The Impact of Aviation on Climate

DAVID S. LEE

1 Introduction

The atmospheric impact of aviation falls into two distinct categories: those upon the global atmosphere and those upon local air quality. Further, the impact upon the global atmosphere can be subdivided between climate change and stratospheric ozone (O) depletion. The latter has only been studied from a hypothetical point of view, since this is a potential effect that might arise from a fleet of supersonic aircraft flying in the mid stratosphere. This chapter will focus on the effects of aviation on the global atmosphere and, in particular, climate change. Whilst the scientific understanding of aviation's impacts on air quality is reasonably well understood, specific details that allow robust assessments of aviation impacts on local air quality — perhaps surprisingly — are only poorly quantified and the reader is directed elsewhere for a brief overview.

Interest in aviation's effects on climate has been provoked by the strong growth of the aviation industry, which has outstripped GDP, long-term growth rates of the order 5% per year being sustained. Particular events have been associated with set-backs to this growth: the Gulf conflict in the early 1990s slowed growth but it picked up quickly and returned to the long-term trend within a few years. More recently, the overall economic downturn of the industry, September 11th, and 'SARS' have taken their toll but there are indications that growth rates are recovering. The seasonal patterns of departures between 1997 and 2003 in Europe (Figure 1) provide evidence of this.

Many forecasts of aviation growth have been made, both by the industry and others; typical is that of the UK Department of Trade and Industry (Figure 2), which shows historical and future projected growth to 2020 in terms of the overall capacity.

In developing this overview, and what current research is telling us, it is worth considering some historical aspects — the origins of interest date back perhaps further than one might suspect. Local air quality was the original driver for the development of aircraft engine emissions regulations by the International Civil Aviation Organization (ICAO), first promulgated in 1981 (although earlier local rules to the USA were introduced by the US Environmental Protection Agency in 1973). However, one of the initial drivers of interest in aviation's atmospheric effects was concern in the late 1960s and early 1970s that emissions of nitrogen oxides (NOx, where NOx = NO + NO2) from a (proposed) fleet of supersonic aircraft flying in the stratosphere would significantly deplete stratospheric ozone (O3), resulting in increased exposure of harmful ultraviolet (UV) radiation at the Earth's surface.' The scientific research programmes that this concern initiated were of quite epic proportions and laid many of the modern foundations of our understanding of stratospheric chemistry and physics. A more detailed account of these research programmes, and their development, is given elsewhere. In fact, the US research programme proceeded after the decision had been taken in the US not to build a supersonic transport (or 'SST') and was partly in response to the intentions of the UK and France to build Concorde, and the USSR the Tupolev TU-144. During this early work it was conjectured that the current subsonic fleet may, in fact, impact upon tropospheric O3, following the proposal of Crutzen that in situ production dominated tropospheric O3.

Interest in the potential effects of subsonic aviation ensued in the 1980s and early 1990s. This interest arose because of the growing realization that the upper troposphere and lower stratosphere, where subsonic aircraft cruise, is a rather sensitive region of the atmosphere in terms of its chemistry. Initially, attention was focussed upon the effects of aircraft NOx emissions on tropospheric O3 production. Whereas O3 in the mid to upper stratosphere provides a protective 'shield' against harmful UV radiation, O3 in the upper troposphere and lower stratosphere acts as a powerful greenhouse gas, warming the Earth's surface. More recently, other effects such as those of contrails (condensation trails) have been studied intensively, although studies of contrails and climate can be traced back to the early 1970s.

Contrails are line-shaped ice clouds caused by the emission of water vapour and particles from the aircraft exhaust. Depending principally on the particular conditions of temperature and humidity (strictly, ice-supersaturation), contrails may be very short-lived or persistent, sometimes spreading by wind-shear, sedimentation and diffusion into cirrus-like clouds that are ultimately unrecognizable as having been caused by aircraft. Other effects on climate from associated particle emissions and the enhancement of cirrus clouds have also been suggested.

In 1996, the Intergovernmental Panel on Climate Change (IPCC), at the request of ICAO, announced its intention to assess aviation's effects on the global atmosphere; this was completed in 1999. However, the IPCC was not the first assessment: other previous assessments and syntheses include, e.g. Schumann (1994), Wahner et al. (1995), Friedl et al. (1997), Brasseur et al. (1998). This period saw tremendous activity originating from national/international research programmes and dedicated efforts for the IPCC report. Shortly before the completion of the IPCC assessment, Boeing announced that it no longer intended to pursue the development of an SST, largely on economic and environmental (noise) grounds. This, along with the overspending and overrunning NASA space station programme, precipitated the termination of NASA's Atmospheric Effects of Aviation Programme (AEAP). Subsequently, some activities were restarted in the US, albeit at a much lower budgetary level, primarily on global modelling and engine emissions. In Europe, however, the IPCC aviation report provided a springboard from which several research programmes into atmospheric science and technology were initiated under the European Commission's Fifth Framework Programme and included: PARTEMIS, NEPAIR, TRADEOFF, INCA, AERO2K, SCENIC and CRYOPLANE. The bulk of the efforts of these programmes were directed at subsonic effects/technology, with the exception of SCENIC and minor components of TRADEOFF, which addressed supersonic impacts.

In Section 2, the emissions from aircraft in terms of species and their global nature are described. Section 3 gives a brief description of the climate metric, radiative forcing, followed by specific aviation impact quantification (Section 4). In Section 5, some potential...