Con Trails

Dec 24, 2009

How aircraft emissions contribute to warming

Aviation contributes up to one-fifth of warming in some areas of the Arctic.

Rex Dalton


Aircraft emissions could be having a dramatic effect on the warming of the Arctic.

The first analysis of emissions from commercial airline flights shows that they are responsible for 4–8% of surface global warming since surface air temperature records began in 1850 — equivalent to a temperature increase of 0.03–0.06 °C overall.

The analysis, by atmospheric scientists at Stanford University in Palo Alto, California, also shows that in the Arctic, aircraft vapour trails produced 15–20% of warming.

The results of this analysis are likely to be studied widely as nations attempt to address the impact of commercial aviation on global warming. There are around 35 million commercial airline flights every year. Studies have been conducted in Europe, with airlines coming under increased pressure as European Union leaders consider levying a carbon tax on aircraft emissions. But little research has been conducted on the topic in the United States.

Previous studies have only estimated the impacts of commercial aviation, but this is the first use of actual emissions data — from 2004 and 2006 — to calculate warming from such flights, says Mark Jacobson, a Stanford engineer who presented the analysis on 17 December at the American Geophysical Union's annual meeting in San Francisco, California.

For the latest study, Jacobson and his team developed a model for aircraft emissions that accounts for atmospheric composition, cloudiness and the physical properties of emissions, particularly of black carbon — a major part of soot.

In his presentation, Jacobson explained how the model was applied to a nine-year simulation covering 2004 to 2013, after breaking up flight routes into 300-kilometre-square grids for analysis. The model was able to calculate the characteristics of vapour trails based on the actual particulate size of emissions and their evolution over time.

Many previous studies have assumed that the impact of aircraft emissions was the same everywhere. But the new analysis reveals that aircraft emissions increased the fraction of cirrus clouds where vapour trails were most abundant, and actually decreased the cirrus fraction in several locations by increasing the temperatures in the lower atmosphere, reducing the relative humidity in such locations.

If black-carbon emissions from aircraft could be reduced 20-fold, warming would be halted and a slight cooling would occur from plane-created vapour trails, Jacobson says.

The team's study is being peer reviewed and is expected to be published soon, Jacobson added.

David Fahey, of the Earth System Research Laboratory in Boulder, Colorado — part of the National Oceanic and Atmospheric Administration — says that studies such as Jacobson's are important to fill the gaps in aircraft-emissions data following the nation's previously "muddled" research course.

Fahey says that now European leaders are calling for carbon taxes to be levied on each commercial airline flight, the United States is being driven to catch up on aircraft-emissions research. Some of the EU proposals suggest taxing a flight for emissions along its entire route. This is "absurd", says Fahey. For a more realistic levy, high-quality research is needed on the actual impact of such emissions, he adds.

http://www.nature.com/news/2009/091221/full/news.2009.1157.html
Con Trails (Condensation trails)