May 21, 2009 - Work by the IPCC (International Panel on Climate Change) has been updated by the same authors. They estimate that aviation accounted for 4.9% of man-made climate impacts in 2005. This contrasts with the 2% figure that is constantly quoted by aviation lobbyists.
Just two years ago the authors came up with a figure of 3% for aviation's worldwide contribution to climate change. They have now revised their estimate for 2005 (David Lee et al 'Aviation and global change in the 21st century'). There are two important changes:
* Including for the first time estimates of cirrus cloud formation
* Allowing for aviation growth between 2000 and 2005
The effect of these is to increase aviation's impacts to:
3.5% without cirrus
4.9% including cirrus
In quite a long and complex paper, the authors estimate the radiative forcing or RF due to aviation emissions and express these as a % of worldwide RF from all sectors. Several gases have climate impacts (some cause cooling rather than warming) and there are considerable uncertainties about the exact impacts and thus wide error limits. The range of uncertainly around the 3.5% figure (excluding cirrus) is given as 1.2% to 10%.
The uncertainties about cirrus formation are particularly great, which is why scientists have previously been reluctant to quote figures. The range of uncertainty around the 4.9% (including cirrus) is 2% to 14%.
The figure of 3.5% (excluding cirrus), includes CO2, O3, CH4, NOx, H2O vapour, contrails, SO4 and soot. The total impact of these is 1.96 times greater than CO2 alone. This illustrates how important it is it to assess the full RF and not just the effect of CO2.
The figure of 4.9% includes cirrus as well as all these other substances. The total impact is then 3.06% greater than CO2. This illustrates even more the importance of looking at all aviation's emissions.
All the figures quoted are for 2005. Because of the high rate of aviation growth, the %s would be higher if re-calculated for 2009.
The relative impact of aviation in the UK is much higher. The government (Department for Transport) estimates that CO2 accounts for 6.3% of total UK emissions and 9.8% of all greenhouse gases, but excluding cirrus. These figures are not on the quite the same basis as the RFs of Lee et al, but they illustrate that aviation is a specially important issue for the UK.
Note - Radiative forcing (RF)
There is no one measure or 'metric' that expresses climate or global warming impacts. Different metrics have different roles and different pros and cons. Radiative forcing (RF) is a measure of the amount of atmospheric warming in a period, eg a year, caused by historical emissions up to that year. Thus the RF due to aviation in 2009 is a function of emissions from aircraft up to 2009. The relationship between emissions and RF is complex because different substances last a different amount of time in the atmosphere. For example, CO2 can last a hundred years or more whereas H2O may only last a matter of days.
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