Wednesday, March 30, 2011

RED FLAG ALERT: More Media Madness on Contrail Disinformation

Contrails Worse for Climate Change Than Airplane Emissions

by Ars Technica Email Author
March 30, 2011
3:23 pm

By John Timmer, Ars Technica

Air travel has come under fire for its potential contributions to climate change. Most people probably assume that its impact comes through carbon emissions, given that aircraft burn significant amounts of fossil fuel to stay aloft. But the carbon released by air travel remains a relatively minor part of the global output—the impact of planes results from where they burn the fuel, not the mere fact that they burn it. A study in the brand-new journal Nature Climate Change reinforces that by suggesting that the clouds currently being generated by air travel have a larger impact on the climate than the cumulative emissions of all aircraft ever flown.

That fact isn’t mentioned in the article at all, however (it’s part of a Nature press release on the paper). What the authors do consider is the fact that carbon emissions are only one of the impacts of aviation. Others include the emissions of particulates high in the atmosphere, the production of nitrogen oxides, and the direct production of clouds through contrail water vapor. Over time, these thin lines of water evolve into “contrail cirrus” clouds that lose their linear features and become indistinguishable from the real thing. Although low-altitude clouds tend to cool the plant by reflecting sunlight, high altitude clouds like cirrus have an insulating effect and actually enhance warming.

To figure out the impact of these cirrus clouds, the authors created a module for an existing climate model (the ECHAM4) that simulated the evolution of aircraft-induced cirrus clouds (they could validate some of the model’s output against satellite images of contrails). They found hotspots of these clouds over the US and Europe, as well as the North Atlantic travel corridor; smaller affects were seen in east Asia and over the northern Pacific. Over central Europe, values peaked at about 10 percent, in part because the output of the North Atlantic corridor drifted in that direction.

On their own, the aircraft-generated cirrus produces a global climate forcing of about 40 milliWatts per square meter (in contrast, the solar cycle results in changes of about a full Watt/M2). But these clouds suppressed the formation of natural cirrus clouds, which partially offset the impact of the aircraft-generated ones, reducing the figure to about 30 mW/M2. That still leaves it among the most significant contribution to the climate produced by aircraft.

Some reports (like one from UPI) have suggested we might focus on making engines that emit less water vapor, but the water is a necessary byproduct of burning hydrocarbon. We’ll almost certainly be accomplishing that as a result of rising fuel prices, and will limit carbon emissions at the same time. The nice thing is that, in contrast to the long atmospheric lifespan of CO2, if we can cause any changes in cloud formation, they’ll have an impact within a matter of days.

Image: Contrails over the southeastern U.S./NASA

Nature Climate Change, 2011. DOI: 10.1038/NCLIMATE1068

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Global radiative forcing from contrail cirrus
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.

Figures at a glance


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.


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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Aircraft contrails stoke global warming, cloud formation

Aircraft condensation trails may be warming the planet on a normal day more than the carbon dioxide emitted by all planes since the first flight in 1903.

By Alister Doyle, Reuters
Tue, Mar 29 2011 at 11:01 AM EST

contrails in the sky

VAPOR: A new study indicates that contrails — white lines of vapor left by jet engines — also have big knock-on effects by adding to the formation of high-altitude, heat-trapping cirrus clouds as the lines break up. (Photo: ZUMA Press)
OSLO - Aircraft condensation trails criss-crossing the sky may be warming the planet on a normal day more than the carbon dioxide emitted by all planes since the Wright Brothers' first flight in 1903, a study said on Tuesday.

It indicated that contrails — white lines of vapor left by jet engines — also have big knock-on effects by adding to the formation of high-altitude, heat-trapping cirrus clouds as the lines break up.

The findings may help governments fix penalties on planes' greenhouse gas emissions in a U.N.-led assault on climate change. Or new engines might be designed to limit Vapor and instead spit out water drops or ice that fall from the sky.

"Aircraft condensation trails and the clouds that form from them may be causing more warming today than all the aircraft-emitted carbon dioxide that has accumulated in the atmosphere since the start of aviation," the journal Nature Climate Change said in a statement of the findings.

The study, by experts at the DLR German Aerospace Center, estimated that the net warming effect for the Earth of contrails and related cirrus clouds at any one time was 31 milliwatts per square meter, more than the warming effect of accumulated CO2 from aviation of 28 milliwatts.
A milliwatt is a thousandth of a watt. Aviation emissions now account for about three percent of annual CO2 emissions from fossil fuels, more than a century since Orville and Wilbur Wright made the first powered airplane flight.

9/11, Iceland

But a key difference is that CO2 lingers for decades while warming from contrails quickly ends if flights are grounded, such as after the September 11, 2001 attacks in the United States, or in Europe after last year's Icelandic volcano eruption.

"You can get rid of contrails very quickly. You can't get rid of CO2 quickly," lead author Ulrike Burkhardt at DLR told Reuters.

The main climate effect of white lines and related cirrus clouds is to trap heat radiating back from the Earth's surface. They also have a smaller, counter-effect by slightly dimming sunlight and so slowing warming. Contrails are especially dense over parts of Europe and eastern United States.
"This is a breakthrough in modeling and understanding of contrails," Olivier Boucher, of the Met Office Hadley Center in England who wrote a related article in Nature, told Reuters.

He said the findings might bring changes in air traffic control, for instance diverting planes from regions or altitudes where air moisture was high and favored cirrus formation.

But a problem was that any benefits of fewer contrails might be canceled out by higher fuel use on longer routes.

He also said that it could spur a novel engine concept that would seek to condense some of the water Vapor "before it leaves the engine. The condensed water could be vented in the form of large ice crystals or droplets that would fall quickly through the atmosphere."

The U.N. panel of climate scientists has estimated that fuel burned at altitude is roughly twice as damaging for the climate as when used at ground level. Boucher said that the study might slightly raise that estimate, adding to potential costs.

(Editing by Mark Heinrich)

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