June 27, 2019

expert reaction to vapour trails and climate change

Research published in Atmospheric Chemistry and Physics shows that Contrail cirrus radiative forcing is expected to increase significantly over time and will have a significant climate impact by 2050.

Prof David Lee, Professor of Atmospheric Science, Manchester Metropolitan University, said:

“This is an excellent piece of research that builds on a solid development of model development and evaluation. However, when interpreting this in terms of policy and regulation, the devil is in the detail. Comparing the present-day warming from contrail cirrus to that from historical emissions of the sector’s CO2 is not necessarily the most useful thing to do when considering the future. The ‘radiative forcing’ (RF) of contrail cirrus is indeed several times larger than that of CO2 from aviation but comparing the RF of a short-lived climate effect to that of a long-lived greenhouse gas is fraught with difficulties as the effect of CO2 last many thousands of years because of its long lifetime; the warming effect of contrail-cirrus would be a few decades at most. Moreover, the uncertainties on contrail cirrus forcing remain much larger than that of CO2, particularly the ‘effective radiative forcing’ which accounts better for the effect on surface temperature – which the paper acknowledges. Early estimates suggest that the effective radiative forcing of contrails could be around 1/2 the radiative forcing. Reducing the soot emissions through changes to the aircraft engine may help decrease the contrail-cirrus signal but this also has large uncertainties and can actually results in small CO2 increases because of engine technology tradeoffs, although reducing the aromatic content of the fuel could achieve soot reductions with no combustion technology changes. The authors’ caution on ‘tactical avoidance’ of contrail formation is well placed as the atmospheric tradeoff between increased fuel and CO2 emissions from such measures to reduce contrail formation is not well quantified or understood, and will vary over time. CORSIA is a first attempt to offset CO2 emissions from international aviation but even this will not reduce emissions sufficiently for the Paris Agreement goals; the non-CO2 signal of contrail-cirrus may add to warming in a more uncertain way but CO2 should remain the number one policy priority because CO2 emissions are so long-lasting and impacts well-understood”

Prof William Collins, Professor of Meteorology, University of Reading, said:

“While the vapour trails (contrails) made by aircraft may look pretty in the sky they have a strong warming effect on climate that is often neglected. This research highlights the importance of contrails if air traffic were allowed to continue to increase unchecked for the next 30 years. These contrails have so far caused more warming than the CO2 emitted by the same aircraft. However contrails only last a short time in the sky whereas the CO2 persists for centuries, so if we are able to reduce air traffic, or reduce the contrails it produces, the climate benefit would appear quickly. Much research is underway to investigate the feasibility of diverting aircraft above or below regions forecast to be conducive to contrails.”

Prof Paul Williams, Professor of Atmospheric Science, University of Reading, said:

“Aviation makes a small but growing contribution to climate change. In return, we expect climate change to cause bumpier flights by increasing the amount of turbulence in the atmosphere. Air travel looks set to be a victim of global warming as well as a cause.”

Prof Keith Shine, Professor of Physical Meteorology, University of Reading, said:

“There is no surprise that contrails are causing a bigger warming than CO2. The IPCC Special Report on “Aviation and the Global Atmosphere” published in 1999 was already saying this, although (as is still true today) the uncertainties in the contrail forcing are larger than that for CO2. The work by Bock and Burkhardt is, in my view, the most advanced set of calculations on the contrail effect to date.

“One thing to note though, is the longevity of the emissions. If aviation was halted today, the contrails would disappear in a matter of hours. The CO2 emitted by aviation would persist for many decades and so is a longer-term commitment to the impact of aviation on the environment. Nevertheless, contrails currently make a very significant contribution to aviation climate effects.”

‘Contrail cirrus radiative forcing for future air traffic’ by Lisa Bock and Ulrike Burkhardt et al. was published in Atmospheric Chemistry and Physics at 1pm UK TIME on Thursday 27 June 2019.

Declared interests

Prof Shine ‘was a reviewer of this paper and have also collaborated with the author’s institute a fair bit over the years’

No others received.