Major scientific progress on understanding aviation’s climate change impact, although uncertainties remain
Contrail-cirrus from transatlantic air traffic over Newfoundland, Canada (photo: NASA)
Fri 28 Aug 2009 – Despite considerable advances in understanding the impact of aviation on climate change, significant uncertainties remain in determining the full non-CO2 effects of aircraft emissions. This is the main conclusion of a major scientific assessment report ‘Transport impacts on atmosphere and climate: Aviation’ just published in the journal Atmospheric Environment. This is the first comprehensive international assessment update on the subject since the groundbreaking Intergovernmental Panel on Climate Change (IPCC) report published in 1999. The new report rejects the use of an emissions ‘multiplier’, or index, first introduced in the IPCC report, as a suitable metric for measuring the total impact of aviation emissions on climate.
The climate impact of aviation is driven by long-term impacts from CO2 emissions and shorter-term impacts from non-CO2 emissions and effects, which include the emissions of water vapour, particles and nitrogen oxides (NOx).
The present day (2005) radiative forcing (RF) – the accepted metric used to quantify the climate impact of aviation – has been re-evaluated over the previous estimate for 2000 and is now assessed to be 55 mW m-2 (excluding induced cirrus formation from contrails), which represents 3.5% of total anthropogenic forcing. With aviation-induced cirrus, the RF increases to 78 mW m-2, representing 4.9% of total anthropogenic forcing. Between 2000 and 2005, there was an increase in passenger traffic (in RPKs) of 22.5% with an increase in fuel usage of 8.4% and RF of 14% (excluding cirrus).
Various scenarios calculate future 2050 aviation RF (excluding cirrus) that show an increase by factors of three to four times over 2000 levels.
Global aircraft fuel usage, and therefore also CO2 emissions, have grown by a factor of 1.8 in 2005 over 1980. In 2005, says the report, aviation represented 2.5% of global CO2 emissions, although this was 2.7% in 2000.
The construction of emission inventories and scenarios has become more refined with the incorporation of non-scheduled air traffic and better estimations of the relatively minor component of military emissions. Nonetheless, challenges remain in the estimations because of the complexity of data handling and some issues remain to be resolved, it reports. None of the bottom-up emissions inventories yet match the recorded sales of jet kerosene by the International Energy Agency (IEA), which calculated aviation CO2 emissions were 733 million tonnes in 2005.
A number of future (2050) scenarios have been constructed since the IPCC report, which estimate CO2 emissions will increase by factors of 2.7 to 3.9 over 2000 levels. These new scenarios generally match well those of the IPCC report although the IPCC and other low-growth scenarios now appear less plausible than the mid- to higher-growth scenarios.
The new assessment says a great deal of progress has been made since the 1999 IPCC report on measuring and understanding the formation of non-CO2 emission species. It has been found, for example, that a significant fraction of aircraft engine NOx emissions is primary NO2 emitted on the ground for low power conditions (idle), around 50% or greater. However, this is not a climate issue but one of air quality.
Aircraft emissions are a dominant source of NOx in the upper troposphere and lower stratosphere (9-12km), along with lightning, and result in the production of ozone, which has a stronger radiative effect at these altitudes than on the ground. Aircraft NOx emissions also result in the destruction of a small fraction of ambient methane. Ozone formation and methane destruction results in positive (warming) and negative (cooling) RFs respectively.
Significant research has been carried out since the IPCC report on the impact of aircraft contrails, which are caused by the emission of water vapour and particles into cold ice-supersaturated air, and therefore their occurrence is primarily controlled by environmental conditions. They are initially linear and may result in an increase of cirrus cloud cover if they are persistent, depending on the traffic density, and cause a local and global change in RF. The occurrence of contrails may be predicted with good accuracy. However, contrail coverage is still poorly quantified since the calculations of coverage involve normalization to observations by satellites, which do not ‘see’ contrails below a certain optical depth. Estimates of contrail RF indicate smaller values than the IPCC presented but this is largely a function of optical depth, which has been reduced. The values of optical depth in contrails are not well known and represent a significant uncertainty, states the assessment.
Contrails may spread into non-linear structures that closely resemble cirrus clouds (contrail-cirrus). A number of independent studies have indicated there is an increase in cirrus cloud coverage in regions of heavy air traffic but the data is uncertain and further studies are required. The radiative properties of contrail cirrus are poorly quantified, and need to be confirmed, but are currently considered similar to those of persistent linear contrails.
The effect of aviation particles on clouds (with and without contrails) may give rise to either a positive or negative forcing. Although modelling processes are highly uncertain, the overall effect of contrails and enhanced cloudiness is considered to be a positive forcing, and could be substantial compared with other effects.
The IPCC 1999 report was the first assessment of the aviation sector that produced estimates of radiative forcing (which was defined specifically as the change in RF since the pre-industrial era) for all the forcing agents known to be pertinent to aviation, both for the current conditions at that time (1992) and future scenarios (2050). In a summary for policymakers, the IPCC report stated that over the period from 1992 to 2050, the overall RF by aircraft (excluding that from changes in cirrus clouds) for all scenarios was a factor of two to four times larger than the forcing from aircraft CO2 emissions alone, which formed the basis for a new measure, the radiative forcing index (RFI) for aviation, which has been used by many as a ‘multiplier’ to calculate the true impact of aviation on climate change.
Unfortunately, says the new assessment, some commentators have misunderstood this as an emission metric, i.e. that total RF can be related directly to an annual rate of CO2 emissions, but “this is erroneous”. The use of RFI for other sectors like coal-fired power plants or maritime shipping would likely have a zero or negative RFI in their first decade of growth because the direct and indirect cooling from the sulphate aerosols would offset the warming effect of CO2 emissions, although the CO2 would eventually emerge as the dominant RF. “For this and other scientific reasons, this report rejects RFI as a suitable emissions index.”
The use of RF or RFI as a measure of aviation’s, or any other sector’s, impact on climate also has other problems, says the report. Firstly, the RF used in the ratio is the instantaneous RF and the history of aviation’s forcing of climate evolves with CO2, as it accumulates in the atmosphere, becoming relatively more important over time. The relationship between RF and global mean surface temperature change only applies in steady-state, when the RF is constant, and thus aviation’s 1992 instantaneous RF is neither a measure of the climate change attributable to aviation up to 1992 nor that in the future. Secondly, the IPCC assessment of the aviation sector included non-Kyoto greenhouse gases.
The previous usage, both legitimate and otherwise, of RF and RFI as a climate metric for aviation illustrates the difficulty of quantifying the net climate impact of an activity with numerous forcing agents, which is a basic property of all industrial sectors, says the new report. Suitable metrics depend entirely on the period of attribution and the time frame considered for climate impact. For specific time horizons, the report says the Kyoto metrics that adopt Global Warming Potentials (GWPs) as emissions equivalences are appropriate.
The new report constitutes the aviation assessment component of the European Sixth Framework project ‘ATTICA’, the ‘European Assessment of Transport Impacts on Climate Change and Ozone Depletion’. Lead author of the report is Professor David Lee, Director, Centre for Air Transport and the Environment (CATE), Department of Environmental and Geographical Sciences at Manchester Metropolitan University.
