Julian Moxon/PARIS
Look at the sky on a clear day and you are likely to see contrails produced by high-flying aircraft, their criss-cross patterns melting slowly to form light, wispy cirrus-type clouds before they disappear.
The phenomenon, triggered by water vapour and particles in the engine exhaust plume, is one of many being studied by scientists grappling with how aircraft may affect the global climate. In the past 40 years, aircraft fuel consumption, the most obvious quantifier of aircraft pollution, has been reduced by 70%, even if the main driver has been commercial, since fuel accounts for by far the largest part of the direct operating costs of an aircraft.
Smoke from exhausts has been virtually banished and engine noise cut considerably.
Major advances in engine and airframe efficiency and in materials technology have been introduced with every new generation of aircraft. Most in the industry agree that as long as it makes commercial sense those advances will continue to be incorporated into new aircraft and the engines that power them.
There is little, it would seem, for the aviation community to worry about. This view is challenged, however, in a recent report carried out for the International Civil Aviation Organisation (ICAO) by the Intergovernmental Panel on Climate Change (IPCC). The report, Aviation and the Global Atmosphere, is more than 300 pages long and is by far the most comprehensive assessment carried out on the effects of aviation on the global atmosphere.
The report considers not only the gases and particles that aircraft engines emit into the atmosphere, but the role they play in modifying its chemical properties. It looks at the potentially damaging effect of the unavoidable increase in contrails as high-flying traffic grows and at the associated formation of long-lived high cirrus clouds, each of which may change the delicate balance between the amount of heat entering or leaving the atmosphere.
The study of that balance is fundamental to assessments of the "greenhouse effect", for which the pollutants are responsible. Different emissions have different chemical and physical effects. If that effect changes the heat exchange process, the emissions become culpable of what atmospheric scientists call "radiative forcing". This is the basic measurement of the greenhouse effect. The report goes into considerable depth on the likely amount of radiative forcing, and therefore the contribution to the greenhouse effect, from the entire range of known aircraft pollutants.
Aircraft are unique in that of all of mankind's machines, they alone operate in the upper troposphere and the lower stratosphere at between 9km and 13km altitude, where weather processes occur and where there are significant amounts of the greenhouse gas, ozone.
Today, aircraft consume only 2-3% of the total of fossil fuels used worldwide, 80% of which is used by civil aviation. This equates to 13% of fossil fuels used by all forms of transport, making it second only to the road transport sector, which consumes 80%.
While the pollutants are known, their effects on the atmosphere are not. Carbon dioxide, for example, is one of the best understood of the global warming pollutants and a major component of aircraft exhaust. It stays in the atmosphere for decades or even centuries - yet the effect of aircraft-induced carbon dioxide on the atmosphere is just beginning to be quantified.
Aircraft may also be unique in a way that could one day fundamentally affect the way they are operated. Aviation is growing at around 5% a year and is predicted to keep expanding at that rate until at least 2015.
Turbine-powered aircraft burn kerosene, and because there are still no alternative fuels that are as energy-intensive and easily stored, will have to continue doing so for the foreseeable future. Hydrogen is being studied, but its use for commercial aviation would require such an enormous change in infrastructure and airline fleets that it is not considered viable as an aviation fuel for several decades at least.
This leaves aviation with an awkward problem: as environmental pressures force changes to less polluting fuels for other fossil fuel-burning machines, it becomes more exposed to future mechanisms designed to limit, or at least charge for, the pollution it is causing. One estimate, contained in the IPCC report, is that aviation's 3% contribution to global warming may increase to between 10% and 20% of the total by 2005. Under the future, more environmentally conscious regime, polluters will have to pay for the share of the environmental damage they cause. Aviation may therefore have to pay proportionately more than other polluters for its unavoidable use of fossil fuels.
"We will have to ask ourselves how important aviation is to the global economy and to our quality of life," says David Griggs, an atmospheric scientist at the UK Meteorological Centre, Bracknell, and one of the report's authors. "Then, we will have three choices: either to leave it alone completely, to try and do something and let it go on growing, or simply to have less aviation."
The report's projections for aircraft pollution take into account the likely cuts in fuel consumption resulting from improved engine and airframe technology over the next few decades and an air traffic management system that allows aircraft to use the most efficient routes. Aviation may, therefore, be left with no choice but to find other ways of reducing its emissions. These include emissions trading, a market-based approach in which an overall level of emissions production is set, within which companies (in this case airlines) are allowed to meet that standard in any way they choose. They would be able to sell credits for any emissions reduction below that level to users who cannot.
Other methods could include direct charges for emissions, ticket taxes, levies on empty aircraft seats, excess traffic to destinations and type of aircraft serving them, and subsidies to provide incentives to reduce pollution.
Whatever decisions are made, the result for aviation is bound to be tougher legislation and increased costs, possibly leading to major changes in the way aircraft are viewed by a public that has become used to low-cost, go anywhere travel. The European Commission is already studying a kerosene tax, which European airlines complain would put them at a "serious disadvantage", while having "negligible environmental benefits". They point out that increases in fuel prices have done nothing to improve environmental performance, calling instead for government action to improve the efficiency of air transport through measures such as putting Europe's air traffic control system under a single authority.
The European Commission has introduced a controversial c125,000 ($131,000) programme to "promote the introduction of price measures to reduce the environmental impact of aviation". This infuriated the industry, which says it has a far better environmental performance than road transport, and is facing disproportionate charges for emissions. Aviation also faces a challenge from high-speed trains.
With fewer options for reducing emissions than other forms of transport, it appears that aviation faces an environmental surcharge for continuing to operate. Not surprisingly, some of the more aware in the industry are beginning to ask what kind of global air transport system will be feasible, or even permitted, in 2050.
Kyoto Protocol
Existing aircraft pollution standards are mainly aimed at improving air quality around airports and there are no specific standards applying to emissions in the cruise. The lead in cutting ground-level emissions was taken by the USA. Subsequently, ICAO developed international standards and recommended practices for fuel venting and emissions of carbon monoxide, hydrocarbons, nitrogen oxides and smoke from engines operating below 3,000ft.
The only global agreement that could affect aviation emissions in the cruise came with the Kyoto Protocol, signed in 1997, which was aimed at stabilising greenhouse gas concentrations at a level that would prevent dangerous long-term damage to the climatic system. Aviation is not specifically mentioned, but there are two areas which are relevant. Firstly, the protocol requires developed countries to reduce their total national emissions from all sources by an average of about 5% compared to 1990 levels. Secondly, there is a provision calling on developed countries to pursue policies and measures for the limitation or reduction of greenhouse gases from aviation bunker fuels (defined as fuels consumed for international marine and aviation transportation).
A major problem with the first area, which is unresolved, is that once aviation emissions have been quantified, the distinction must be made whether they are from domestic or international flights. If the latter, then how are the emissions allocated to any particular country? Who, for example, is responsible for an aircraft registered in Germany which loads fuel in Montreal and flies to Frankfurt, producing greenhouse gases over several countries on the way?
ICAO has called the Kyoto Protocol a "welcome clarification" of the respective roles of ICAO and the United Nations Framework Convention on Climate Change (UNFCCC). "It is clear that ICAO is the forum where emissions from international aviation are to be addressed," it says. Within ICAO, progress on reducing emissions is the responsibility of its Committee on Aviation Environmental Protection (CAEP), which last year agreed on a revised work programme taking the Kyoto Protocol into account. It has set up groups to study the technical and operational aspects of aircraft emissions and market-based options for reducing them.
The pollutants
Aircraft affect the atmosphere by introducing gases and particles into it and by forming contrails. The emissions include greenhouse gases, such as carbon dioxide and water, that trap terrestrial radiation, as well as chemically active gases that alter natural greenhouse gases, such as ozone and carbon monoxide. Particles may interact directly with the earth's radiation balance or influence the formation of clouds.
The IPCC report points out that detecting aircraft-induced climate change is difficult because the proportion of radiative forcing caused by aircraft is tiny. Considerable research is being done to evaluate this contribution.
Carbon dioxide
The largest and the best understood man-produced agent affecting climate change is carbon dioxide, the production of which has increased almost exponentially since the beginning of the industrial revolution. Because of its exceptionally long lifetime in the atmosphere, carbon dioxide becomes thoroughly mixed and evenly distributed around the planet. It is therefore difficult to separate the contribution made by any particular polluter.
Along with water, carbon dioxide is the most abundant of the products of jet fuel combustion, being an unavoidable product of it. Today, aircraft account for 2.4% of the total produced by all man-made sources, a figure which, because of the increase in traffic, is projected to rise to more than 7% by 2050, even though the production per engine has reduced significantly with the introduction of new technology.
Water
Water emission by aircraft engines leads directly to the formation of the contrails that are characteristic of high-flying aircraft in the cruise.
These emissions are extremely small compared to those from natural atmospheric processes such as convection from land and sea, cloud formation and rainfall, but there is concern that contrails may have a disproportionate effect on global warming. Their effects are only beginning to be understood, even if the amount of global warming they may cause is unclear.
Water remains in the troposphere for around nine days. In the stratosphere it can stay for months and even years, so there is a build-up of aircraft-produced water vapour that might upset the natural hydrological balance. The report highlights two main consequences: a direct effect on the heat exchange process in the atmosphere, and a chemical impact on stratospheric ozone which could increase the occurrence of polar stratospheric clouds at high altitudes.
Contrails are expected to increase more rapidly than fuel consumption because of the rise in the numbers of aircraft flying in the upper troposphere, where they are most likely to form. The IPCC report admits there is still much uncertainty about the increase in greenhouse effect resulting from extra contrail cover. One estimate puts it at around 0.5% of total global coverage (see diagram), six times what it is today. The understanding of the effect on formation of cirrus cloud is even less certain, the best estimate being that it will rise in proportion to the extra fuel burned in the upper troposphere.
Nitrogen oxides
Nitrogen oxides are influential in the chemistry of the atmosphere and in the production and destruction of ozone. The processes by which NOx affects that chemistry are complex and differ according to factors such as season and location. At cruise altitudes, increases in ozone lead to an increased greenhouse effect. The IPCC report found that in 1992, NOx emissions from subsonic aircraft were estimated to have increased ozone concentrations at cruise altitudes by up to 6% and are projected to grow to about 13% of the total by 2050.
NOx production by aircraft is related to combustion temperature and has increased as operating temperatures and pressures have gone up. Improved combustor technology has helped reverse the trend, but as the report points out, the production of NOx is linked to that of carbon dioxide, such that attempts to reduce NOx can increase the amount of carbon dioxide present in the exhaust. New, dual annular staged combustors provide more control over a greater operating range, but are more expensive and suffer from extra weight and complexity. Further research is under way in the USA and Europe to develop "ultra-low NOx" engines which retain low emissions of other pollutants.
Aerosol particles
Engines emit invisible aerosol particles, including soot, metals, sulphuric acid, water vapour and possibly nitric acid and unburned hydrocarbons. These may stimulate chemical reactions in the atmosphere, absorb or scatter radiation and change cloud properties. They can seed contrails and cirrus clouds and may be a factor in increasing cloud cover. The chemistry of aerosol production and its interaction with the atmosphere is little understood.
Source: Flight International
