Saving ozone with a no-go zone: Jumbo jets are spending more time in the stratosphere than previously realised, and their emissions are damaging the ozone layer. Should flights in the stratosphere be banned?
By FRED PEARCE The world’s busiest aircraft flight corridor between North America and Europe borders one of the most threatened sectors of the ozone layer, over the North Atlantic, which thins by up to 25 per cent in late winter. Every day, up to 500 civil aircraft in the corridor burst out of the troposphere and up into the stratosphere, with its fragile ozone layer. About 15 per cent of emissions from all civil flights come from these aircraft, and at least half their emissions issue straight into the stratosphere. What makes these figures significant is the increasing evidence that nitrogen oxides (NOx) and water vapour in these emissions play a key role in destroying ozone. At a symposium in Germany last week, atmospheric chemists debated for the first time whether aircraft should be banned from the stratosphere in order to protect the ozone layer. This could be done by flying lower, they said, or, in the North Atlantic corridor, by taking more southerly routes, where the tropopause – the boundary between the troposphere and stratosphere – is higher. Bad altitude? Robert Sausen of the German government’s Institute for Air and Space Travel (DLR), near Munich, told New Scientist that ‘within five years I think we will be able to draw up flight paths that would avoid emissions from aircraft reaching the stratosphere. After that it will be a political process of persuading the air industry to act.’ NASA and the European Union are now funding research on subsonic aircraft emissions, in an attempt to improve models of how they react in the atmosphere. ‘Nobody has the answer yet,’ says Ann Thomson, who heads the NASA study. ‘But we’ll certainly be studying the issue of different routeings, and especially whether flights should be above or below the tropopause, that is the major issue.’ Finding a solution will not be easy, not least because in the troposphere aircraft emissions can act as greenhouse gases. Then there are the demands of the airlines, which want to keep fuel costs down. The lower edge of the stratosphere, bounded by the tropopause, varies in altitude from about 18 kilometres at the equator to between 6 and 8 kilometres over the poles. Concern that aircraft crossing this boundary destroy ozone dates back more than two decades to the construction of the first Concordes, which cruise at around 16.5 kilometres (55 000 feet). But the dispute subsided when Britain and France failed to sell the jet widely and it was all but forgotten in the mid-1980s when halogen compounds such as CFCs were shown to destroy ozone. The recent announcement by European aircraft manufacturers of plans to build a new supersonic aircraft has renewed concern that a fleet of ‘daughters of Concorde’ could trigger greater destruction of ozone. NASA has launched a research programme to consider how supersonic aircraft could be designed to minimise their impact on the stratosphere (‘Green designs on supersonic flight’, New Scientist, 14 August 1993). But it has largely escaped the attention of atmospheric chemists until recently that the present generation of jumbo jets is flying higher than ever to capitalise on the lower fuel costs that come from cruising in thinner air. Today about 40 per cent of all aircraft fuel is burned in the stratosphere. For the North Atlantic corridor, this figure may reach as high as 75 per cent, according to recent estimates by Jouke Peper of the Dutch National Aerospace Laboratory. As late as 1992, British Airways put the figure at between 15 and 20 per cent. Aircraft emissions probably play a crucial role in ozone destruction by fuelling the formation of polar stratospheric clouds. Tiny particles in these frozen clouds provide surfaces on which the critical chemical reactions take place that destroy ozone. The clouds are made either of water or nitric acid, derived from the nitrogen oxides. Both NOx and water vapour are thrown out by jet engines. Two years ago, NASA estimated that about 25 per cent of the NOx in the stratosphere originated from aircraft. And in the lower stratosphere, where the ozone destruction is concentrated, this figure may rise to 60 per cent. Aircraft engines also discharge an estimated 80 million tonnes of water vapour into the stratosphere every year. At temperate latitudes in the northern hemisphere, air traffic has increased the amount of water vapour in the lower stratosphere by about 10 per cent over the past thirty years. While excluding aircraft from the stratosphere may improve conditions there, one key question is what damage lower flight paths would do to the troposphere. Emissions of NOx here do not destroy ozone, but create it. Recent studies suggest that aircraft generate their own version of the photochemical smog that motor vehicles create at ground level. According to Ulrich Schumann of the DLR, who organised last week’s colloquium, NOx concentrations at middle latitudes in the upper troposphere may have doubled in recent years, and the air lanes over the North Atlantic and Europe have around 20 per cent more ozone than other areas. This ozone will help to compensate for any losses in stratospheric ozone, but at the cost of causing global warming. Ozone within the troposphere is an increasingly important greenhouse gas, stopping infrared radiation escaping to space. While aircraft produce only about 3 per cent of the NOx in the troposphere, NOx at this altitude stays around longer than at ground level – several days rather than a few minutes or hours. Moreover, in the extreme cold of the upper troposphere, ozone’s radiation-trapping properties are thirty times greater than at ground level, says Colin Johnson at AEA Technology near Oxford. Robert Egli, a consultant in atmospheric chemistry from Schaff-hausen, Switzerland, argues that aircraft emissions could cause up to 8 per cent of the warming caused by greenhouse gases. Water vapour emitted by aircraft is also a powerful greenhouse gas. A molecule of water is two hundred times more effective at trapping radiation from Earth than a molecule of carbon dioxide. In addition, around the tropopause the air is close to saturation with water and a small increase of vapour from aircraft can create wide expanses of thin cirrus clouds that cause even stronger warming. Schumann says at these altitudes, a tiny increase in water vapour concentration that nudges the atmosphere into saturation ’causes changes in local heating rate about 100 times larger than the effect of doubling carbon dioxide’. Such a drastic change will not occur globally, he says, but ‘it nevertheless demonstrates the strong radiative sensitivity of the atmosphere here’. The problem about where aircraft should fly for optimum environmental protection is further complicated because, when flying at lower levels, aircraft burn more fuel. This is partly because the air is thinner and partly because modern aircraft, with small swept-back wings, are designed for optimum performance in the stratosphere. Imposing a ban on flying in the stratosphere would increase the total output of NOx and other pollutants from aircraft by about 7 per cent, according to Lufthansa. Rigid restrictions Even so, last summer Peper said that the stratosphere might be most at risk from emissions. While NOx may stay longer in the upper troposphere than at ground level, it lasts even longer – up to a year – in the stratosphere. Because of this, and the lower density of air in the stratosphere, he says, ‘stratospheric emissions generally cause larger perturbations’ and so cause more concern. Peper estimated the likely impact of rigid restrictions on flying at particular heights. For instance, a ceiling of 10.5 kilometres (35 000 feet) over the North Atlantic would reduce emissions in the stratosphere by only 10 per cent, whereas a 9 kilometre (31 000 feet) restriction would cut emissions there by 40 per cent. Only a restriction to below 5.5 kilometre (18 000 feet) would eliminate pollution in the stratosphere. The uncertainty about how much flying time takes place in the stratosphere arises partly because the altitude of the tropopause varies greatly with latitude, season and the weather in the troposphere. In the main North Atlantic air corridor, a sharp difference also exists between the height of the tropopause north and south of the jet stream, a kind of natural high-speed wind tunnel in the upper atmosphere. North of the jet stream, the tropopause is between 8 and 10 kilometres high; south, it is between 12 and 16 kilometres. Peper found that, during one day when all transatlantic flights were plotted in detail, almost all aircraft flying to America from Europe flew in the stratosphere. Eastbound, half the flying time took place in the upper troposphere, because airliners took advantage of the strong tailwind provided by the jet stream, which is strongest just below the tropopause. Even if flights are restricted to the troposphere, pollution can rise into the stratosphere. Likewise, gases emitted in the stratosphere can fall, depending on weather conditions. Mixing of gases across the tropopause, says Peper, is most efficient in the mid-latitudes where most aircraft fly. And, for this reason, Peper concludes that altitude restrictions on aircraft could be ‘counterproductive’. The result might be that more fuel is burnt, with no net gain to the environment. Sausen suggests that a more sophisticated approach, gearing height restrictions for individual flights to real-time conditions in the atmosphere, could be worthwhile. In a project funded by the German environment ministry, he has attempted to model the environmental impact of a series of flights from Frankfurt to New York, following the fate of emissions for ten days. ‘If there is low pressure below,’ says Sausen, ‘then air is rising and pollution from planes flying near the top of the troposphere could still rise into the stratosphere. But if there is high pressure below, then pollution will descend and planes could fly higher without damaging the ozone layer.’ In future, he says, aircraft might be ordered to 7.5 kilometres (25 000 feet) when flying over low-pressure systems, but permitted higher, up to 11 kilometres (37 000 feet), when flying over high-pressure zones. Peper suggests that the tendency to concentrate aircraft into relatively narrow flight corridors, such as that over the North Atlantic, might also have to end. Concentrations of pollution in particular areas could trigger larger effects than more dispersed pollution. Polar regions might have to be avoided because of their greater sensitivity to pollution. This summer, Airbus, the European aircraft manufacturing consortium, will install equipment on five airliners to monitor concentrations of ozone and water vapour in the atmosphere as they travel the world over the next two years (This Week, 26 March). But so far, airlines and aircraft manufacturers have been reluctant to consider funding research into whether they should change their flight paths to protect the environment. Fast growing The question will not go away, however. The number of civil aircraft flights is projected to grow faster than almost any other part of the world economy. At present, more than two trillion passenger-kilometres are flown each year, a figure that is expected to double in 12 years. Even assuming that half this increase continues to be compensated for by increases in fuel efficiency, this will mean that NOx emissions will grow at 3 per cent a year. Egli says the first priority should be to impose international taxes on aircraft fuel, which is now exempt in virtually every country. That would slow down the growth of air travel by removing its unfair price advantage over other forms of transport. After that, he says,