Climate impact of aircraft contrails much larger than was previously thought, says aviation climate scientist
Fri 30 Oct 2009 – According to a leading climate scientist, recent studies have shown the net warming effect of aviation induced cloudiness (AIC) is far larger, up to 30 times more, than had previously been thought and immediate attention should be given to mitigate the impact. Professor Ulrich Schumann, Head of the Institute of Atmospheric Physics at the German Aerospace Centre (DLR), said more focus should be placed on reducing aircraft contrails through improved air traffic management and better weather forecasting. Depending on temperature and humidity, contrails occur inside thin cirrus in the upper troposphere, which is the cruising altitude of most aircraft. By adjusting the flight level, more than 50 percent of contrails could be avoided, said Prof Schumann.
Despite considerable scientific progress in predicting the climate impact of aviation, major uncertainties remain, in particular with respect to contrail cirrus. The range of radiative forcing from aviation induced contrails scatter by a factor of larger than 10. However, the understanding has improved significantly over the past two years.
Contrail and cirrus formation is a highly non-linear process and depends strongly on the scale transition from the plumes emanating from an aircraft’s engines with fresh soot and young contrails into spread cirrus layers. Early studies concentrated on line-shaped contrails, but contrails develop dynamically into cirrus in time-scales of hours during which the line-shaped structure is lost. This scale transition lacked a model that would follow the history of all contrails from the global fleet of aircraft from origin shortly after engine exit until the end of their lifespan.
Schumann and his team have now developed a new Contrail Cirrus Prediction Tool (CoCiP) that can simulate contrail cirrus resulting from a single flight as well as from a fleet of cruising aircraft, flight by flight, regionally or globally. The method predicts contrail cirrus for given air traffic and weather prediction data and describes the life-cycle of each contrail individually.
Contrails are initiated when the ambient atmosphere is humid enough to allow for contrail persistence and the initial plume properties reflect the properties of the originating aircraft. The evolution of individual contrails of cruising aircraft can then be computed using wind, temperature, humidity and ice water content from numerical weather prediction (NWP) output. Radiative cloud forcing can be estimated for the sum of all contrails using radiative fluxes (the amount of energy moving a certain distance from the source in a unit of time) without contrails from NWP output.
So far, CoCiP has been tested in parameter studies in comparison with in-situ, lidar (technology that can measure a diffuse object such as a smoke plume or clouds) and satellite data. The model will next be used for a global evaluation of contrail cirrus radiative forcing and in the future it may be coupled to other global models (with aerosols and chemistry) to assess the total aviation climate impact.
Prof Schumann said the evidence had been collected over two years. “We haven’t published it yet because we want to complete a global assessment and we are waiting right now for input data from Eurocontrol,” he told GreenAir Online at this week’s Greener by Design conference ‘Aviation Faces up to Climate Change’. “However, we do have satellite observations of cloud changes and models to explain in detail what we observe. Altogether, this tells us that the climate impact of aviation is larger than assessed so far.”
Although the impact of CO2 was larger in the longer term, say up to 100 years, the effects of aviation-induced cloudiness had a much stronger impact than CO2 in the short to medium term, even up to the 50-year mark, he said. Even though the largest concentrations of AIC were in Europe and the US at present, the effects are much wider.
Like other leading climate scientists, Prof Schumann is against using a multiplier on CO2 emissions to take account of the impact of non-CO2 effects like contrail cirrus but believes a suitable metric can be found.
Adjusting the flight levels of cruising aircraft to avoid causing contrails can lead to an aircraft flying at a less than optimum fuel performance and so lead to higher emissions of CO2. However, Prof Schumann believes the procedure would provide better and immediate climate gains.