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24 NATURE CLIMATE CHANGE | VOL 1 | APRIL 2011 | www.nature.com/natureclimatechange
news & views
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 aviation’s climate impact
results from a number of dierent factors,
as well as by the large uncertainty in the
eect that some of these factors have on
climate. Writing in Nature Climate Change,
Burkhardt and Kärcher 3 present a global
modelling study that quanties the climate
eect of ‘spreading contrails’ — the least well
quantied 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 eect 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 eect on climate. Although there
are robust case studies of this spreading
phenomenon using satellite observations2
(Fig.1), its relevance to the climate system
remainsunknown.
Both ground- and satellite-based cloud
observations have suggested a small but
noticeable increase in cirrus cloud cover in
regions of high air-trac 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 dicult 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 diculties, and thus oers a way of
tackling this thornyproblem.
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 38mW m–2. is
can be compared with a radiative forcing of
4mW m–2 from young contrails alone and
28mWm–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 eects 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
(-7mWm–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 inuence
for much longer than contrails, should all
aircra be grounded indenitely. ese
results are intrinsically dicult to validate
against observations, but the authors have
performed a sensitivity study that shows
their results are not signicantly aected by
the contrail spreading rate (±5mWm–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 dierence in atmospheric lifetime of
carbon dioxide and contrails, the associated
carbon dioxide penalty does not oset in the
longer term the gain obtained by avoiding
contrailformation10.
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 eect from contrail formation.
Although the work of Burkhardt and
Kärcher3 oers 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 Oce Hadley Centre,
Fitzroy Road, Exeter EX1 3PB, UK.
e-mail: olivier.boucher@metoce.gov.uk;
olivier.boucher@lmd.jussieu.fr
References
1. Lee, D.S. etal. Atmos. Environ. 43, 3520–3537 (2009).
2. Haywood, J.M. etal.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. etal. Atmos. Chem. Phys. 3, 1633–1644 (2003).
6. Stubenrauch, C.J. & Schumann, U. Geophys. Res. Lett.
32, L14813 (2005).
7. Hendricks, J. etal. Geophys. Res. Lett. 32, L12814 (2005).
8. Mannstein, H. etal. Transport. Res. D 10, 421–426 (2005).
9. Williams, V. etal. Clim. Policy 3, 207–219 (2003).
10. Forster, P.M. etal. 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 sacrice. Social
scientists have attributed such reluctance to
engage in energy-ecient 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 eects 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
climatechange.
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 dicult, 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