada, and the European Union
(mandating a cap of 0.1 % sulfur) or the potential for ECAs
to be declared in additional regions of the world.
2.1.2 International aviation
International aviation CO2emissions data for the years 2010
to 2050 are taken from Present and Future Trends in Air-
craft Noise and Emissions (ICAO, 2019a). We extrapolate
the aviation CO2emissions for the Low Aircraft Technology
and Moderate Operational Improvement Scenario through
the year 2100 following the 2020 and 2050 trend.
Given that there is a range of reasonable growth patterns
for aviation emissions in particular (Lee, 2018; Skeie et al.,
2009), and our results depend on this baseline, we ran a set
of sensitivity tests to evaluate the influence of different CO2
BAU projection growth patterns on the perceived avoided
warming impacts. The two sensitivity tests considered are
based on an exponential growth rate pattern through 2100
following the 2005–2050 trend for the high and low demand
forecasts as depicted in a previous version of Present and Fu-
ture Trends in Aircraft Noise and Emissions (ICAO, 2013).
These emissions estimates are scaled down to calculate the
corresponding Low Aircraft Technology and Moderate Op-
erational Improvement Scenario proportionally to the latest
ICAO forecast (2019a), resulting in declining growth rate
patterns in which growth rates follow their 2020–2050 de-
clining trend until plateauing at 0 % – as is the case for the
low demand scenario. These sensitivity tests are analyzed in
addition to the Low Aircraft Technology and Moderate Oper-
ational Improvement Scenario noted above for a total of three
analyzed BAU scenarios for aviation. Because of uncertain-
ties associated with how the emissions of non-CO2pollutants
are linked to these different CO2growth rates, we focused
our sensitivity tests on emissions of CO2in particular. The
CO2emissions profiles used for this sensitivity analysis are
shown in Fig. 2. We note that all other figures in the paper re-
flect the Low Aircraft Technology and Moderate Operational
Improvement Scenario, which depicts a limited growth pat-
tern for international aviation as this provides a middle-of-
the-road estimation.
Figure 2. Projected future emissions from international aviation
used for sensitivity analysis. The sensitivity tests (dashed lines) are
based on an exponential growth rate pattern through 2100 following
the 2005–2050 trend for the high and low demand forecasts as de-
picted in a previous version of Present and Future Trends in Aircraft
Noise and Emissions (ICAO, 2013).
Aviation emissions data for black carbon and nitrogen ox-
ides are also taken from the RCP database using scenario
RCP8.5. Given that the RCP data include emissions projec-
tions for both international and domestic aviation, we use
historical data from the Emissions Database for Global At-
mospheric Research (EDGAR; Crippa et al., 2016) to esti-
mate the percent of total emissions from global aviation at-
tributed to international flights (using the most recent data
from 2012). Historical international aviation emissions data
for sulfur dioxide and carbon monoxide are taken from the
EDGAR database and are linearly extrapolated for each gas
in order to match the growth patterns for the other non-CO2
climate pollutant emissions associated with aviation. We es-
timate international aviation organic carbon emissions based
on the RCP black carbon data and using the organic-to-black-
carbon ratio (0.49) provided by EDGAR for international
aviation emissions (Crippa et al., 2016), again adjusted to re-
flect only the emissions from international flights. The BAU
projections for international aviation sulfur dioxide, carbon
monoxide, and organic carbon are added to the business-as-
usual scenario including all natural and anthropogenic cli-
mate forcings in order to account for their original absence
in the RCP8.5 database.
2.2 Mitigation scenarios
The mitigation emissions pathways are developed based on
a series of agreed upon, proposed, or prospective policy sce-
narios for international shipping and aviation (Table 1). For
international shipping, we analyze two mitigation scenarios:
(i) the IMO’s recently agreed upon minimum ambition mit-
igation target of reducing carbon intensity by at least 40 %
below 2008 levels by 2030 and total emissions by 50% be-
low 2008 levels by 2050, followed by full decarbonization
of the sector; and (ii) the maximum ambition scenario con-
sistent with pathways to achieve the 1.5 C target (Intergov-
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C. C. Ivanovich et al.: Climate benefits of proposed CO2mitigation strategies 14953
Table 1. Descriptions of mitigation scenarios analyzed in this study for international aviation and shipping.
Mitigation scenario description Abbreviation
Aviation
Cap emissions at 2020 levels CAP
CORSIA (including offsets, biofuel use, and improve-
ments to aircraft technology and air traffic manage-
ment) ends after 2035, followed by business-as-usual
emissions growth
CORSIA
CORSIA emissions reductions sustained through 2100 CORSIA–EXT
CORSIA ends in 2035, followed by decarbonization in
2100
CORSIA–DECARB2100
CORSIA ends in 2035, followed by decarbonization in
2050
CORSIA–DECARB2050
Shipping
IMO Greenhouse Gas Targets: 50 % reduction from
2008 levels by 2050, decarbonization by 2100; does not
affect non-CO2pollutants
IMO–MIN AMBITION, CO2ONLY
Linear decrease in emissions starting in 2020, leading to
decarbonization in 2050; does not affect non-CO2pol-
lutants
IMO–MAX AMBITION, CO2ONLY
IMO Greenhouse Gas Targets: 50 % reduction from
2008 levels by 2050, decarbonization by 2100; propor-
tional emissions reductions for all non-CO2pollutants
IMO–MIN AMBITION, ALL POLLUTANTS
Linear decrease in emissions starting in 2020, leading
to decarbonization in 2050; proportional emissions re-
ductions for all non-CO2pollutants
IMO–MAX AMBITION, ALL POLLUTANTS
ernmental Panel on Climate Change, 2018) in which a 40 %
reduction in emissions by 2030 is followed by decarboniza-
tion of the sector by the year 2050. These scenarios assume
a linear reduction in emissions between target years, specifi-
cally 2015, 2030, 2050, and 2100 for the minimum ambition
target and 2015, 2030, and 2050 for the maximum ambition
target.
While these policies are motivated by the intention to re-
duce emissions of CO2, non-CO2climate pollutant emis-
sions will likely be impacted as well – although how will
depend on the specific methods used to achieve the CO2tar-
gets (Balacombe et al., 2019; Bouman et al., 2017), which
are currently undecided. Therefore, we analyze scenarios in
which the CO2mitigation methods do not affect other pol-
lutants and scenarios in which the CO2mitigation methods
affect other pollutants proportionally; the desire is to capture
a range of plausible climate benefits.
Several policy measures have been suggested to reduce
carbon dioxide emissions from international aviation. Here
we analyze a scenario with the emissions reductions neces-
sary in order to maintain a cap on net emissions of interna-
tional flights to year-2020 levels and four scenarios based on
the adoption of CORSIA. The CORSIA-based scenarios in-
clude the following: (i) emissions reductions due to offsets,
biofuel use, and improvements in aircraft technology and air
traffic management through 2035; (ii) an extension of COR-
SIA through 2100; (iii) full decarbonization of the interna-
tional aviation sector by 2100 following CORSIAs comple-
tion in 2035; and (iv) full decarbonization of the international
aviation sector by 2050 following CORSIAs completion in
2035.
All CORSIA-based scenarios include the maximum po-
tential contribution of improved technology and management
(and this maximum potential encompasses the anticipated
effects of the ICAO CO2standard); however, the CORSIA
component of the scenario only affects CO2emissions given
that this is an offsetting program. Projections for the CO2
emissions reductions associated with CORSIA through the
year 2035 are based on the latest list of participating member
countries from the ICAO (ICAO, 2016, 2019b) and using the
Environmental Defense Fund’s aviation emissions interactive
tool (Environmental Defense Fund, 2019). While CORSIA
aims to offset international aviation emissions to the point of
capping emissions at year-2020 levels, country exemptions
to the program allow a small portion of emissions above this
cap to remain uncovered. Because no policies currently ex-
ist to limit the emissions attributed to these exempt countries,
emissions projections for the CORSIA–EXT scenario extend
their current growth rate through the end of the century. Pro-
jections concerning how both biofuel use and the improve-
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14954 C. C. Ivanovich et al.: Climate benefits of proposed CO2mitigation strategies
ments to aircraft technology and air traffic management will
contribute to the future emission of non-CO2climate pollu-
tants in the aviation sector are very limited and contain high
levels of uncertainty, so this trade-off is not considered in the
presented analysis.
2.3 Climate model
We employ the reduced-complexity climate model, Model
for the Assessment of Greenhouse-gas Induced Climate
Change (MAGICC) version 6, because of its widespread
and prominent use and its ability to reliably model cli-
mate responses to small forcing changes (Meinshausen et
al., 2011a; Ocko et al., 2018). Decades of research have
been devoted to improving model parameterizations, and
model results demonstrate consistency with sophisticated
Coupled Model Intercomparison Project CMIP atmosphere–
ocean and C4MIP carbon cycle models (Meinshausen et al.,
2011a).
MAGICC contains a hemispherically averaged upwelling–
diffusion ocean coupled to a four-box atmosphere (one over
land and one over ocean for each hemisphere) and a car-
bon cycle model, with an average equilibrium climate sen-
sitivity (ECS) of 3 C. Between 1765 and 2005, radiative
forcings are determined by historical greenhouse gas con-
centrations (Meinshausen et al., 2011b), prescribed aerosol
forcings and land-use historical forcings (National Aeronau-
tics and Space Administration (NASA) GISS model; http:
//data.giss.nasa.gov/, last access: 1 November 2019), solar ir-
radiance (Lean, 2010), and historical emissions of ozone pre-
cursors (Lamarque et al., 2010). After 2005, radiative forc-
ings are calculated from greenhouse gas emissions (carbon
dioxide, methane, nitrous oxide, ozone-depleting substances,
and their replacements), tropospheric ozone precursor emis-
sions (carbon monoxide, nitrogen oxides, and non-methane
volatile organic carbon), aerosol emissions (sulfate, black
and organic carbon, sea salt, and mineral dust), and the in-
direct effects (first and second) of aerosols.
Whereas the radiative impacts of well-mixed greenhouse
gases (such as CO2and methane) are fairly well understood
due to our knowledge of gas absorption, aerosol radiative ef-
fects are more complex and uncertain. This is due to the spa-
tial and temporal heterogeneity complicating observations;
a variety of possible microphysical and optical properties
based on varying sizes, shapes, structures, mixtures, and hu-
midity levels; and interactions with clouds that can impact
the lifetime and brightness of the clouds. Given that aerosols
are quite relevant to both the aviation and shipping sectors
(e.g., Unger et al., 2010), we include their direct and indi-
rect effects in our simulations, noting that caution must be
applied in interpreting the results. Aerosol direct forcings
are approximated by simple linear forcing–abundance rela-
tionships. The indirect effects of sulfate, black carbon, or-
ganic carbon, nitrate, and sea salt aerosols are also included.
The effect on cloud droplet size is determined by scaling the
optical thickness patterns of each species (as described by
Hansen et al., 2005) by their respective emissions. The effect
of aerosols on cloud cover and lifetime is modeled as a pre-
scribed change in the efficacy of the cloud albedo (for full
parameterization details, see Meinshausen et al., 2011a).
We note that all emissions are treated as surface emis-
sions. Aviation emissions in flight occur at higher elevations,
and this can affect atmospheric chemistry and radiation pro-
cesses. For example, when sulfate is located above clouds,
the radiative efficiency can be halved (less cooling); in con-
trast, the radiative efficiency of black carbon can be doubled
(more warming) when it is located above clouds (Ocko et
al., 2012). On the other hand, using more sophisticated cli-
mate models that can resolve horizontal and vertical granu-
larities is often complicated by unforced internal variability
that makes isolating the climate impact of relatively small ra-
diative perturbations difficult if not impossible (Ocko et al.,
2018).
The latest version of MAGICC is not calibrated for the
inclusion of linear contrails and induced cirrus cloudiness
from aviation, phenomena in which water vapor and im-
purities released in aircraft exhaust form cirrus-like clouds.
This is an active area of research and significant progress has
been made in recent years to better understand these uncer-
tain processes (e.g., Lee et al., 2009; Schumann et al., 2015;
Brasseur, 2016; Bock and Burkhardt, 2016). In the absence
of these parameterizations in MAGICC, we include a sen-
sitivity analysis to show their potential impact on the BAU
radiative forcings and temperature responses to aviation.
We use default MAGICC properties with the exception of
a few updates to reflect the most recent state of the science.
Specifically, we modify methane’s radiative efficiency (ac-
counting for shortwave in addition to longwave absorption)
and atmospheric lifetime, as well as tropospheric ozone’s
radiative efficiency (Etminan et al., 2016; Stevenson et al.,
2013). As with climate models of any complexity level,
there are limitations in our knowledge of climate and car-
bon cycle processes, radiative forcings, and especially in-
direct aerosol effects, which introduce uncertainties within
the model. While MAGICC uses several calibration meth-
ods to determine its parameters from a large collection of
sophisticated models, the comprehensive models will pass
along their own uncertainties to MAGICC. Further, due to
MAGICC’s relative simplicity, parameters are averaged over
large spatial scales. This is particularly important to acknowl-
edge as the recent literature has demonstrated that radiative
forcings associated with the transport sector can differ based
on the regional location at which the transport takes place
(Berntsen et al., 2006; Fuglestvedt et al., 2014; Kohler et al.,
2013; Fromming et al., 2012; Lund et al., 2017; Skowron et
al., 2015), particularly for the impact of non-CO2emissions.
Other major sources of uncertainty stem from the innate
inability to accurately project future emissions due to uncer-
tainties in both the human and the climate components of
prediction. All mitigation scenarios are compared to an esti-
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C. C. Ivanovich et al.: Climate benefits of proposed CO2mitigation strategies 14955
mated baseline, and the social and economic data utilized in
order to inform this estimated baseline cannot be expected
to perfectly match the unpredictable nature of human ac-
tion. Further, the large spatial scales and parameterizations
involved in climate modeling contribute to some degree of
uncertainty. A full discussion of model uncertainties can be
found in Meinshausen et al. (2011a).
2.4 Climate model simulations
We run 335 year-to-year integrations from the years 1765
to 2100 for a set of 14 different simulations. These simula-
tions are comprised of five BAU pathways and nine mitiga-
tion pathways based on current and potential policy scenarios
within the international aviation and shipping sectors. For fu-
ture emissions from sectors other than international aviation
and shipping, we use RCP8.5 emissions data, but the climate
impacts are subtracted out as described below.
The five BAU scenarios account for warming impacts due
to the following: all natural and anthropogenic forcings; iso-
lation of the CO2emissions from international shipping; iso-
lation of the CO2emissions from international aviation; iso-
lation of the CO2, black carbon, methane, nitrogen oxide,
sulfur dioxide, organic carbon, and carbon monoxide emis-
sions from international shipping; and isolation of the CO2,
black carbon, nitrogen oxide, sulfur dioxide, organic carbon,
and carbon monoxide emissions from international aviation.
The nine mitigation simulations account for the future emis-
sions pathways for the nine policy scenarios outlined in Ta-
ble 1.
In order to isolate sector emissions in each BAU and
mitigation scenario, we subtract the total emissions of all
gases and aerosols associated with each sector from the to-
tal RCP8.5 emissions of all gases and aerosols in the all-
forcings scenario driven by all natural and anthropogenic
forcings (Eq. 1). The annual average mean surface temper-
ature changes from these emissions profiles are subtracted
from the temperature changes in the all-forcings scenario
in order to determine the contribution to future tempera-
ture change from each sector (Eq. 2). It is important to note
that the background temperature response to other forcings
(anthropogenic and natural) can affect the temperature re-
sponses to shipping and aviation. Therefore, even though
they are ultimately subtracted out in our calculation, they do
impact our results, and uncertainties in BAU emissions from
other sectors and the resulting temperature effects need to be
acknowledged.
Emissionsall-forcings without sector =Emissionsall-forcings
Emissionssector (1)
1Tsector =1Tall-forcings
1Tall-forcings without sector emissions (2)
Comparisons of each sector’s baseline scenario to its respec-
tive mitigation scenarios are analyzed independently from
other potential mitigation efforts that may occur in the fu-
ture. Thus, isolating the temperature impacts of a given miti-
gation scenario does not mandate that all other anthropogenic
emissions continue unabated. The same methodology can be
used to isolate temperature changes due to individual gases
or aerosols for each sector.
3 Results
3.1 BAU climate responses
Both the shipping and aviation sectors emit a combination of
warming and cooling climate pollutants and precursors. The
net temperature impact depends on the magnitude of emis-
sions, the radiative efficiencies, and the atmospheric life-
times of the individual species. CO2builds up in the atmo-
sphere over time and thus its forcing increases gradually with
constant emissions, whereas short-lived species such as all
aerosols would yield constant annual forcings with constant
emissions. Given that we are analyzing the climate impacts
of future emissions from international aviation and shipping
(year 2020 through 2100), the near-term radiative forcings
(defined as the forcing at the tropopause after stratospheric
temperature adjustment) are dominated by non-CO2pollu-
tants, and the long-term radiative forcings are dominated by
CO2(Fig. 3).
The net radiative forcing for international shipping is
47 mW m2in 2020 and +48 mW m2in 2100. The shift
from negative to positive is due to the large increase in CO2
emissions and their accumulation over time in the atmo-
sphere. A considerable amount of the positive radiative forc-
ing from CO2emissions in 2100 (+127 mW m2) is offset
by a relatively large negative radiative forcing in 2100 from
NOxemissions (66 mW m2). Net radiative forcing due to
NOxemissions is a combination of negative and positive ra-
diative forcings from indirect effects; negative forcings arise
from reductions in methane, the production of nitrate, and
nitrate’s effect on clouds, and positive forcings arise from
the production of tropospheric ozone. Indirect aerosol effects
from all species yield a radiative forcing of 32 mW m2in
2100.
Radiative forcings derived in this study from shipping
emissions of CO2and NOxare consistent with the litera-
ture. Previous estimates of CO2s present-day (early 2000s)
impact range from +26 to +43 mW m2, corresponding to
emissions of 500 and 800 Tg CO2yr1(Eyring et al., 2010).
This is consistent with this analysis when accounting for
the anticipated growth in CO2emissions of more than 5-
fold by 2100 since the early 2000s (IMO, 2014). Previ-
ous studies estimate radiative forcings from NOxthat range
from +8 to +41 mW m2for indirect effects on tropospheric
ozone (compared to our value of +25 mW m2in 2100) and
56 to 11 mW m2for indirect effects on methane (com-
pared to our value of 22 mW m2in 2100) for present-day
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14956 C. C. Ivanovich et al.: Climate benefits of proposed CO2mitigation strategies
Figure 3. Contribution of emittants to radiative forcing in 2100 (defined as the forcing at the tropopause after stratospheric temperature
adjustment) from business-as-usual emissions from (a) international shipping and (b) international aviation. Radiative forcings are presented
for the year 2020 (hashed), which represent forcings from emissions that year, and the year 2100 (solid), which represent the change in
forcings from 2020 to 2100.
Figure 4. Contribution of future emissions to surface air temperature change (C) associated with business-as-usual emissions starting in
2020 and continuing through the end of the century from international shipping. Future temperature impacts are presented (a) for emissions
of CO2only (thin line) and all pollutants (thick line), as well as (b) the contribution of individual pollutants in the year 2100.
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C. C. Ivanovich et al.: Climate benefits of proposed CO2mitigation strategies 14957
Figure 5. Contribution of future emissions to surface air temperature change (C) associated with business-as-usual emissions starting in
2020 and continuing through the end of the century from international aviation. Future temperature impacts are presented (a) for emissions
of CO2only (thin line) and all pollutants (thick line), as well as (b) the contribution of individual pollutants in 2100.
emissions of around 2.9 to 6.5 Tg N yr1(we assume NOx
emissions of 5.6 Tg N yr1in the year 2100) (Eyring et al.,
2010). For SO2emissions from shipping, previous studies
estimate direct radiative forcings from 47 to 12 mW m2
due to the production of sulfate; our estimate is 14 mW m2
in 2100 from emissions that are lower (2.0 Tg S yr1) than
present-day values in the literature (3.4 to 6.0 Tg S yr1)
(Eyring et al., 2010). Our estimate of direct radiative forcing
from black carbon (+5 mW m2in 2100 from emissions of
0.2 Tg BC yr1) is slightly higher than estimates in the liter-
ature (+1.1 to +2.9 mW m2in 2000–2005 from emissions
of 0.05 to 0.3 Tg BC yr1) (Eyring et al., 2010). Indirect ef-
fects of aerosols have enormous ranges in estimates in the
literature (Righi et al., 2011), but we note that our estimate
appears to be on the lower end.
The net radiative forcing for international aviation emis-
sions (note: not including impacts on contrails and cirrus
clouds) is 1.4 mW m2in 2020 and +62 mW m2in 2100.
Although radiative forcings are smaller for CO2for aviation
compared to shipping due to slightly less emissions, there are
proportionally less emissions of the negative forcing precur-
sors NOxand SO2, yielding higher net radiative forcing from
aviation. As with the shipping forcings, the large CO2radia-
tive forcing in 2100 (+87 mW m2) is partially offset by the
strong negative forcing from NOxemissions (24 mW m2).
Indirect aerosol effects from all species yield a radiative forc-
ing of 10 mW m2in 2100.
Estimates of present-day radiative forcing from aviation
in the literature include both domestic and international
emissions, whereas our estimates of future radiative forc-
ings exclude domestic travel. Our estimates of radiative
forcing from CO2emissions are in agreement with previ-
ous estimates when accounting for different emissions in-
puts (such as +87 mW m2in 2100 from emissions of
3670 Tg CO2yr1compared to +28 mW m2in 2005 from
emissions of 641 Tg CO2yr1in Lee et al., 2009). Our es-
timates for radiative forcings from NOx, SO2(direct), and
black carbon (direct) are slightly smaller than what is pre-
sented in the literature, despite larger emissions projected
for the year 2100 compared to the present day, but there
are large uncertainties associated with these estimates and
a low level of scientific understanding (Sausen et al., 2005;
Fuglestvedt et al., 2008; Lee et al., 2009). For example,
Brasseur (2016) estimate +6 to +37 mW m2for indirect ef-
fects of NOxemissions on tropospheric ozone (compared to
our value of +11 mW m2in 2100) and 8 to 12 mW m2
for indirect effects on methane (compared to our value of
8 mW m2in 2100). Gettelman and Chen (2013) conduct
a more sophisticated assessment of the climate impact of avi-
ation aerosols than what is presented here and report an esti-
mate of 46 mW m2from combined sulfate direct and in-
direct effects; this is considerably larger than our estimate of
3 mW m2in 2100.
Radiative forcings directly impact temperatures – a net
positive forcing has a warming tendency, and a net nega-
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14958 C. C. Ivanovich et al.: Climate benefits of proposed CO2mitigation strategies
tive forcing has a cooling tendency. Figures 4 and 5 show
the temperature responses over time to projected emissions
from both sectors. Given that the ambition for the proposed
and agreed upon mitigation policies within the international
shipping and aviation sectors is based on the need to cut CO2
emissions from each sector, we isolate the temperature im-
pacts from the CO2emissions in addition to the net effect
from all emitted climate pollutants.
Figure 4a shows the impact of future international ship-
ping emissions (beginning in 2020) on surface air tempera-
ture change throughout the 21st century. In the year 2020,
the impact on temperature represents the contribution from
that year’s worth of emissions only, and then every year
forward represents the cumulative effect as some pollutants
build up in the atmosphere over time from continuous emis-
sions. While the CO2effect is always that of warming and
grows over time from both growing emissions and accumu-
lating concentrations due to the long atmospheric lifetime of
CO2, the inclusion of all climate pollutants yields a net cool-
ing effect in the near term consistent with the net negative
forcings discussed above. It is not until the 2080s that the
shipping net effect shifts to warming. This is consistent with
Unger et al. (2010), who show strong near-term cooling ten-
dencies from the shipping sector that lessen over time as CO2
builds up in the atmosphere. However, note that their study
analyzed perpetual year-2000 emissions and not a BAU sce-
nario. This is also consistent with Fuglestvedt et al. (2009),
which predicts that the accepted regulations in the shipping
sector’s emissions of sulfur dioxide and nitrogen oxides will
lead to the sector having a net cooling effect for about 70
years, after which the sector switches to warming. Our analy-
sis predicts a slightly more rapid shift to warming (after about
65 years in 2085), likely due to our inclusion of the warming
climate pollutant black carbon, which is not featured in the
analysis by Fuglestvedt et al. (2009).
Based on our BAU projections, future CO2emissions from
international shipping result in an additional warming of
0.07 C by the year 2100. However, when all pollutants are
considered, the net warming from shipping in 2100 drops to
0.01 C due to the net warming and cooling effects from non-
CO2pollutants (Fig. 4a).
For the year 2100, the temperature impacts attributed
individually to emissions of CO2, black carbon, methane,
nitrogen oxides, sulfur dioxide, organic carbon, and car-
bon monoxide are shown in Fig. 4b. The indirect effects
of aerosols are included in the analysis of the temperature
impacts for each isolated pollutant. Specifically, shipping’s
cooling effect, which offsets the CO2warming effect, is dom-
inated by the cooling pollutant precursor nitrogen oxides.
The net cooling from nitrogen oxides arises from nitrate
formation, indirect aerosol effects from nitrates, the forma-
tion of tropospheric ozone, the reduction of methane, and ef-
fects of the net forcings on the carbon cycle (cooling in the
ocean suppresses CO2diffusion from the ocean into the at-
mosphere). Given that sulfur dioxide emissions – a precursor
to the cooling pollutant sulfate – are projected to decrease
significantly due to the sulfur fuel regulation newly adopted
by the IMO, sulfur dioxide from shipping contributes less
significantly to cooling. Recent studies have demonstrated
the potential for low-sulfur shipping scenarios to reduce the
indirect aerosol effect from shipping sulfur emissions (Lauer
et al., 2009; Righi et al., 2011). However, the remaining emis-
sions of sulfur dioxide from the shipping sector throughout
the century are still responsible for about 0.02 C of cooling
by the year 2100.
While BAU organic carbon, carbon monoxide, and
methane have nearly negligible contributions to shipping’s
influence on end of century temperatures, shipping’s black
carbon emissions are responsible for 0.01 C warming and
add to CO2’s warming effects (Fig. 4b). We note that for
both sectors, our calculations assume no change in the ge-
ographical distribution of emissions. Recent literature has
demonstrated that the location of non-CO2emissions can
have a large influence on their subsequent climate impact
(Fuglestvedt et al., 2014; Kohler et al., 2013; Fromming et
al., 2012; Lund et al., 2017; Skowron et al., 2015).
Figure 5a shows the impact of future international avia-
tion emissions (beginning in 2020) on surface air temperature
change throughout the 21st century. The contribution of CO2
emissions to future warming over time and in the year 2100
is slightly lower than that from the shipping sector (0.05 C
by 2100). However, the inclusion of non-CO2climate pollu-
tant emissions does not yield a net cooling effect for several
decades as it does with shipping and reduces warming by the
end of the century to 0.03 C (note that we do not include
the impacts on contrails and cirrus clouds here). For a few
years, the net temperature impact from future aviation emis-
sions is cooling but then quickly switches to warming and
increases steadily through the end of the century (consistent
with radiative forcing calculations in Unger et al. (2010) for
constant year-2000 emissions). Similar to shipping, the cool-
ing effect is dominated by the cooling precursor gas nitrogen
oxide (Fig. 5b). By 2100, nitrogen oxides are responsible for
a cooling of 0.02 C, while the end-of-century contributions
from all other non-CO2climate pollutants (sulfur dioxide, or-
ganic carbon, carbon monoxide, and black carbon) are negli-
gible. Recall that the indirect effects of aerosols are included
in the analysis of the temperature impacts for each isolated
pollutant. We note that some studies have investigated the ef-
fect of aviation soot on natural cirrus clouds (Penner et al.,
2019) or the effect on warm clouds (Gettelman and Chen,
2013; Righi et al., 2013; Kapadia et al., 2016). MAGICC
takes into account indirect effects of soot, such as simplified
parameterizations of impacts on cloud brightness and life-
time, but does not include more sophisticated treatments as
analyzed in previous studies.
Our projections for the contribution to future warming
from international shipping and aviation are slightly lower
than the estimated range for each sector’s 2100 share of
warming estimated in Skeie et al. (2009). Our estimate of
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C. C. Ivanovich et al.: Climate benefits of proposed CO2mitigation strategies 14959
international aviation’s contribution to warming is below the
0.11 to 0.28 C range, but Skeie et al. (2009) analyzed com-
bined domestic and international transport emissions. Fur-
ther, the new emissions projections generated by the ICAO
in 2019 suggest lower projected emissions from aviation over
the next century (ICAO, 2019a). Our shipping warming im-
pact estimates are at the lower end of the 0.01 to 0.25 C
range, attributed to the differences in methodology discussed
below.
First, our model includes indirect aerosol effects, particu-
larly the climate impact associated with the nitrogen oxide
production of nitrate aerosols, which yield negative forcings
that are not considered in the analysis by Skeie et al. (2009).
This inclusion also explains why nitrogen oxides yield net
cooling impacts in our analysis, while they yield net warm-
ing impacts by Skeie et al. (2009) due mainly to warming
from the production of tropospheric ozone not canceled out
by cooling from indirect effects.
Second, our shipping estimates are also lower because
Skeie et al. (2009) only consider the emissions of CO2, ni-
trogen oxides, and sulfur dioxide for each sector, and emis-
sions profiles are based on older projections. In particular,
the projected emissions of CO2and nitrogen oxides in Skeie
et al. (2009) are both higher than our projected emissions
(which both yield more warming impacts in the absence of
indirect aerosol effects), while their emissions of sulfur diox-
ide are lower than our projections (which means less cooling
from sulfate). Acknowledging these differences in methodol-
ogy, we observe the same general warming trends within our
scenarios and the literature, wherein aviation emissions ex-
hibit an increasing net warming effect, while shipping emis-
sions result in a declining cooling trend until the end of the
century.
In the RCP scenarios presented by Lund et al. (2012), ship-
ping is projected to cause a cooling of between 0.02 and
0.04 C by mid-century. Our analysis estimates that ship-
ping is responsible for 0.03 C in the year 2050, which
falls within this range. Further, the authors’ findings are in
agreement with those presented in this analysis through their
observation of warming later in the century once the accu-
mulating CO2emissions impact overruns the cooling impact
of nitrous oxides and sulfur dioxide, particularly due to the
reduced sulfur dioxide emissions associated with the imple-
mented fuel regulations.
Our estimates for the contribution to global average tem-
perature in the year 2100 from the aviation sector’s CO2
emissions of 0.05 C falls at the lower end of the range pre-
sented by Terrenoire et al. (2019), between 0.04 and 0.1C,
based on a set of eight CO2emissions projections contrast-
ing in traffic growth and efficiency gains. We note that this
analysis includes the impact of both domestic and interna-
tional aviation. Our estimate of the impact of the aviation
sector is also less than that of Huszar et al. (2013), at 0.2
or 0.1 C with and without the impact of the non-CO2signal,
respectively. This analysis also does not account for aviation-
produced aerosols but does include the impact of water vapor
emissions (as well as that of contrail–cirrus), leading to an el-
evated warming associated with the sector in comparison to
our analysis.
Our model does not include radiative effects from lin-
ear contrails or contrail-induced cirrus cloudiness. Although
studies suggest a low level of scientific understanding for the
climate impacts of linear contrails and a very low level of sci-
entific understanding of induced cirrus cloudiness (Lee et al.,
2009), considerable work has been done recently towards im-
proving our understanding of these effects. Estimates of the
present-day radiative impact of linear contrails range from
+3 to +12 mW m2(Lee et al., 2009; Brasseur, 2016) and
of cirrus cloudiness range from +12 to +63 mW m2(Lee
et al., 2009; Schumann et al., 2015; Brasseur, 2016; Bock
and Burkhardt, 2016); for context, this is compared to around
30 mW m2from CO2emissions – note these values are for
both domestic and international aviation. As air traffic rates
increase, we expect the radiative forcings from contrails and
changes in cirrus cloudiness to increase as well; Bock and
Burkhardt (2019) suggest an increase in contrail–cirrus ra-
diative forcing by a factor of 3 from the present day through
2050 due to increases in air traffic and also a slight shift to-
wards higher altitudes.
Without growth in air traffic, the inclusion of these ef-
fects would increase our radiative forcing estimates in 2100
by 15 % to 75 % based on the lower and upper estimates of
both linear contrails and cirrus cloudiness. Assuming 5-fold
growth in air traffic from 2005 to 2100, our radiative forcing
estimate from international aviation could increase by 75 %
to 350 %. The resulting impact on temperature responses to
BAU international aviation could therefore be considerably
higher than our projection of 0.05 C in 2100: 0.06 to 0.09 C
based on current air traffic patterns and 0.09 to 0.23 C for a
5-fold increase in air traffic.
3.2 Avoided warming from mitigation measures
The policy scenarios analyzed have significant potential to
reduce future temperature impacts associated with emis-
sions from the international shipping and aviation sectors
(Fig. 6). The IMO greenhouse gas target of a 50 % reduc-
tion in CO2emissions below 2008 levels by 2050 and full
decarbonization of the industry by 2100 results in an avoided
future warming of 0.06 C by 2100. This avoided warming
reduces the shipping sector’s contribution to future warm-
ing from CO2by almost 85 % at the end of the century. A
more stringent mitigation scenario in which decarbonization
is achieved by mid-century (consistent with a 1.5 C warm-
ing cap) increases avoided warming to 0.07 C by 2100, or
almost 100 % of the original unabated contribution to warm-
ing from the sector’s CO2emissions.
Because the non-CO2climate pollutants emitted by the
shipping sector yield a net cooling, the scenarios that re-
duce their emissions proportional to the reductions in CO2
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14960 C. C. Ivanovich et al.: Climate benefits of proposed CO2mitigation strategies
Figure 6. Surface air temperature changes associated with various policy scenarios for emissions mitigation in international (a) shipping and
(b) aviation. Each business-as-usual scenario presents the contribution to future surface air temperature from the emissions of all climate
pollutants starting in 2020 and continuing through the end of the century.
outlined in each policy increase each scenario’s contribu-
tion to future warming and consequently reduce their relative
avoided warming. Specifically, the IMO minimum and maxi-
mum ambition greenhouse gas targets reduce the anticipated
BAU warming from the shipping sector by about 0.02 and
0.01 C by the end of the century, respectively (in compari-
son to 0.06 and 0.07 C in the CO2-only scenarios, respec-
tively). We expect that the true warming mitigation provided
by these policies lies within these bounds.
The various mitigation scenarios outlined in Table 1 for
the international aviation sector result in an avoided future
warming of 0.01 to 0.05 C by 2100 relative to a BAU base-
line. Full implementation of CORSIA under current guide-
lines (ending in 2035 and then allowing emissions to increase
along a business-as-usual pathway) results in an avoided
warming of 0.01 C by 2100 (a 20 % reduction of warm-
ing from a CO2BAU baseline). However, extending COR-
SIA’s offsetting and reduction program through the end of
the century more than doubles the climate benefit (0.02 C
avoided warming), avoiding about 40 % of the CO2BAU
baseline warming. The scenario that follows CORSIA and
then decarbonizes the sector by the year 2100 (CORSIA-
DECARB2100) reduces future warming by 0.04 C by the
end of the century, avoiding about 80 % of the CO2BAU
warming. The most aggressive mitigation policy, completing
CORSIA followed by decarbonization of the sector by 2050,
results in an avoided warming of 0.05 C by 2100 (about a
90 % reduction of warming from a CO2BAU baseline). The
avoided warming in the year 2100 associated with each in-
vestigated policy scenario for the emissions mitigation of in-
ternational shipping and aviation is outlined in Fig. 7.
The warming mitigation potential of the various policy
scenarios associated with aviation were evaluated based on
three CO2BAU growth patterns. While the BAU pathway
dictates the magnitude of projected future warming, the asso-
ciated avoided warming from each policy scenario is relative
to the BAU baseline. For international aviation, in compari-
son to the 0.05 C contribution to future warming from CO2
only expected from the central growth rate pattern, 0.10 and
0.02 C of future warming are expected from the emission
of CO2in the upper and lower emissions growth rate pat-
terns, respectively. The mitigation scenario that mimics the
structure of the IMO minimum ambition greenhouse gas tar-
get (CORSIA–DECARB2100), for example, is expected to
reduce the warming attributed to the emissions of CO2from
the sector by 0.04 C by the end of the century in the limited
growth rate pattern and is expected to avoid 0.09 and 0.01 C
by the end of the century in the upper and lower growth rate
patterns, respectively. These avoided temperatures from the
upper, central, and lower growth scenarios represent 87 %,
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C. C. Ivanovich et al.: Climate benefits of proposed CO2mitigation strategies 14961
Figure 7. Avoided warming in the year 2100 associated with various policy scenarios for emissions mitigation in international (a) shipping
and (b) aviation.
84 %, and 66 % of the unabated warming levels from CO2
emissions, respectively. While the expected BAU warming
from the sector’s CO2emissions varies significantly between
each pattern of growth, the potential to reduce this warm-
ing through proposed, stringent mitigation scenarios scales
proportionally for the two higher emissions growth rate sce-
narios. In contrast, the BAU lower growth scenario demon-
strates a future in which emissions remain relatively close to
2020 levels throughout the century. Because the most strin-
gent policy scenarios investigated in this analysis focus on
emissions reductions taking place mid-century to late cen-
tury, a lower percent of warming reduction is observed for
each policy within the lower growth scenario in comparison
to the upper and central scenarios.
Although we do not expect that the offsetting programs
analyzed here will affect the amount of contrail and cirrus
cloud formation and will therefore not impact the avoided
warming potential, improvements to aircraft technology and
management practices may reduce the prevalence and thick-
ness of these clouds, for example due to increased fuel effi-
ciency. Similarly, we do not consider the reduction of non-
CO2climate pollutants emitted by the aviation sector in the
mitigation scenarios. However, offsetting schemes such as
CORSIA do implement the use of biofuels as well as aircraft
technology and air traffic management improvements, both
of which have the potential to impact future emissions of
non-CO2climate pollutants and the density of contrail–cirrus
(Bock and Burkhardt, 2019; Caiazzo et al., 2017; Burkhardt
et al., 2018). While the influence of these changes on the non-
CO2impact of international aviation is currently not well es-
timated, their impact should be considered in future analyses
as understanding develops.
4 Conclusions
Quantifying the temperature impacts of future international
aviation and shipping emissions – both for business-as-usual
pathways and mitigation scenarios – is essential to under-
standing the benefits of proposed policies and targets. Given
that international aviation and shipping are important con-
tributors to the emission of climate pollutants, earlier stud-
ies have analyzed their current and BAU future climate im-
pacts using a variety of methods. To build upon these pre-
vious analyses, we analyzed the climate benefits over time
associated with accepted, proposed, and prospective mitiga-
tion policies for each sector. We use a reduced-complexity
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14962 C. C. Ivanovich et al.: Climate benefits of proposed CO2mitigation strategies
climate model to determine the BAU temperature contribu-
tion due to the future emissions of international aviation and
shipping from all emitted climate pollutants and the potential
to avoid future warming based on a series of realistic mitiga-
tion scenarios.
Using the reduced-complexity climate model MAGICC,
we estimate that under BAU conditions, future CO2emis-
sions (2020 through the end of the century) from the in-
ternational shipping and aviation sectors would be respon-
sible for 0.07 and 0.05 C of future warming by 2100, re-
spectively (0.01 and 0.03 C when including the sectors’
emissions of non-CO2climate pollutants; additional inclu-
sion of aviation-induced contrails and clouds could increase
the warming associated with international aviation by up to
600 %). Planned and proposed mitigation policies in each
sector that specifically target CO2emissions have the po-
tential to significantly reduce this climate impact. However,
policies that target the mitigation of non-CO2climate pollu-
tants, often through air quality management, result in emis-
sions reductions that may not always avoid future warming
(Kapadia et al., 2016; Sofiev et al., 2018; Yim et al., 2015).
For example, if the emissions of all shipping-produced cool-
ing agents (sulfur dioxide, nitrogen oxides, and organic car-
bon) were immediately halted and the shipping sector suc-
cessfully decarbonized by mid-century, the sector would in-
crease the world’s temperatures through the end of the cen-
tury (Fuglestvedt et al., 2009).
Given that we have already reached a global warming level
of around 1 C above preindustrial levels (Intergovernmen-
tal Panel on Climate Change, 2018), there is an “allowable
warming” of 0.5 to 1.0 C of additional warming should we
wish to stabilize at the 1.5 or 2 C threshold, respectively.
Together, future warming from the CO2emissions of inter-
national shipping and aviation reach about 0.12 C by the
end of the century, which is 12%–24 % of this remaining al-
lowable warming. However, certain policy measures have the
potential to significantly avoid the vast majority of this future
warming. The IMO minimum ambition greenhouse gas tar-
get (decarbonize the sector by 2100) and its mirrored aviation
scenario (CORSIA offsetting program extended followed by
decarbonizing by 2100) have the potential to reduce future
warming associated with the CO2emissions from each sec-
tor by more than 80 %, with even further reductions should
both sectors decarbonize by mid-century in comparison to
2100 (a trajectory consistent with achieving 1.5 C of maxi-
mum warming).
For context, achieving the Paris Agreement committed
pledges and targets is projected to avoid 0.3C of warm-
ing by the end of the century compared to current policies
(Climate Action Tracker, 2019). Adding the avoided warm-
ing from the already agreed upon international shipping tar-
get of decarbonization by the end of the century (0.06 C)
and the extension of the CORSIA aviation offsetting program
(0.02 C) increases this potential by over 25 %. Further, pur-
suing the most ambitious, yet feasible, mitigation measures
for international shipping and aviation could increase the
avoided warming from the Paris Agreement by nearly 50%.
Overall, the proposed and prospective mitigation measures
for both of these sectors have considerable climate benefits
in the context of achieving international temperature goals.
Code availability. The MAGICC v6 model executable is available
for download at https://www.magicc.org/ (last access: 1 Novem-
ber 2019), although the model itself is closed source. The user
manual can be accessed at http://wiki.magicc.org/index.php?title=
Manual_MAGICC6_Executable (last access: 1 November 2019).
Full model details along with 19 sets of Atmosphere–Ocean Gen-
eral Circulation Model (AOGCM)-calibrated parameters used here
for ensemble members are found in Meinshausen et al. (2011a). We
update the default values of methane, tropospheric ozone radiative
efficiency, and methane atmospheric lifetime to values in Myhre et
al. (2013) and Etminan et al. (2016).
Data availability. Results from MAGICC are available from
Catherine C. Ivanovich (civanovich@edf.org) upon request.
Author contributions. CCI and IBO designed the experiments, and
CCI carried them out. AP and PPC curated data and provided guid-
ance on the policies. CCI and IBO prepared the paper with contri-
butions from all coauthors.
Competing interests. The authors declare that they have no conflict
of interest.
Acknowledgements. We thank Nathaniel Keohane and
Steven Hamburg for reviewing our paper.
Financial support. Catherine C. Ivanovich was funded by the High
Meadows Foundation. Ilissa B. Ocko was funded by the Heising
Simons Foundation and the Robertson Foundation.
Review statement. This paper was edited by Mathias Palm and re-
viewed by two anonymous referees.
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