Eco-Aviation Channel

Uncertainty persists on airline impact on atmosphere, climate

By Geoffrey Thomas
Eco-Aviation Today, September 2009, p.2

"Despite the progress made on modelling aviation's impacts on tropospheric chemistry, there remains a significant spread in model results, and while much progress has been made in the last ten years on characterizing emissions, major uncertainties remain over the nature of particles." This is the key finding of a new report, "Transport impacts on atmosphere and climate: Aviation," published in Atmospheric Environment.

The report states that the last major international assessment of environmental impacts was done by the Intergovernmental Panel on Climate Change in 1999. It adds that scientific advances since the 1999 assessment have reduced key uncertainties, sharpening the quantitative evaluation, yet the basic conclusions remain the same: The climate impact of aviation is driven by long-term impacts from CO2 emissions and shorter-term impacts from non-CO2 emissions and effects, which include emissions of water vapor, particles and nitrogen oxides.

The present-day radiative forcing from aviation is estimated to be some 3.5% of current anthropogenic forcing excluding cirrus cloud enhancement. Including cirrus cloud enhancement, it is estimated to be 4.9% of current forcing.

Citing two IPCC Special Report on Emissions Scenarios compatible scenarios, the report states that future forcings may increase by factors of 3-4 over 2000 levels in 2050. The effect of aviation CO2 emissions on global mean surface temperature lasts for many hundreds of years (in common with other sources), while its non-CO2 effect on temperature lasts for decades.

According to the report, understanding of aviation's impacts on cloudiness also has improved. A limited number of studies have demonstrated an increase in cirrus cloud attributable to aviation, although the magnitude varies.

The authors added that the effect of aviation particles on clouds (with and without contrails) may give rise to either a positive forcing or a negative forcing. The modeling and underlying processes are highly uncertain, although the overall effect of contrails and enhanced cloudiness is considered to be a positive forcing and could be substantial compared with other effects.

The debate over quantification of aviation impacts has progressed toward studying potential mitigation and the technological and atmospheric tradeoffs, the authors noted. However, they found that "current studies are still relatively immature and more work is required to determine optimal technological development paths, which is an aspect that atmospheric science has much to contribute."

They found that total global aircraft fuel usage grew by a factor of 1.8 in 2005 over 1980. According to the report, aviation represented 2.5% of global CO2 emissions in 2005, a slight reduction on the 2.7% in 2000. It warns that a number of future scenarios for 2050 have been constructed since the IPCC 1999 report estimating that CO2 emissions will increase by factors of 2.7 to 3.9 over 2000 levels. While these scenarios match the IPCC report, they are at the higher end of the growth scenarios.

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