New Scientist article April 11 1998 (Aircraft emissions)



By Fred Pearce

Excluding aircraft emissions from the Kyoto agreement on global warming is turning out to be an even bigger mistake than some scientists thought at the time.

The draft of a new study by the Inter-governmental Panel on Climate Change (IPCC), circulated for review by selected scientists last week, agrees with earlier estimates that aircraft may be responsible for 5 to 6 per cent of the warming caused by greenhouse gases. But some of the study's authors acknowledge that this figure is based on outdated models of atmospheric chemistry, and that the true figure could be 10 per cent or more. This would mean that aircraft emissions have more than half the global warming potential of emissions from road transport.

The claims will force governments to take aircraft emissions -- which are doubling every 10 years -- more seriously. At present, they are not covered my international agreements limiting emissions of carbon dioxide and other gases thought to cause global warming. The Kyoto Protocol signed last December excludes them because negotiators could not agree on how to allocate responsibility for emissions made during international flights.

Aircraft influence global warming in two main ways.

First, they emit carbon dioxide -- aircraft consume around 3 per cent of the fossil fuels burned worldwide, a sixth as much as motor vehicles.

Second, they produce nitrogen oxides (NOx) that are converted to ozone in the upper troposphere, between 9 and 13 kilometres above the ground, where most aircraft cruise.

While ozone in the stratosphere blocks harmful ultraviolet radiation, lower down in the troposphere it also acts as a powerful greenhouse gas. According to the IPCC draft, the ozone produced by aircraft in the troposphere has as much effect on global warming as the CO2 they produce.

But Paul Wennberg of the California Institute of Technology in Pasadena, a coauthor of the IPCC report's chapter on ozone, says that "production of ozone is occurring faster, probably twice as fast" as assumed in the report. This claim is based on his analysis of data collected by the NASA research ER-2 in 1995 and 1996. The flights found that the upper troposphere contained much greater than expected concentrations of the hydroxyl radical, which oxidises NOx to ozone in a series of reactions driven by light. Using this data, Wennberg and 23 coauthors warned this year in "Science" (vol 279, p49) that current atmospheric models -- including those used in the IPCC review -- were seriously flawed.

Chemists had assumed that hydroxyl in the atmosphere was created largely from water. Because there is little water vapour in the upper atmosphere, hydroxyl concentrations would therefore be low. But Wennberg says that additional hydroxyl is generated by reactions involving a range of

chemicals besides water.

The chair of the group writing the chapter of the IPCC report on ozone, Dick Derwent of the British Meteorological Office in Bracknell, describes Wennberg's paper as "absolutely excellent". He points out, however, that the increased contributions of aircraft emissions to global warming

calculated by Wennberg might be offset by another recently discovered effect -- some researchers have suggested that reactions involving NOx from aircraft emissions destroy methane, a potent greenhouse gas.

(NB: A graph also appears in the article showing relative annual CO2 emissions (in billion tonnes). Reading off the chart, these are, approximately:

Electricity 7.0

Other energy 1.5

Industry 5.0

Heating 3.3

Road transport 4.9

Ships 0.4

Aircraft (There are 3 levels given for aircraft emissions. The first figure is the current 'official' one and it is about 0.6. The IPCC figure is about 1.4, and the Wennberg et al figure is about 2.1)

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