by Ars Technica
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
http://www.wired.com/wiredscience/2011/03/contrails-climate/
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Nature Climate Change
- Nature Climate Change
- 1,
- 54–58
- (2011)
- doi:10.1038/nclimate1068
- Received
- Accepted
- Published online
Abstract
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.
Subject terms:
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 1 h, the contrail’s vertical extent is set in CCMod to the model’s layer depth, approximately 700 m, 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 5 h 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 35 yr with the ECHAM4–CCMod climate model (using a time step of 30 min, 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.
http://www.nature.com/nclimate/journal/v1/n1/full/nclimate1068.html#/contrail-cirrus-coverage
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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.
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)
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.
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