Science Articles

Uplifting Insights into Aviation and Climate

Sat, 17th May 2008

Why the policy makers might be "plane" wrong...

Helen Rogers

On the 13th November 2007 theAeroplane European Parliament voted to include the aviation industry's carbon dioxide (CO2) emissions in its Emissions Trading Scheme (ETS) from 2011.

The ETS, which has been in place since 2005, is one of the key policies introduced by the European Union to help meet the EU's greenhouse gas emissions reduction target under the Kyoto Protocol, the international convention aimed at reducing greenhouse gases that cause climate change. The approach of the ETS is to put a price on carbon that businesses use or produce, and create a market for carbon trading.

The scheme is divided into phases for which member states must develop a National Allocation Plan (NAP) approved by the European Commission. These plans must set an overall 'cap' on the total amount of emissions allowed from all the installations covered by the scheme. This is converted to allowances - 1 allowance is equivalent to 1 tonne of CO2. The allowances are then distributed by member states to installations in the scheme.

Installations covered by the Scheme are required to monitor and report their emissions. At the end of each year they are required to surrender allowances to account for their installation's actual emissions. They may use all or part of their allocation and have the flexibility to buy additional allowances or to sell any surplus allowances generated from reducing their emissions below their allocation.

The installations are covered by the EU ETS on the basis of CO2 emitting activities they carry out, and include heavy industries such as electricity generation, iron and steel manufacture, mineral processing such as cement making, and pulp and paper processing. Aeroplanes were a glaring omission.

Aviation affects the Earth's climate in a variety of ways. The exhaust gases pumped out by planes contain CO2, which is a well-known climate-change culprit, as well as nitrogen oxides (NOx) that can influence the levels of other greenhouse gases including methane and ozone. In humid, cold air aeroplanes can also form contrails (condensation trails), which affect global temperatures by reflecting the sun’s radiation back into space, or by acting as a blanket, trapping some of the Earth’s heat in our atmosphere.

The European Parliament has attempted to account for these effects through the use of a measure called an uplift factor (UF), where the impact of aviation on the future climate is assumed to be a multiple of its CO2 emissions. In this case they've adopted an uplift factor of 2 – attributing to aviation a climate impact double the CO2 emitted.

So what's the problem? Chiefly, it's that the different components in aircraft emissions act over many different time-scales, so basing an impact calculation on just one time scale can produce misleading results.

For instance, whilst the white contrails that streak across the sky in a plane's wake will be gone in a few hours, the effects of NOx will persist for decades, and the CO2 will circulate for centuries.

This means that when totting up the full climate impact of the emissions, the CO2 should be summed over 100s of years, whereas the NOx and contrails should be considered over a decade or so. In other words, the relative climate importance of CO2 compared with NOx or contrails depends strongly on the time period, or time horizon, over which the emissions are summed.

The uplift factor of 2 used by the European Parliament appears to be based on the so-called “radiative forcing” due to aviation around the year 2000. Radiative forcing is one possible way of quantifying climate change and is based on the impact of all aircraft emissions prior to 2000. It is defined as the difference between incoming solar radiation and the Earth's outgoing radiation due to changes in the atmosphere, and is measured in Watts per square meter. A positive forcing tends to warm the climate, while a negative forcing tends to cool it.

In the case of CO2, emissions will continue to cause climate effects many decades into the future. For the short-lived emissions, on the other hand, the forcing impact will die away rapidly. This approach therefore accounts for the expected impact of the short-lived aviation emissions, but fails to fully account for the long-term impact of CO2 and will therefore underplay the total potential climate effects of aviation.

Aviation Uplift Factors

Aviation: Schematic showing the total climate impact over 100 years of two example species - 'A' (shown in red) and 'B' (shown in blue). The uplift factor (UF, shown in green) is the total climate impact of A+B divided by the climate impact of B. The lifetime of the species B is 10 times longer than the lifetime of species A. Species A and B are indicative of aviation emissions / effects.

Shipping Uplift Factors

Shipping: Schematic showing the total climate impact over 100 years of two example species - 'A' (shown in red) and 'B' (shown in blue). The uplift factor (UF, shown in green) is the total climate impact of A+B divided by the climate impact of B. The lifetime of the species B is 10 times longer than the lifetime of species A. Species A and B are indicative of shipping emissions / effects where, for example, non-CO2 emissions may result in a cooling of the atmosphere and therefore a negative climate impact and uplift factor.

The choice of the time horizon is therefore crucial, but in the case of the radiative forcing approach used by the European Parliament it has not been applied consistently across the different aviation emissions. If the uplift factor were instead to be calculated using the 100-year time horizon adopted in the Kyoto Protocol, and applied systematically across aviation emissions, its value would almost certainly be much less than the factor 2 adopted by the European Parliament.

So can the uplift factor approach be valid? Yes, provided it is understood that a single time horizon must be chosen and applied across all emissions, and then only if aviation’s contribution to climate change is adequately understood.

Taking all of this into consideration, the choice of an uplift factor of 2 by the European Parliament is scientifically flawed and should be reviewed. In fact, an inappropriate choice of an uplift factor could lead to inappropriate design and technology measures being applied within the aviation industry, which may paradoxically result in an increased climate impact.

There are also wider aspects. For example, emissions from international shipping are now becoming part of international negotiations. If the European Parliament were to adopt the same methodology for calculating the uplift factor for shipping, they would obtain a negative uplift factor! This is mainly because sulphur emissions from shipping lead to the formation of short-lived particles that cause a strong cooling. However, as above, over long time horizons the role of CO2 becomes dominant.

The recognition that climate impacts extend beyond CO2 alone is critically important. The key is to ensure that a scientifically rigorous methodology is applied wherever and whatever emissions impact on climate. It appears that this is far from being the case.

This article was written jointly by the following authors (listed alphabetically):

Piers Forster (Director of Research and Roberts Research Fellow in the School of Earth and Environment at the University of Leeds), Rod Jones (Professor of Atmospheric Science and Director of the Institute for Aviation and the Environment at the Department of Chemistry, University of Cambridge), Helen Rogers (Senior Research Associate at the Institute for Aviation and the Environment at the University of Cambridge) and Keith Shine (Professor of Physical Meteorology in the Department of Meteorology at the University of Reading).

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