Published: 28th May 2009
A GROUP of experts reporting to the Intergovernmental Panel on Climate Change (IPCC) has increased dramatically the figure it believes aviation contributes to climate change.
In a report published last month, the eight international scientists put aviation's total contribution -'radiative forcing' - in 2005 at 4.9%.
This is well over the 3% in the IPCC's Fourth Assessment Report on the state of global warming, and the 2% often quoted by the industry.
The estimates, based on a comprehensive range of models, include for the first time cirrus cloud formation.
Professor David Lee of Manchester Metropolitan University, a lead author, said: "The IPCC underestimated the impact of aviation by basing their results on 2000 when the 'base year' of all other ‘forcings’ was 2005. When you account for the large increase in traffic, there are significant differences.
"In addition, we did two 2050 scenarios to indicate where we might be going. This is in advance of a much more extensive scenario analysis that we are currently preparing."
The paper ‘Aviation and Global Climate Change in the 21st Century’ paints a complex picture of impacts, with some gases causing cooling and others warming and considerable uncertainties about the exact impacts.
Excluding pollution-induced cloud cover, aviation causes 3.5% of ‘forcing’ via a cocktail of gases - CO2, O3, CH4, NOx, H2O vapour, contrails, SO4 and soot. The total impact of these is 1.96 times greater than CO2 alone. The figure of 4.9% includes cirrus as well as all these other substances. The total impact is then 3.06% greater than CO2.
Most scientists agree that ‘manageable’ climate change means a rise of no more than two degrees celsius rise by the end of the 21st Century.
But Lee and colleagues are concerned about future scenarios of aviation emissions for 2050 that show an increase of fuel usage by factors of 2.7–3.9 over 2000. Simplified calculations of total aviation RF in 2050 indicate increases by factors of 3.0–4.0 over the 2000 value, representing 4–4.7% of total RF (excluding induced cirrus).
Professor Lee added: "Aviation traffic showed incredible recovery after the shock of 2001, growing 38% between 2000-07. Even with the recent financial crisis, air traffic was not affected until mid 2008 and the growth in 2007 was around 5%. I expect that post-financial crisis, we can see swift recovery."
Notes - Radiative forcing (RF)
Radiative forcing is a measure of climate change since the industrial revolution and has changed by 1.6 W/m2 since then, equating to an increase in global mean temperatures of ~0.7 degC. 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.
Aviation and global climate change in the 21st century, published in the Atmospheric Environment journal is authored by David S. Lee a,*, David W. Fahey b, Piers M. Forster c, Peter J. Newton d, Ron C.N. Wit e, Ling L. Lim a, Bethan Owena, Robert Sausen f
a Dalton Research Institute, Manchester Metropolitan University, John Dalton Building, Chester Street, Manchester M1 5GD, United Kingdom
b NOAA Earth System Research Laboratory, Chemical Sciences Division, Boulder, CO, USA
c School of Earth and Environment, University of Leeds, Leeds LS2 9JT, United Kingdom
d Department for Business, Enterprise and Regulatory Reform, Aviation Directorate, United Kingdom
e Natuur en Milieu, Donkerstraat 17, Utrecht, The Netherlands
f Deutsches Zentrum fu¨r Luft- und Raumfahrt (DLR), Institut fu¨r Physik der Atmospha¨re, Oberpfaffenhofen, Germany