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Contrails formed by aircraft can evolve into cirrus clouds indistinguishable from those formed naturally. These 'spreading contrails' may be causing more climate warming today than all the carbon dioxide emitted by aircraft since the start of aviation.
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24 NATURE CLIMATE CHANGE | VOL 1 | APRIL 2011 | www.nature.com/natureclimatechange
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Aviation is at present responsible for
about 3% of all fossil fuel carbon
dioxide emissions, but an estimated
2–14% of anthropogenic climate forcing1.
Furthermore, its contribution to climate
forcing could triple by 2050, according to
some scenarios1. As such, mitigating the
impact of aviation on climate has become a
subject of considerable public and political
interest. e debate is complicated, however,
by the fact that aviations climate impact
results from a number of dierent factors,
as well as by the large uncertainty in the
eect that some of these factors have on
climate. Writing in Nature Climate Change,
Burkhardt and Kärcher 3 present a global
modelling study that quanties the climate
eect of ‘spreading contrails’ — the least well
quantied of all the aviation-related climate-
forcing agents.
Aircra-engine emissions are mostly
composed of carbon dioxide, water vapour,
nitrogen oxides, sulphur oxides and aerosol
particles. As well as the direct eect that
these emissions have on climate, aviation has
an added impact induced by the formation
of condensation trails (contrails) in the
wake of the aircra. ese line-shaped trails
are formed by the mixing of hot, moist air
coming out of the engine with cold ambient
air. When the atmosphere is supersaturated
with respect to ice, the line-shaped contrails
can spread to form cirrus cloud, which has
a warming eect on climate. Although there
are robust case studies of this spreading
phenomenon using satellite observations2
(Fig.1), its relevance to the climate system
remainsunknown.
Both ground- and satellite-based cloud
observations have suggested a small but
noticeable increase in cirrus cloud cover in
regions of high air-trac density relative
to adjacent regions4–6. However, contrail
spreading is not the only mechanism that
could explain this increase. It has also been
suggested that aircra-emitted aerosols
could serve as ice nuclei and facilitate the
formation of cirrus cloud7. To understand
the impact of aviation on climate, it is
necessary to quantify the importance of
these two mechanisms. is, however, is not
a straightforward task.
In situ observations of aerosols and ice
nuclei in the upper troposphere are still very
scarce. ere are also multiple confounding
factors that make the observations dicult to
interpret. For instance, when a line-shaped
contrail spreads into a large cirrus cloud, it is
virtually impossible to tell from observations
alone whether a cirrus cloud would have
formed naturally (that is, without having
being triggered by the aircra) at some point
in time. Climate modelling does not have
these diculties, and thus oers a way of
tackling this thornyproblem.
Burkhardt and Kärcher 3 developed
a process-based model of how contrails
form, grow (through the depletion of water
vapour in the surrounding air), spread
and nally disappear (through mixing and
fall-out of the ice crystals). By tracking
the fate of contrail and natural cirrus
separately, the authors can quantify the
radiative forcing from spreading contrails
(including young line-shaped contrails),
which they estimate to be 38mW m–2. is
can be compared with a radiative forcing of
4mW m–2 from young contrails alone and
28mWm–2 from aviation carbon dioxide.
Interestingly, spreading-contrail cirrus
ATMOSPHERIC SCIENCE
Seeing through contrails
Contrails formed by aircraft can evolve into cirrus clouds indistinguishable from those formed naturally. These
‘spreading contrails’ may be causing more climate warming today than all the carbon dioxide emitted by aircraft
since the start of aviation.
Olivier Boucher
10:06: NOAA17 10:40: METOP 11:30: TERRA 12:02: NOAA18
13:42: NOAA18 15:26: NOAA15 17:08: NOAA15 19:48: NOAA17
Figure 1 | Satellite infrared images of contrails spreading into cirrus clouds over the UK. The young
contrails, which appear as a spring shape and sharp lines in the first image, gradually spread into cirrus
clouds, which appear as bright white areas in the lower images. The time of each image and the satellite
used to take it are shown in the inset of each frame. Burkhardt and Kärcher3 used a model that simulates
this spreading process to assess the warming eects of contrails and the cirrus clouds that form from
them. Their results indicate that so-called spreading contrails cause an order of magnitude more climate
warming than the line-shaped contrails alone, and are the largest single climate-forcing agent associated
with aviation. Image reproduced with permission from ref.2, © 2009 AGU.
© 2011 Macmillan Publishers Limited. All rights reserved
NATURE CLIMATE CHANGE | VOL 1 | APRIL 2011 | www.nature.com/natureclimatechange 25
news & views
clouds cause a reduction in natural cirrus,
because they modify the water budget
in the upper troposphere; however, this
reduction in natural cirrus is relatively small
(-7mWm–2).
Overall, and despite their short lifetime,
contrails may have more radiative impact at
any one time than all of the aviation-emitted
carbon dioxide that has accumulated in
the atmosphere since the beginning of
commercial aviation. It is important to note,
however, that the emitted carbon dioxide
would continue to exert a warming inuence
for much longer than contrails, should all
aircra be grounded indenitely. ese
results are intrinsically dicult to validate
against observations, but the authors have
performed a sensitivity study that shows
their results are not signicantly aected by
the contrail spreading rate (±5mWm–2).
is is a conservative estimate of the
uncertainty and more work is needed to
assess the robustness of the results.
ese ndings are important, because if
the calculations of Burkhardt and Kärcher
are correct, they provide a basis to develop
mitigation strategies to reduce the impact
of aviation on climate. For instance, it
has been suggested that ight routes or
ight altitudes could be planned and
altered in real time to avoid parts of the
atmosphere that are supersaturated with
respect to ice8,9. Even though this would
help to reduce both young and spreading
contrails, such a strategy is likely to lead to
an increase in fuel consumption. It would
be important to make sure that, given the
large dierence in atmospheric lifetime of
carbon dioxide and contrails, the associated
carbon dioxide penalty does not oset in the
longer term the gain obtained by avoiding
contrailformation10.
e results by Burkhardt and Kärcher
might also justify the development of a
novel engine concept that seeks to condense
a fraction of the water vapour in aircra
emissions in a cooling unit before it leaves
the engine11. e condensed water could be
vented in the form of large ice crystals or
droplets that would fall quickly through the
atmosphere. Reducing the content of water
vapour in the engine exhaust would make
contrail formation less likely.
Alternatively, one could make use of the
nding that spreading contrails suppress
the formation of natural cirrus clouds. It
may be possible to accelerate the deposition
of ambient water vapour onto the contrail
ice crystals either by modifying the aircra
wake dynamics or the aerosol and cloud
microphysics in the exhaust plume. If the
lifetime of the contrail cirrus can be reduced
several-fold for the same suppression of
natural cirrus, there could be a net climate-
cooling eect from contrail formation.
Although the work of Burkhardt and
Kärcher3 oers some exciting pointers as to
how the impacts of aviation on the climate
system might be reduced, the uncertainties
remain large. Given the urgency of the issue,
it is important that research on the climate
impacts of contrails and on how contrails
could be mitigated through technological
advances or operational changes in the
aviation industry are pursued in parallel.
Olivier Boucher is at the Met Oce Hadley Centre,
Fitzroy Road, Exeter EX1 3PB, UK.
e-mail: olivier.boucher@metoce.gov.uk;
olivier.boucher@lmd.jussieu.fr
References
1. Lee, D.S. etal. Atmos. Environ. 43, 3520–3537 (2009).
2. Haywood, J.M. etal.J.Geophys. Res.114, D24201 (2009).
3. Burkhardt, U. & Kärcher, B. Nature Clim. Change
1, 54–58 (2011).
4. Boucher, O. Nature 397, 30–31 (1999).
5. Zerefos, C.S. etal. Atmos. Chem. Phys. 3, 1633–1644 (2003).
6. Stubenrauch, C.J. & Schumann, U. Geophys. Res. Lett.
32, L14813 (2005).
7. Hendricks, J. etal. Geophys. Res. Lett. 32, L12814 (2005).
8. Mannstein, H. etal. Transport. Res. D 10, 421–426 (2005).
9. Williams, V. etal. Clim. Policy 3, 207–219 (2003).
10. Forster, P.M. etal. Atmos. Environ. 40, 1117–1121 (2006).
11. Noppel, F. & Singh, R. J.Aircra 44, 1721–1726 (2007).
In the face of political obstacles to
achieving domestic and international
agreements on the reduction of
greenhouse-gas emissions, policymakers
are increasingly looking to individuals to
voluntarily cut their energy use to curb
emissions in the near term1. Unfortunately,
most people living in western countries
fail to install energy-saving technologies,
even if doing so would save them money
in the long run2. Furthermore, they show
little motivation to change their lifestyles in
ways that require personal sacrice. Social
scientists have attributed such reluctance to
engage in energy-ecient behaviour at least
in part to a lack of personal experience of
the impacts of climate change3. Empirical
evidence to support this hypothesis has,
however, been scarce. Writing in Nature
Climate Change, Spence and colleagues4
provide welcome evidence that direct
experience of adverse climate impacts
increases people’s concern about climate
change, as well as their perceived ability to
tackle it and their willingness to act.
In most western countries, people lack
personal experience of climate change,
which is considered to have direct impacts
on people’s lives only in far-away places
or the distant future. is situation
contrasts with that of climate scientists,
whose work can take them to locations
where the impacts of climate change are
clear, and whose training may also make
them less reliant on personal experience
to appreciate the risks. It is plausible that
these eects explain the discrepancy in
views about the magnitude and severity of
the risks associated with climate change
between the general public and climate
scientists5— the majority of whom see the
risks as growing and believe that concerted
action is needed to reduce them6. However,
empirical evidence that personal experience
of a risk motivates action to reduce it has
been thin and inconclusive in the context of
climatechange.
Spence and co-workers4 surveyed a
representative sample of the UK population
to assess their perceptions and beliefs about
climate change, as well as their willingness
to conserve energy. Intense rainstorms have
caused a number of severe oods in the
UK over the past decade or so, and about a
PSYCHOLOGY
Climate change hits home
Engaging the public with climate change has proved dicult, in part because they see the problem as remote. New
evidence suggests that direct experience of one anticipated impact — flooding — increases people’s concern and
willingness to save energy.
Elke U. Weber
© 2011 Macmillan Publishers Limited. All rights reserved
... No CO2 or SOx emissions are released when combusting hydrogen, but NOx is still produced, as explained above. Previous research has identified that 57% of the climatic impact of aviation is a result of contrails, these are condensation trails from aircraft engines that are responsible for capturing and absorbing heat that would otherwise escape into space, and therefore, would constitute a major effect on global warming [39]. ...
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Aviation makes a significant contribution to anthropogenic climate forcing. The impacts arise from emissions of greenhouse gases, aerosols and nitrogen oxides, and from changes in cloudiness in the upper troposphere. An important but poorly understood component of this forcing is caused by ‘contrail cirrus’—a type of cloud that consist of young line-shaped contrails and the older irregularly shaped contrails that arise from them. Here we use a global climate model that captures the whole life cycle of these man-made clouds to simulate their global coverage, as well as the changes in natural cloudiness that they induce. We show that the radiative forcing associated with contrail cirrus as a whole is about nine times larger than that from line-shaped contrails alone. We also find that contrail cirrus cause a significant decrease in natural cloudiness, which partly offsets their warming effect. Nevertheless, net radiative forcing due to contrail cirrus remains the largest single radiative-forcing component associated with aviation. Our findings regarding global radiative forcing by contrail cirrus will allow their effects to be included in studies assessing the impacts of aviation on climate and appropriate mitigation options.
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Aviation emissions contribute to the radiative forcing (RF) of climate. Of importance are emissions of carbon dioxide (CO2), nitrogen oxides (NOx), aerosols and their precursors (soot and sulphate), and increased cloudiness in the form of persistent linear contrails and induced-cirrus cloudiness. The recent Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC) quantified aviation’s RF contribution for 2005 based upon 2000 operations data. Aviation has grown strongly over the past years, despite world-changing events in the early 2000s; the average annual passenger traffic growth rate was 5.3% yr-1 between 2000 and 2007, resulting in an increase of passenger traffic of 38%. Presented here are updated values of aviation RF for 2005 based upon new operations data that show an increase in traffic of 22.5%, fuel use of 8.4% and total aviation RF of 14% (excluding induced-cirrus enhancement) over the period 2000–2005. The lack of physical process models and adequate observational data for aviationinduced cirrus effects limit confidence in quantifying their RF contribution. Total aviation RF (excluding induced cirrus) in 2005 was ~55mW m-2 (23–87mW m-2, 90% likelihood range), whichwas 3.5% (range 1.3–10%, 90% likelihood range) of total anthropogenic forcing. Including estimates for aviation-induced cirrus RF increases the total aviation RF in 2005–78 mW m-2 (38–139 mW m-2, 90% likelihood range), which represents 4.9% of total anthropogenic forcing (2–14%, 90% likelihood range). Future scenarios of aviation emissions for 2050 that are consistent with IPCC SRES A1 and B2 scenario assumptions have been presented that show an increase of fuel usage by factors of 2.7–3.9 over 2000. Simplified calculations of total aviation RF in 2050 indicate increases by factors of 3.0–4.0 over the 2000 value, representing 4–4.7% of total RF (excluding induced cirrus). An examination of a range of future technological options shows that substantive reductions in aviation fuel usage are possible only with the introduction of radical technologies. Incorporation of aviation into an emissions trading system offers the potential for overall (i.e., beyond the aviation sector) CO2 emissions reductions. Proposals exist for introduction of such a system at a European level, but no agreement has been reached at a global level.
Article
TOVS Path-B satellite data provide information on effective high cloud amount and relative humidity for the period 1987 to 1995. Differences in trends of seasonal mean effective high cloud amount between situations favorable for contrails and for cirrus are analyzed in regions with high and low air traffic density. In regions with especially high air traffic density, a significantly stronger increase of effective high cloud amount is found for situations with sufficiently cold and humid air masses favorable for contrails than for all situations in general or for situations favorable for cirrus. Situations of potential contrails occur in about 5 to 10% of all situations, and their seasonal effective high cloud amount averages lie between about 7% and 22%. Indicators of cirrus increase due to air traffic corresponding to the difference in trends of effective high cloud amount between potential contrail situations and cirrus or all situations are about 2.8%–3.5% and 1.6%–4.7% per decade over Europe and the North Atlantic flight corridor, respectively. Weighted by frequency of potential contrail occurrence, the overall increase amounts to at least 0.20%–0.25% and 0.08%–0.24% per decade over regions with very high air traffic (Europe and the North Atlantic flight corridor, respectively).
Article
Potential cirrus modifications caused by aircraft-produced black carbon (BC) particles via heterogeneous ice nucleation were studied with a general circulation model. Since the role of BC in cirrus cloud formation is currently not well known, hypothetical scenarios based on various assumptions on the ice nucleation efficiency of background and aircraft-induced BC particles were considered. Using these scenarios, the sensitivity of ice cloud microphysics to aviation-induced BC perturbations is studied. The model results suggest that cloud modifications induced by aircraft BC particles could change the ice crystal number concentration at northern midlatitudes significantly (10–40% changes of annual mean zonal averages at main flight altitudes), provided that such BC particles serve as efficient ice nuclei. The sign of the effect depends on the specific assumptions on aerosol-induced ice nucleation. These results demonstrate that, based on the current knowledge, significant cirrus modifications by BC from aircraft cannot be excluded.