Tuesday, June 24, 2014


ScienceAlert Staff Monday, 23 June 2014

Using photons, scientists have simulated a way that quantum particles could travel through a wormhole back in time.

Image: edobric/Shutterstock
Physicists at the University of Queensland in Australia have used photons - single particles of light - to simulate quantum particles travelling through time and study their behaviour.
They were hoping to find out more about whether time travel would be possible at the quantum level - a theory first predicted in 1991.
In the study, the researchers simulated the behaviour of a single photon that travels through a wormhole and interacts with its older self. This is known as a closed timelike curve - a closed path in space-time that returns to the same starting point in space but at an earlier time. Their study is published in Nature Communications.
They did this by making use of a mathematical equivalence between two cases, lead author Martin Ringbauer told The Speaker
In the first case, photon one "travels trough a wormhole into the past, then interacts with its older version,” Ringbauer explained. And in the second case, photon two travels through normal space-time, but interacts with another photon that is trapped inside a closed timelike curve forever.
"We used single photons to do this but the time-travel was simulated by using a second photon to play the part of the past incarnation of the time travelling photon," said University of Queensland physics professor Tim Ralph
The research will hopefully help researchers bridge the gap between two critical theories, said Ringbuaer.
"The question of time travel features at the interface between two of our most successful yet incompatible physical theories – Einstein's general relativity and quantum mechanics," Ringbuaer explained.
"Einstein's theory describes the world at the very large scale of stars and galaxies, while quantum mechanics is an excellent description of the world at the very small scale of atoms and molecules,” he added.
According to Einstein’s theory, it could be possible to travel back in time by following a closed timelike curve. However physicists and philosophers have struggled with this theory given the paradoxes such as the grandparents paradox, where a time traveller could prevent their grandparents from meeting, thus preventing the time traveller’s birth in the first place.
But in 1991 it was suggested that time travel in the quantum world would avoid these kinds of paradoxes because the properties of quantum particles are “fuzzy” and “uncertain” - and this is the one of the first times anyone has simulated the behaviour of such a scenario.
“We see in our simulation (as was predicted in 1991) how many effects become possible, which are forbidden in standard quantum mechanics,” said Ringbauer. “For example it is possible to perfectly distinguish different states of a quantum system, which are usually only partially distinguishable. This makes quantum cryptography breakable and violates Heisenberg’s uncertainty principle. We also show that photons behave differently, depending on how they were created in the first place.”



Featured Image
Jason Lamb
Scientists from the Netherlands have done what Einstein said was impossible; they’ve achieved quantum teleportation.

In a paper published in the Journal of Science, physicists at the Kavli Institute of Nanoscience at the Delft University of Technology said they were able to “transport information between two quantum bits separated by three meters, or about 10 feet.” This directly contradicts Einstein’s notion of particle entanglements, and is a breakthrough in quantum mechanics and information delivery.
So what is quantum transportation you ask? Unlike Star Trek-esque transportation, quantum transportation utilizes entanglement, where two objects separated by distance effect one another as if they are part of the same system. In this experiment, scientists were able to send the spin state of an electron, from one electron to another 10 feet away. With hopes to increase this distance to a kilometer, the researchers admit that there are five or six groups currently competing to successfully harness this technology and make it useful for practical application.
Once successfully harnessed, quantum teleportation will revolutionize the way we transmit data and use computers in our daily lives. Without the need of wires or any physical objects to transmit the data, you could instantaneously send and receive information from around the world. That 30GB game download? Instant. That medical information you need so desperately from Russia? It’s here in an instant.
google quantum computer Scientists Achieve Quantum Teleportation for the First Time
[via Slashgear]
It will also create much more powerful computers, and allow us to process calculations that are currently impossible. This would assist medical and scientific research, by allowing us to run simulations much quicker than we are able to at present.
When and if this technology is ready, we’ll be able to access information across the world (and even universe) instantly, without the need to wait. While it’s not quite Star Trek-style transportation, it’s in no way a lesser thing.


Wednesday, June 4, 2014


Nature Climate Change

Published online


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.

At a glance


  1. Contrail-cirrus and young-contrail coverage for the year 2002 as simulated by ECHAM4-CCMod.
    Figure 1
  2. Fraction of contrail-cirrus coverage that is identified as coverage due to young (age [le]5[thinsp]h) contrails.
    Figure 2
  3. Contrail-cirrus radiative forcing and optical depth at 250[thinsp]hPa for the year 2002.
    Figure 3
  4. Change in natural-cirrus coverage due to the presence of contrail cirrus.
    Figure 4


Aviation-induced cloudiness consists of contrail cirrus (of which a subset is line-shaped) and of changes in the occurrence or properties of natural cirrus arising from both the presence of contrail cirrus and increased ice-nuclei concentrations in the upper-troposphere due to aircraft soot emissions. Observations indicate that these changes may have a significant effect on cirrus cloudiness1. Radiative forcing—a measure of the radiative imbalance of the atmosphere caused by a particular forcing agent—due to aircraft-induced cloudiness has been estimated from observed trends in cirrus cloudiness to range approximately between 10 and 80mWm−2 for the year 2005 (refs 2, 3, 4).
Contrail cirrus initially form behind cruising aircraft as line-shaped contrails and transform into cirrus-like clouds or cloud clusters in favourable meteorological conditions, occasionally covering large horizontal areas5, 6, 7. They have been tracked for up to 17h in satellite observations6. They remain line-shaped, and therefore easily distinguishable from natural cirrus, for only a fraction of their lifetime. The impact of aircraft soot emissions on cirrus in the absence of contrails depends on the ice-nucleating properties and the ice-active number concentration of soot-particle emissions. Both of these parameters are highly uncertain8, and whereas the impact of aircraft soot on cirrus has been shown to be statistically significant in terms of cirrus ice-particle-number concentrations9 in a climate model, at present this can not be shown for radiative forcing10.
Contrail cirrus are composed of ice crystals that—similarly to natural cirrus—reflect solar short-wave radiation and trap outgoing long-wave radiation11. For fixed ambient conditions, their radiative effect is mainly determined by their coverage and optical depth12. Contrail cirrus form and persist in air that is ice-saturated13, 14, whereas natural cirrus often require high ice supersaturation to form15. This implies that in a substantial fraction of the upper troposphere, contrail cirrus can persist in supersaturated air that is cloud-free16, 17, thus increasing high cloud coverage1, 11, 18. Remote-sensing studies have estimated line-shaped-contrail coverages as large as a few per cent in regions in which the levels of air traffic are high19, 20, 21. The coverage due to contrail cirrus is as yet unknown because they are difficult to distinguish from natural cirrus in satellite observations11.
The global radiative forcing due to line-shaped contrails has been estimated to amount to 10mWm−2 (6–15mWm−2) for 2005, with a low level of scientific understanding4. The global radiative-forcing estimates for line-shaped contrails22 rely on the scaling of simulated contrail-formation frequency to an observed regional contrail coverage. Assuming the scaling coefficient to be spatially and temporally constant, global contrail coverage can be inferred16, 23. This methodology is not suited to studying the effect of contrail cirrus24. Present studies have been unable to provide a best estimate for the contrail-cirrus radiative forcing.
We have developed a process-based contrail-cirrus module17, 25 (CCMod) in a global climate model, ECHAM4 (ref. 26; see Methods), which enables the simulation of the life cycle of persistent contrails. Contrail cirrus exist alongside and interact with natural clouds and, depending on their overlap with natural clouds, can increase overall cloud coverage. Here, we use our contrail-cirrus module to simulate contrail-cirrus coverage, the associated radiative forcing and resulting changes in the natural cirrus clouds.

Contrail-cirrus coverage

Coverage due to contrail cirrus and that due to young contrails (defined here as up to 5h old) are shown in Fig. 1. Coverage due to young contrails may be compared to the coverage inferred from satellite observations, because young contrails are most likely to be still line-shaped. Using a maximum random overlap scheme, contrail-cirrus coverage (Fig. 1a) amounts to several per cent over large parts of the Northern Hemisphere. Coverage due to persistent young contrails reaches 2% over Europe and exceeds 1% over large parts of the US (Fig. 1b), which is in line with earlier estimates of line-shaped-contrail coverage obtained by an independent technique22. Over central Europe, contrail-cirrus coverage is largest, reaching up to 10%. Although the level of air traffic over the east coast of northern America is as large as over central Europe, contrail-cirrus coverage in the former region is lower, reaching 6%. It is mainly the coverage due to contrails older than 5h that is smaller over the USA than over Europe (see below). This is mainly caused by the fact that many old contrails are advected into central Europe from the North Atlantic flight corridor, an area favourable to contrail formation and persistence, whereas there is little contrail advection towards the eastern part of the US. Furthermore, warmer temperatures over the US reduce the probability of contrail formation so that coverage due to young contrails is also slightly smaller than over central Europe. Over the east coast of southeast Asia, the area in which air traffic density is third largest, young-contrail coverage reaches 0.2% and contrail-cirrus coverage exceeds 1%. Globally averaged contrail-cirrus and young-contrail coverage amount to 0.61% and 0.07%, respectively. Contrail-cirrus coverage is therefore approximately nine times larger than coverage due to young contrails alone.

Figure 1: Contrail-cirrus and young-contrail coverage for the year 2002 as simulated by ECHAM4–CCMod.
Contrail-cirrus and young-contrail coverage for the year 2002 as simulated by ECHAM4-CCMod.
a,b, Coverage due to contrail cirrus (a) and due to persistent young contrails with ages of up to 5h (b), considering young contrails and contrail cirrus of any optical depth. c, Coverage due to visible contrail cirrus with a solar optical depth >0.02. Coverages have been calculated by assuming maximum random overlap among contrails or contrail cirrus alone. Only part of the contrail/contrail-cirrus coverage leads to an increase in overall cloud coverage.

Contrail-cirrus coverage (Fig. 1a) exceeds the coverage due to young contrails (Fig. 1b) significantly. The fraction of the total contrail-cirrus coverage that is due to young contrails (Fig. 2) amounts to 0.11 globally and is spatially very variable. In the area of the North Atlantic flight corridor only a small fraction (0.1–0.15) of the contrail-cirrus coverage is due to young contrails. The fraction of young contrails from the contrail-cirrus coverage lies between 0.15 and 0.25 over central Europe and between 0.25 and 0.4 over the eastern US. Over southeast Asia, a large part of contrail-cirrus coverage is due to young contrails.

Figure 2: Fraction of contrail-cirrus coverage that is identified as coverage due to young (age ≤5h) contrails.
A large fraction of contrail cirrus is optically very thin (solar optical depth <0.02) and can therefore neither be detected by a satellite nor seen with the human eye from the ground27. Owing to their abundance, the radiative effect of such optically thin contrail cirrus may not be negligible28, similar to the effect of optically thin natural cirrus29. The fraction of optically thin contrail cirrus is larger in colder areas (farther north), where ice-supersaturation frequency, at the main flight level (230hPa), is usually larger and the water content of the air is lower. This means that the contrail-formation criterion is more frequently met, but contrails forming and persisting in those areas are less likely to become optically thick. Therefore, the fraction of optically thin contrail cirrus is larger over Europe than over the US and especially over southeast Asia. When considering only contrail cirrus that exceed a threshold optical depth of 0.02, the coverage due to these visible contrail cirrus is fairly similar over the eastern US and central Europe and amounts to above 3% and up to 4%, respectively (Fig. 1c). Over southeast Asia, most contrail cirrus are visible because of the higher specific water content, and coverage due to visible contrail cirrus amounts to 0.5%, exceeding locally 1%. Coverage due to visible young contrails reaches 1% over the southeast US and over central Europe and exceeds 0.2% over southeast Asia25. Globally, coverage due to visible contrail cirrus amounts to 0.23%, whereas coverage associated with visible young contrails amounts to 0.04%.

Contrail-cirrus optical depth and radiative forcing

Stratosphere-adjusted radiative forcing due to both young contrails and contrail cirrus was calculated online within the climate model (see Methods). The former estimate serves solely for comparison with previous results. As CCMod does not allow the calculation of the optical depth of young contrails alone, we assume that the optical depth of young contrails (averaging over 5h) is equal to that of contrail cirrus (averaging over all ages). There are no data available to estimate the difference in optical depth between young contrails and contrail cirrus, but we conjecture that this assumption is likely to lead to an underestimation of young-contrail optical depth and the associated net radiative forcing.
Globally, the long-wave radiative forcing due to contrail cirrus (after correcting the scattering component of the long-wave forcing from the model30) amounts to 47.1mWm−2 and short-wave radiative forcing to −9.6mWm−2, resulting in a net radiative forcing of 37.5mWm−2. This includes the effect of line-shaped contrails. Globally averaged contrail-cirrus optical depth is 0.05. Net radiative forcing of contrail cirrus (Fig. 3a) reaches values larger than 300mWm−2 over the eastern US and central Europe. Over most of the US, Europe, over the North Atlantic flight corridor and also over parts of southeast Asia, net radiative forcing exceeds 100mWm−2. Over much of the northern mid-latitudes contrail-cirrus radiative forcing exceeds 30mWm−2. Maxima in radiative forcing are found in areas of maxima in contrail-cirrus coverage, but radiative forcing is enhanced in areas with large contrail-cirrus optical depth (Fig. 3b). This means that for a fixed contrail-cirrus coverage, radiative forcing is larger over southeast Asia than in the northern mid-latitudes and slightly larger over the eastern US than over central Europe or the North Atlantic flight corridor.

Figure 3: Contrail-cirrus radiative forcing and optical depth at 250hPa for the year 2002.
Contrail-cirrus radiative forcing and optical depth at 250[thinsp]hPa for the year 2002.
Net radiative forcing (without correcting for long-wave scattering) (a) and solar optical depth (b) of contrail cirrus.
Globally the long-wave radiative forcing due to young contrails amounts to 5.5mWm−2. This estimate is in line with an earlier estimate of line-shaped-contrail long-wave radiative forcing using the same model but an independent technique22. The latter estimate is slightly lower because the randomly overlapped visible line-shaped-contrail coverage in the earlier study (0.06%) is slightly smaller than our visible young-contrail coverage using the same overlap assumption (0.07%). Short-wave radiative forcing due to young contrails amounts globally to −1.2mWm−2. This value is about 50% larger than the value we obtain when neglecting the diurnal cycle of air traffic, resulting from the fact that most flights are daytime flights31, 32. Net radiative forcing due to young contrails amounts globally to 4.3mWm−2, which is at the lower bound of the range of radiative-forcing estimates for line-shaped contrails3, 31. This may imply that our contrail-cirrus radiative forcing constitutes a low estimate as well. Differences between our short-wave and long-wave forcing estimates and those in the literature are likely to be due to differences in: (1) the spatial and temporal distribution of contrail coverage and its optical depth resulting from the differences in the parameterization schemes; (2) the background cloud fields and their overlap with the contrails; (3) the flight inventories and their reference years used.
The global net radiative forcing of contrail cirrus is roughly nine times that of young contrails, making it the single largest radiative-forcing component connected with aviation. It is important to note that contrail cirrus have a much shorter lifetime than long-lived greenhouse gases. This difference in lifetime influences the relative importance of contrail cirrus and other forcing agents for climate change when estimating their impact for remote time horizons33.
The uncertainty in the spreading rate effective in ECHAM4–CCMod (ref.  25) introduces an uncertainty in the estimate of contrail-cirrus net radiative forcing of ±5mWm−2. The sensitivity of contrail-cirrus radiative forcing to optical depth and ice-particle shape of contrail cirrus, to the flight inventory and to the model’s radiation code is likely to be similar to that of line-shaped contrails. Therefore, we estimate that the uncertainty related to the first two variables amounts in ECHAM4 to an uncertainty in contrail-cirrus radiative forcing of ~25% (ref.  34) and ~15% (ref. 35), respectively. The sensitivity of global radiative forcing to the flight inventory is probably small35. Furthermore, it has been shown that the radiative response due to contrails varies by ±22% around a multi-model mean36. An estimate of the combined uncertainty of radiative forcing would need to take into account the interdependence between the different uncertainties.

Reduction of natural cirrus coverage and optical depth

Contrail cirrus change the water budget of the surrounding atmosphere and therefore can have an impact on natural clouds. Water vapour that is deposited on ice particles within contrail cirrus is not available for formation and deposition in natural cirrus any longer. Therefore, contrail cirrus have the potential to modulate the optical properties of natural clouds, delaying their onset and replacing them, which may partly offset the direct climate impact of contrail cirrus. Virtually nothing is known about those cloud and humidity changes.
We have carried out long-term integrations using ECHAM4–CCMod with and without prescribing air traffic. At the main flight level at 230hPa, where we would expect a maximum impact of contrail cirrus on the natural-cloud coverage due to the maximum in contrail-cirrus coverage, the two simulations exhibit differences in natural-cirrus coverage in the main traffic areas (Fig. 4a). Reductions in the natural-cirrus coverage of around 2% can be found over northeastern Europe and over the east coast of the US. Over the western part of the main transatlantic air-traffic route, off the west coast of northwest Africa and west of the main contrail-cirrus area of southeast Asia, reductions range between 1 and 1.5%. These changes are different from zero at the 95% significance level (when taking into account serial correlations), whereas the local decrease over southeast Asia is mostly significant only at the 90% level. Furthermore, a statistically significant increase in cirrus coverage can be found over and north of eastern Siberia. In those areas, the 230hPa level lies frequently within the stratosphere. Owing to a cold and moist bias in the polar lower stratosphere, common to many climate models37, the model has a higher sensitivity to the moisture input than would be expected in nature. Therefore, we will not discuss this signal any further. Maxima of decrease in natural-cirrus coverage lie consistently downstream of the maxima in contrail-cirrus coverage (compare with Fig. 1a). The change in cirrus coverage, calculated by vertically overlapping all ice clouds in a column (Fig. 4b), confirms that the natural-cirrus coverage is decreased owing to the presence of contrail cirrus. The decrease is slightly less significant owing to the large natural variability in cloud coverage. We note that over the North Pacific flight corridor natural-cirrus coverage is also significantly changed and that the increase in cirrus coverage over Siberia at 230hPa does not translate into an increase in overlapped cirrus coverage in the area.

Figure 4: Change in natural-cirrus coverage due to the presence of contrail cirrus.
Change in natural-cirrus coverage due to the presence of contrail cirrus.
Change in natural-cirrus coverage at the main flight level of 230hPa (a) and when overlapping all ice clouds within one vertical column (b). Dotted lines indicate the 90% confidence interval and solid lines the 95% confidence interval.
Locally, the decrease in natural-cirrus coverage (over Europe and the US) amounts to up to 10% of the natural-cirrus coverage or up to 20% of the contrail-cirrus coverage. Furthermore, in the main contrail-cirrus areas of North America and Europe, the optical depth of natural clouds is significantly (at the 95% significance level) reduced by up to 10% owing to the presence of contrail cirrus.
Both changes in natural-cirrus coverage and optical depth exert a negative net radiative forcing (cooling), partly offsetting the positive net radiative forcing due to contrail cirrus. The large natural variability in albedo, sea surface temperature, natural clouds and so on, all of which influence the radiative fluxes in the atmosphere, impedes the isolation of the relatively small change in natural-cirrus radiative forcing. Assuming that the decrease in natural-cirrus coverage amounts to approximately one fifth of the global contrail-cirrus coverage (as we have found a maximum of 2% (1.5%) decrease of natural-cirrus coverage downstream of the areas of 10% (6%) contrail-cirrus coverage over Europe (US)), the feedback due to this change in natural-cirrus cloudiness would induce a cooling of approximately a fifth of contrail-cirrus radiative forcing, that is −7mWm−2. This estimate is very uncertain and further work is needed to more reliably quantify the feedback.

Implications for evaluating the impact of aviation

We report a model-based estimate of the global climate impact of contrail cirrus, comprising not only young or line-shaped but also aged, irregularly shaped contrails, and including resulting changes in cirrus cloudiness. Defining the radiative forcing due to contrail-induced cloudiness (CIC) as the contrail-cirrus radiative forcing offset by the natural-cloud feedback results in a radiative forcing by CIC of about 31mWm−2. Net radiative forcing due to CIC constitutes one of the largest single aviation-related radiative-forcing components. This radiative forcing due to CIC together with the timescale on which the climate impact is to be evaluated or reduced needs to be considered33 in aviation climate-impact assessments or mitigation studies, respectively.
Besides the uncertainty due to the treatment of contrail cirrus, our radiative-forcing estimates are also affected particularly by uncertainties related to the model’s representation of upper-tropospheric humidity and clouds. Clouds are influenced by small-scale processes that cannot be resolved by a large-scale climate model and which therefore need to be parametrized. The representation of clouds is a major source of uncertainty in climate simulations4. The same problems also affect the representation of contrail cirrus.
The uncertainty of the radiative-forcing estimates should be evaluated from independent studies based on different models and contrail-cirrus parameterizations. Reducing the uncertainty in the evaluation of contrail-cirrus radiative forcing requires more and better observational data sets24, 38. In the long term, progress in this research area requires advanced representation of natural clouds and humidity in climate models and appropriate data sets for their validation39, 40.


The contrail-cirrus module, CCMod, introduces a new cloud class ‘contrail cirrus’ in the global climate model ECHAM4. It is based on a prognostic treatment of fractional coverage, length and ice water mixing ratio of contrail cirrus25. The processes controlling contrail-cirrus coverage and properties, which are contrail formation below a threshold temperature14, advection, spreading and water deposition, sublimation and precipitation, are parametrized physically consistent with the parametrization of natural clouds25. Of the flight distance, only a fraction (given by the supersaturated area fraction) results in persistent contrails. CCMod simulates the life cycle of those persistent contrails. Contrails are advected by the wind field and remain in (and are limited by) the ice-supersaturated fraction of a grid box, assuming that persistent contrail cirrus predominantly form in large persistent ice-supersaturated areas, such as prefrontal areas, in which they remain for a long time. Supersaturated areas are inferred from the assumptions of subgrid-scale variability given by the cloud scheme17. Contrail cirrus spread proportional to the vertical wind shear and their vertical extent. In nature, the vertical extent is dependent on ice-particle sedimentation and is limited by the thickness of a supersaturated layer. After 1h, the contrail’s vertical extent is set in CCMod to the model’s layer depth, approximately 700m, which is roughly in line with observations41. Contrails dissipate as their ice water content is reduced by sublimation and precipitation. Within the contrail-cirrus cloud class, fractional coverage and length of young contrails (up to 5h old) are tracked independently, allowing the analysis of the coverage due to young contrails for purely validational purposes. The ice water content due to young contrails has not been tracked independently, prohibiting the analysis of the optical depth of young contrails.
The ECHAM4 diagnostic cloud-coverage scheme is relative-humidity based and the cloud water content is prognostic26. Cloud particle fall speeds are dependent on the cloud water content. The model’s water budget was changed to accommodate for the new cloud class25, enabling the simulation of the competition for available water vapour between natural and contrail cirrus. Water vapour deposition, sublimation, precipitation and optical depth of natural cirrus and contrail cirrus are dependent on their respective ice water content. CCMod has been evaluated using satellite and in situ measurements of ice supersaturation (ref. 17 and N. Lamquin et al., manuscript in preparation) and regional observations of line-shaped-contrail coverage25. As only observational data sets of line-shaped contrails and none of contrail cirrus are available, coverage and optical properties of contrail cirrus could not be validated.
Stratosphere-adjusted radiative forcing has been calculated as a difference between different calls of the radiation scheme at each time step in a model run42, allowing the online calculation of radiative forcing due to contrail cirrus. For the radiation calculations, natural clouds and contrail cirrus have been randomly overlapped in the vertical at each model time step, except when clouds existed in neighbouring model levels, in which case clouds were stacked above each other (maximum random overlap). This allows natural clouds and contrail cirrus to overlap each other in the vertical. The coverage due to contrail cirrus shown in Fig. 1 was calculated by assuming maximum random overlap among contrail cirrus alone. Only part of this coverage leads to an increase in overall cloud coverage.
Simulations have been conducted using an hourly resolved version of the global air traffic inventory AERO2k (ref. 43) for the year 2002. Integrations of 10 and 35yr with the ECHAM4–CCMod climate model (using a time step of 30min, a horizontal resolution of T30 and 39 vertical levels) have been carried out to estimate contrail-cirrus coverage and radiative forcing and the feedback of contrail cirrus on natural clouds.


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We thank M. Ponater for providing us with a code for calculating stratosphere-adjusted radiative forcing and for comments and U. Schumann for the diurnal cycle of air traffic. This work was carried out within the DLR project ‘Climate compatible air transport system’.

Author information


  1. Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Physik der Atmosphäre, Oberpfaffenhofen, 82234 Weßling, Germany

    • Ulrike Burkhardt &
    • Bernd Kärcher


The concepts of the parametrization were jointly developed and discussed by U.B. and B.K. U.B. carried out the research and wrote the paper.

Competing financial interests

The authors declare no competing financial interests.

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