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.

[partner id="arstechnica" align="right"] 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 planet 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 hot spots of these clouds over the United States 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, 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 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 United States/NASA

Nature Climate Change, 2011. DOI: 10.1038/NCLIMATE1068

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