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1998 Fuel Use and Emissions for Danish IFR Flights

1. Environmental effects

Air pollutant emissions have local, regional and global environmental effects. Local effects are mainly associated with the deterioration of air quality in residential or working areas, while effects on a regional scale are e.g. acidification and eutrophication. On a global level the greenhouse effect and ozone depletion are recognised as the most important environmental problems. The two latter themes are also receiving most attention, when the impact from aviation on the atmospheric environment is investigated.

The greenhouse gases - from both anthropogenic and natural sources - are able to absorb infrared radiation. In this way the emissions change the natural balance of incoming energy from the sun and energy escaping back to space. The amount of greenhouse gas emissions emitted until now and the present emission rate will probably lead to a global warming of the Earth’s surface. The environmental end effects could be raised sea level, flooding of low-lying areas, new climatic stresses to forest, deserts, rangelands and other unmanaged eco-systems. The eco-systems could decline or fragmentize and some specific flora or fauna could be subject to extinction.

Ozone can be measured throughout most of the atmosphere, but are found in high concentrations in the stratosphere especially in a layer situated about 20 km above the Earth’s surface. Stratospheric ozone is very important to life on Earth by blocking most of the harmful ultraviolet light (UV-B) radiated by the sun. Depletion or even removal of the protective stratospheric ozone layer would have severe consequences. Unnaturally high levels of UV-B can cause skin cancer on humans and may reduce crop yields.

When global warming and the depletion of the ozone layer are considered, aircraft emissions in the upper troposphere and lower stratosphere (8-13 km) are met with special concern. At these cruise altitudes the emissions alter the atmospheric concentration levels of the greenhouse gases CO₂, ozone (O₃), methane (CH₄) and water vapour (H₂O); they trigger formation of condensation trails (contrails) and may increase cirrus cloudiness. All these disturbances of the normal atmospheric composition - arising from direct emissions, pollutants formed during different atmospheric reactions or cloud formation - have an effect on the heating of the Earth’s surface. In addition aviation emissions pertubate the ultraviolet radiative balance and cause changes in the total ozone column.

The importance to a potential climate change mechanism can be explained by the concept of radiative forcing. It expresses the perturbation or change to the energy balance of the Earth-atmosphere system in watts per square meter (W m^-2). Positive values of radiative forcing imply a net warming while negative values imply a cooling. A measure for the harmful effects of UV-B is the erythemal dose rate, defined as UV irradiance weighted according to how effectively it causes sunburn.

An evaluation of the environmental effects from aircraft has been made in a special report from the Intergovernmental Panel on Climate Changes (IPCC) "Aviation and the Global Atmosphere" (IPCC, 1999). The report considers all gases and particles emitted by aircraft in the upper atmosphere, their role in modifying the chemical properties of the atmosphere and their ability to trigger the formation of condensation trails (contrails) and cirrus clouds. Subsequently it is explained first how the radiative properties can be modified, as a result possibly leading to climate change, and secondly how the ozone layer could be modified, causing changes in ultraviolet radiation (UV-B) reaching the Earth’s surface.

To put aircraft emissions into future perspectives the report also describes the environmental effect for the years to come as a result of potential changes in aircraft technology, air transport operations, and the institutional, regulatory and economic frameworks. This is done by examining 7 different emission scenarios for the time period 1990-2050. In the following a brief description of the substances contributing to global radiative forcing and UV-B perturbations from subsonic aircraft and the contributor’s predicted end level will be given, according to the IPCC reference scenario for the years 1990 to 2050.

1.1 CO₂

The radiative forcing from CO₂ is the result of the build-up in concentrations from CO₂ emitted in the last 100 years or so. Aviation’s accumulated CO₂ concentration share in 1992 was a little more than 1% of the total concentration increase coming from all anthropogenic emissions. The share is smaller than the actual 1992-emission share, because the emissions only occurred in the last 50 years. The accumulated aviation share is in the IPCC reference scenario predicted to be 4% in 2050.

1.2 Ozone

In 1992 the NO_x emissions from aircraft at cruise altitudes (upper troposphere and lower stratosphere) are estimated to have caused a 6% increase in ozone concentrations in northern mid-latitudes compared with an atmosphere without aircraft emissions. Furthermore the IPCC reference scenario predicts the ozone concentrations to increase to about 13% in 2050. The increase in ozone concentration is substantially smaller in other regions of the world, but will in total tend to heat up the Earth’s surface.

The same quantity of NO_x emissions is more effective at producing ozone in upper-tropospheric and lower-stratospheric altitudes than at surface level. In addition the radiative forcing of the same amount of ozone is stronger at cruise altitudes than at lower altitudes. Taking this into account the reference scenario predicts a 0.4 and 1.2% increase in the total ozone column at northern mid-latitudes in 1992 and 2050, respectively. Adversely, stratospheric aircraft emissions of sulphur and water tend to deplete ozone and this to some degree outbalances the NO_x-induced ozone increase, but how strong this effect is is still not quantified.

1.3 Methane

Tropospheric NO_x emissions decrease the concentration of methane, while ozone is being formed. The fall in methane concentrations tends to cool the Earth’s surface. The methane concentration is 2% smaller in 1992 compared to an atmosphere without aircraft, and according to the IPCC reference scenario the concentrations will be 5% smaller in 2050 compared to an atmosphere without aircraft. However, this decrease in methane concentrations is very small compared to the observed 2.5-fold overall increase since pre-industrial times.

1.4 Water vapor

Most of the water vapour emissions from subsonic aircraft occur in the troposphere. At these flying altitudes the water vapour is removed by precipitation within 1 or 2 weeks. A smaller part of the water vapour is injected into the lower stratosphere. Here it can build up to larger concentrations. Being a greenhouse gas water vapour tend to heat the Earth’s surface, but the overall effect is smaller than for CO₂ and ozone as far as subsonic aircraft are concerned.

1.5 Contrails

Contrails mainly form in the upper troposphere and are initiated by the water vapour emitted by aircraft flying at these cruise altitudes. Contrails have radiative forcings - which similar to high thin clouds - tend to heat up the Earth’s surface. In 1992 the average contrail cover was about 0.1% and this cover is expected to increase to 0.5% in the IPPC reference scenario year 2050. The increase in contrail cover is higher than the projected increase in global fuel consumption. The future aircraft will become more fuel efficient, causing the air traffic to a relatively larger growth in the upper troposphere compared to the growth in global fuel consumption. The radiative effects of contrails are still uncertain, but are dependent upon their optical properties and global cover. The optical properties are determined by the particles emitted or formed in the aircraft plume and the ambient atmospheric conditions.

1.6 Cirrus clouds

Extensive cirrus clouds have been observed to develop after the formation of persistent contrails. A limited number of studies find that the formation of cirrus clouds (beyond those identified as line-shaped contrails) is positively correlated with aircraft emissions. An increase in cirrus cloud cover tends to heat up the Earth’s surface. The knowledge of the mechanisms behind cirrus cloud formation is still very limited, but preliminary estimates of aircraft-induced cirrus cloud cover are 0 to 0.2% of the Earth’s surface and this share are projected to increase by a factor 4 in 2050 according to the IPCC reference scenario.

1.7 Sulfate and soot aerosols

Particulate emissions related to aviation are in principle sulphate and soot aerosols. The total amount of these components is small compared to the emissions from sources at surface level. Even though the particle emissions from aircraft in the reference scenario are projected to increase with the global fuel consumption, their relative emission share of total particulate emissions remains small in the future. Soot tends to cool while sulphate tends to heat the Earth’s surface. However, their direct radiative forcings are small compared with those of other aircraft emissions. Because aerosols influence the cloud formation, the accumulation of aerosols may play a role in advanced cloud formation and may also change the radiative properties of clouds.

1.8 Overall climate effects of subsonic aircraft

The figure 1.1 and 1.2 show the radiative forcing from aircraft in 1992 and in 2050 taken from IPCC (1999), the latter presentation showing the IPCC reference scenario results. Note the difference in axis scaling for the two presentations. The two-third uncertainty ranges of the estimates are also presented, indicating that the true value of radiative forcing lies within the uncertainty range with a probability of 67%. Included in the totals are the effects from changes in concentrations of CO₂, ozone, CH₄, water vapour, contrails, sulphate and soot aerosols, while the possible change in cirrus cloud cover are left out. To each component a relative appraisal of the scientific evidence is made.

In 1992 the best estimate of radiative forcing from subsonic aircraft in total is 0.05 Wm^-2 (true values between 0.01 and 0.1 Wm^-2) or 3.5% of the total radiative forcing by all anthropogenic activities. Largest uncertainties are related to CH₄ and contrails. According to the reference scenario the best estimate of the total radiative forcing would rise to 0.19 Wm^-2 in 2050 or 3.8 times the level in 1992. The 6 remaining IPCC scenarios have best estimates of radiative forcings between 0.13 and 0.56 Wm^-2. These results are a factor of 1.5 less to a factor of 3 greater than that of the reference scenario and 2.6 to 11 times the value in 1992. For the 7 IPCC scenarios the total radiative forcings from subsonic aircraft are between a factor of 2 to a factor of 4 stronger than the radiative forcing from aircraft-induced CO₂ alone. Taking all radiative forcing from anthropogenic activities into account, the effect would be a factor of 1.5 stronger than the effect from CO₂ alone.

Figure 1.1
Radiative forcing from aircraft in 1992 (IPCC, 1999)

Figure 1.2
Radiative forcing from aircraft in 2050 (IPCC, 1999)

1.9 Overall effects of subsonic aircraft on UV-B

Stratospheric ozone blocks most of the harmful ultraviolet light (UV-B) radiated from the sun. The erythemal dose rate is defined as the UV irradiance weighted according to how effectively it causes sunburn. In 1992 the erythemal dose rate is estimated to decrease with 0.5% at 45 ^oN in July by sub-sonic aircraft emissions (mainly due to NO_x) compared with an atmosphere without aircraft. This should be held up against a calculated erythemal dose increase of 4% from 1970 to 1992 due to the overall ozone depletion. Aircraft contrails, aerosols and induced cloudiness give much smaller changes to UV-B. The decrease in UV-B is estimated to be a factor of 4 lower in the Southern Hemisphere than in the Northern Hemisphere.

In the IPCC reference scenario the change in UV-B is -1.3% in 2050 compared to a situation with no aircraft (with a two-thirds uncertainty range from -0.7 to -2.6%).

The change in UV-B from other sources is estimated to be -3% at 45 ^oN from 1970 to 2050. The latter decrease is caused by 1) the incomplete recovery of the ozone layer in 2050 back to the level of 1970 and 2) the expected increase of ozone-precursor emissions in the same period.

Table 1.1 repeats the overview of the emitted components, their role and major environmental effects at Earth’s surface given in IPCC’s Table 1-1 (1999).

Table 1.1
Emission components contributing to climate and ozone change (IPCC, 1999)

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