Air traffic is expected to double by 2010, leading to significant increases in emissions of carbon dioxide and nitrogen oxides. There is particular concern over NOx emissions because, while aircraft account for less than 3% of anthropogenic releases, at high altitude NOx causes the formation of ozone, a powerful greenhouse gas.
Attempts to quantify aviation's contribution to global warming have been hindered by uncertainty about the photochemistry of the upper troposphere and lower stratosphere - the regions where most aircraft emissions occur - and the variability of ozone concentrations (ENDS Report 237, pp 14-18 ). The latest research, led by Paul Wennberg of Harvard University, sheds new light on the subject.1
Ozone is produced in the troposphere by the oxidation of hydrocarbons by hydrogen radicals. The rate of ozone production increases with the concentration of NOx. The new research, involving detailed atmospheric measurements between October 1995 and August 1996, found far higher concentrations of hydrogen radicals than expected - indicating greater potential to form ozone.
"Increases in the concentrations of NOx from aircraft or biomass burning will lead to the production of significantly more ozone than previously thought," Dr Wennberg concluded.
The researchers identify aircraft and large-scale forest burning in the tropics as the two key anthropogenic sources of NOx in this region of the atmosphere.
The findings raise questions about aviation's contribution to tropospheric ozone formation - currently put at about 10% - and the accuracy of climate change models. Current IPCC estimates are that changes in tropospheric ozone since pre-industrial times have contributed 0.2-0.6 Wm-2 of global radiative forcing. This compares with a combined value of 2.45 Wm-2 for the other main greenhouse gases. Aircraft are also responsible for 2-3% of global carbon dioxide emissions.
In 1996, the IPCC began work on a special report into the contribution of aviation to global warming. This research is largely complete, but a lengthy scientific and political review process means that it will not be published until 1999.
A modest tightening of controls on aircraft NOx emissions within the EC was proposed late last year (ENDS Report 275, p 46 ). But other measures on a broader international front, including a tax on aviation fuel now under discussion, may be needed to curb the industry's contribution to climate change.