Brussels should hold off imposing new rules governing engine emissions of nitrogen oxides until there is a greater body of scientific knowledge about their global warming impact, say its own expert advisers.
Late last year the European Commission told a high-level group of the 42-member state European Civil Aviation Conference that it had received the cautionary advice from Dutch environmental consultancy CE Delft.
CE Delft, the original architect of the EU Emissions Trading Scheme, was commissioned by the EC to conduct a thorough review of the science as well as any future regulatory framework regarding aviation NOx emissions as a prelude to additional rules for airlines.
These so-called flanking measures were to accompany newly minted rules on carbon dioxide emissions contained within the trading scheme by the end of 2008. Their delay is raising the possibility that they may well have been shelved, with one senior source saying: "I could imagine people like the green MEPs being extremely disappointed by that."
A legislative proposal on the preferred mechanism to deal with NOx was always on the cards following the compromise reached last July between the European Parliament and the Council of Ministers on the precise way carbon trading would be imposed on aviation from 2012.
The issue has come under the acute political pressure, with some parliamentary members demanding that the value of carbon credits bought by airlines be doubled in the absence of specific new NOx legislation.
CE Delft concludes that doubt remains over how to measure NOx with any degree of effectiveness even though it is well known that at cruise altitudes NOx increases ozone in the upper troposphere and lower stratosphere with a consequent warming effect, while it also destroys a small amount of ambient methane, creating a cooling effect.
However, although ozone has a much shorter lifetime than methane, the warming effects of ozone dominate areas such as the northern hemisphere, where there are higher levels of aviation activity, whereas the cooling effects of methane decay are global. As a result, NOx's climate impact warms in the northern hemisphere and cools in the southern hemisphere.
Also, because the metric used to compare the climate impact of NOx to CO2 is essentially backward looking, measuring the forcing from the historic atmospheric build-up of aviation CO2, it is not a robust policy metric to gauge global warming potential.
The report authors say a robust, consensus analysis of aviation NOx is needed urgently. "The outcome cannot be predicted of such a hypothetical study, but all things being equal, if such a study were performed, it is likely to take the order of three years," says CE Delft.