Estimates of Climate Response to Aircraft Emission Scenarios

Presently, the total amount of CO2 emissions from aviation is small compared to the emissions from all anthropogenic activities. Aviation accounts for up to 180 Tg/year of fuel or 2.2% of global anthropogenic CO2 emissions. Higher surface temperatures and a sea level rise are linked to the increase in atmospheric CO2 concentration. Global air traffic emits about 1.9 Tg of nitrogen oxides per year; that is far less than emitted by other sources. Small as they are they nonetheless have notable effects on the NOx concentration near the tropopause and influence the ozone concentration. It seems that aviation induced ozone is particularly effective in heating the troposphere. Computational studies have shown that ozone changes cause regional surface temperature changes large enough to be discernible from climatic noise at the time scale of a few decades. Aircraft also emit water vapor, a greenhouse gas, which increases the relative humidity and hence the tendency to form clouds. On a regional scale that may lead to the formation of long-lived condensation trails which in turn may trigger an increase in cirrus cloud cover. Computations with the global climate model ECHAM show that in regions with heavy air traffic and a typical mean cirrus cloudiness of 20% a 5% increase to 25% causes a noticeable regional temperature change of about IK. At present and on a global scale, surface temperature changes due to the combined effects of aviation emissions are small but they are distinguishable regionally. However, the increase in aviation fuel consumption is substantially larger than the increase in consumption of all fossil fuels, and future growth is expected to continue to surpass all improvements in fuel efficiency. Model calculations were made to evaluate the long time impact of aviation emissions of CO2 and NOx on the atmospheric composition and the climate. The calculations were based on different scenarios reaching from 1992 to 2050 and beyond. (c)l999 American Institute of Aeronautics & Astronautics Past fuel consumption was estimated according to aviation production values provided for 1960 to 1995 by the International Energy Agency, and extrapolated backwards to 1940. Over the past decades, aviation fuel consumption grew on an average by 4% annually. The integral fuel consumption was estimated to have reached 4.6 Pg (4 Pg of carbon) in 1995. During the same time frame CO2 emissions from air traffic added about 1.4 ppmv to the atmospheric CO2 concentration or 1.7% of the total CO2 increase since 1800. Calculations indicate that, by 1995, the global mean surface temperature increased by about 0.004 K and the sea level rose by 0.024 cm due to CO2 emissions, whereas the transient responses to ozone change amounted to 0.03 K in surface temperature and 0.15 cm in sea level in 1995. Future emissions of subsonic air traffic were based on an aviation fuel consumption of 309 and 488 Tg/yr in 2015 and 2050, respectively, and an annual increase rate of I% until 2100. The emission index of NOx was assumed either to stay constant or to vary according to different scenarios. The model then predicts a CO2 concentration change of 13 ppmv by 2100, causing temperature increases of 0.01, 0.02, 0.05 K, and sea level increases of 0.06, 0.15, 0.34 cm in the years 2015, 2050, 2100, respectively, due to CO2 emissions. Including the radiative forcing due to the ozone effects linearly with fuel consumption additionally raises temperatures and sea levels to 0. 11 (0.22) K and 0.71 (1.53) cm by 2050 (2100). The aviation share in anthropogenic CO2 loading of the atmosphere increases from 1.7 % at present to 3.3 % by 2100 in this scenario. The estimated effects on the climate of ozone change due to aircraft NOx emissions amount to up to 5 times that of C02. Therefore ozone changes dominate the climate effects. Accordingly, the reduction of Nox emissions through the introduction of low NOx combustor technology will have a strong impact on mitigating adverse climatic effects. (c)l999 American Institute of Aeronautics & Astronautics ;erman Aerospace Center Aircraft Emissions can Modify the Climate n aircraft emit substances which are radiatiely active (e.g. CO*) l they emit substances which produce or destroy radiatkly active substances (e.g. NO, which modifies the 0, concentration) n the emissions trigger the generation of additional clouds (e.g. contrails). 1.24 kg 3.15 kg 6-30 g O.l-2g 0.7-2.5 g 0.1-0.7 g O.Ol-O.lg Combustlon products per kg kerosene at cruise altltuds (depending on operating conditions) n.Ym.9~.~-ierman Aerospace Center Aircraft Emissions H Aircratt contributed 1.9% to all an 1995. The mean a between 1940 and factors associated with economic growth. E For comparison, U.S. aviation fuel use is growing three times faster than use of motor gasoline. 1 Aircraft emissions of NO, account for similar growth, and emission of aerosol and particulates are also of concern.