Contextualización de los casos de investigación

Aircraft emissions of gases and particles directly into the upper troposphere and lower stratosphere impact atmospheric composition and hence the climate. These gases and particles alter the chemical composition of the atmosphere in a variety of ways, both directly and indirectly. Direct effects of aircraft emissions result from the emissions of CO₂, water vapor (H₂O), nitrogen oxides (NO_x), particulates (mainly soot), sulphur oxides (SOx), carbon monoxide (CO), and various hydrocarbons (HCs). In addition, some of these emissions react with other molecules in the atmosphere. These indirect effects include production of O₃, alteration of methane (CH₄) lifetime, formation of contrails, and cirrus cloudiness (IPCC, 1999).

Research into the potential effects of aviation on the upper atmosphere was synthesized in a special report of the IPCC, Aviation and the Global Atmosphere (IPCC, 1999). The IPCC report was a landmark in the discussion of the effects of air

transport on the global atmosphere. It also gave an estimate of radiative forcing (i.e.

measure of climate change) from various aircraft emissions and their effects. Besides the IPCC report, other assessments and syntheses have been made. Several assessments of the impacts of air transport in Europe and the United States of America (USA) are available (Stolarski et al., 1995; Friedl et al., 1997; Schumann et al., 1997;

Brasseur et al., 1998; Rogers et al, 2002; Schumann, 1994). The impact of air transport on climate was also analysed briefly in the IPCC’s (2001) Third Assessment Report.

These results have been updated by Sausen et al. (2005) based on a recent European Commission (EC) research project, TRADEOFF. The research since the IPCC assessment (1999) has been largely concentrated on affects relating to contrails and cirrus enhancement, although not exclusively. This can be explained by the fact that, at present, the largest aircraft radiative forcing of climate is thought to be through CO₂, NOx, and contrail and cirrus formation, with the largest uncertainties associated with contrail and cirrus effects. Much of this research is still underway.

The impact of air transport on the climate combines with that caused by other anthropogenic greenhouse gas emissions and particulates, and also by natural variability. It is therefore not possible to separate the effect of air transport on climate change from that of all other anthropogenic activities. It is assumed, however, that air transport's contribution to climate change is roughly proportional to its contribution towards radiative forcing (IPCC, 1999). The effect of the air transport impact, which is within the scope of this study (CO2), is explained and quantified below to provide a detailed effect analysis. There are several reasons behind limiting the scope of this study to CO₂ emissions. First of all CO₂ impacts have a long lifetime relative to NO_x and contrail. Second, the impacts of NOx and contrail depend largely on location/altitude and background conditions, and therefore are difficult to assess. Moreover, there is a link in broad terms, between CO2 reductions and NOx reductions for operational measures such as direct routing. Finally, the policy focus so far has been on reduction of CO2 emissions.

Due to the uncertainty of the effects of certain emissions it is difficult to predict the long-term effect of air transport emissions. In addition, climate policies regarding the reduction or limitation of air transport impact on climate change will affect future emission scenarios. Nevertheless, the current trend of air traffic growth will mean that the amount of emissions in the atmosphere will increase drastically. The IPCC report (1999) featured different models of air traffic growth, including high growth, normal growth and low growth scenarios. However, it has to be noted that the reference

Chapter 3 Climate change and air transport

scenario used by IPCC had a traffic growth of 3.1 percent per annum that is less than that seen in the last decade. For example, UK air transport has experienced a 6.6 percent annual growth rate in total domestic and international traffic between 1985 and 1998 (ATAG, 2000) and 8 percent between 2003 and 2004 (DfT, 2006b).

The possible effects that the projected increase in traffic will have on the environment and some of the possibilities we are facing are listed below. However, with the current uncertainties over the effects of many emissions, estimates for 2050 are far from accurate. Issues relevant to this research with 2050 set as a target date are as follows (adapted from IPCC (1999) and GAO (2000)):

• The radiative forcing caused by air transport may increase from 3.5 percent of total radiative forcing at present to as high as 15 percent of the total.

• The earth’s average temperature could rise by approximately 1.6 Celsius by 2050, of which 0.09 degrees would be attributable to air transport. Regional temperature rises could differ greatly from the global mean.

• It is estimated that air transport CO2 emissions in 2050 may represent as much as 3-11 percent of manmade CO2 emissions compared to 2-2.5 percent today.

In addition, in order to highlight the UK White Paper’s low priority to the anticipated environmental impacts of air transport expansion, Upham (2003) shows that with the recommended expansion by 2020, over a quarter of the UK’s CO₂ emissions could be due to air transport alone. While in 2000, aviation was responsible for 6 percent of UK emissions. The conflict between the UK’s current climate and air transport policies has been analysed and presented in the recent paper by the Tyndall Centre for Climate Change (Bows and Anderson, 2007). Analysing the UK Government’s projections of emissions from air transport and other sectors, their research shows how aviation could increasingly become a dominant emissions source.

3.3.1.1 Carbon-dioxide (CO2)

Carbon dioxide (CO₂) is the major greenhouse gas contributing to anthropogenic climate change and air transport is one of the fastest growing sources globally. The amount of CO2 formed from the combustion of aircraft jet fuel is determined by the total amount of carbon in the fuel. Every mass unit of fuel burned produces approximately 3.15 mass units of CO2. The CO2 is an unavoidable end product of the combustion process. CO₂ contributes to the warming of the Earth’s atmosphere by retaining part of the solar radiation reflected off the Earth’s surface. Overall, the way CO2 behaves within the atmosphere is relatively simple and well understood today.

Global air transport used 188 million tonnes (Mt) of fuel in 2004 (Kim et al. 2005), emitting 592 Mt of CO_2. In that year, global CO₂ emissions from fossil fuels were 29000 Mt (Marland et al. 2007), giving an air transport share of two percent. In the UK, the share is 6.9 percent (Defra, 2007). With worldwide growth in air transport estimated at five percent per year, this share of total CO₂ emissions is likely to grow despite improvements in efficiency, particularly if other industry sectors are constrained. The research into the potential impacts of continuing current levels of growth in the UK’s air transport industry on other sectors of the economy, indicates that under current stabilisation profiles all other sectors of the economy “will need to significantly, possibly completely, decarbonise by 2050 if the respective carbon-reduction target is not to be exceeded” (Bows, Anderson and Upham., 2005).

However, as mentioned before, when non-CO₂ effects (such as NO_x and contrail-cirrus) are considered, air transport’s share of anthropogenic climate impact (as measured using radiative forcing) is even higher. IPCC (1999) estimates that the radiative forcing of air transport as a whole is two to four times greater than the radiative forcing of CO2

alone. This estimate means that CO2 makes up only about one-third of forcing from air transport emissions.

In the context of this research, as discussed in Chapter 1, any factors influencing the efficient use of fuel will alter the amount of emitted CO₂. Wind speed and direction can have a significant impact on flight times and trajectories, and hence on the fuel required. The first research objective addresses the impact of the future wind field on CO2 emissions by aircraft. This issue will be addressed in Chapter 5.