April 1, 2011 — andyextance

The “contrails” that decorate the sky as aeroplanes fly by on a clear day are surprisingly powerful drivers of climate change, German scientists have found this week. “Contrails cause more warming today than all the accumulated CO2 from aviation,” said Bernd Kärcher from the German Aerospace Centre. However, Kärcher and his colleague Ulrike Burkhardt emphasise that the warming effect, known as radiative forcing, of contrails and the cirrus clouds they form will be much shorter-lasting than for CO2. “At the moment, warming is more influenced by the contrail cirrus than by the accumulated CO2,” Burkhardt told Simple Climate. “It does not mean the contrail cirrus is more important than the accumulated CO2, because CO2 will be still forcing in 50 or 100 years.”

Contrails are line-shaped ice clouds that form directly behind aircraft jet engines. “Because they appear so quickly, you usually can associate them with an aeroplane, when you look into the sky,” Kärcher explained. “When they age, they might lose their line shape and change their physical properties, evolving into cirrus-like clouds. Together we call them contrail cirrus.” Contrail cirrus has two main competing effects on the Earth’s temperature: reflecting warming radiation from the sun back out into space, and blocking Earth’s outgoing thermal radiation, Kärcher said. “The latter is overwhelming, it’s more important, on average,” he underlined. “Contrail cirrus is acting as a blanket.”

The German scientists are able to say this so confidently because they’ve overcome a major obstacle in understanding contrail cirrus. While line-shaped contrails are easy to observe, it’s harder to distinguish the older cirrus-like contrails from naturally-forming cirrus. As a result, previously the radiative forcing, the net flow of energy into or out of Earth’s atmosphere, had only been calculated for the younger contrails.

Bringing climate into aviation planning

Now Burkhardt and Kärcher, writing in new journal Nature Climate Change, describe how to work out the radiative forcing for all contrail cirrus. They exploit a climate model developed by the German research community, known as ECHAM, which is one of many used by the Intergovernmental Panel on Climate Change. “We take the physical processes that control the evolution of contrail cirrus and treat them as a separate class of cirrus clouds,” Kärcher said. “We integrate a new cloud type into the climate model, treat it similarly to the natural cirrus clouds, and let them interact. This had not been done before.” With contrail cirrus being otherwise difficult to separate from other cirrus clouds, Kärcher believes modelling approaches are the most promising way to study their impact on climate.

The researchers found that contrail-induced cirrus is responsible for almost half of the radiative forcing caused by aviation. However, because clouds are much shorter-lived than CO2 contrail cirrus will only remain one of the largest forcing agents if the number of flights continues to grow. “If aviation is declining, the contrail cirrus radiative forcing will decrease, but the CO2 is still accumulating and will become more important,” Kärcher said. But with aviation currently contributing about 5 percent of all human-caused climate change, and air traffic volume set to increase, contrail cirrus will remain important.

This is the first study to fully evaluate radiative forcing from contrail cirrus, so the German scientists now aim to iron out the uncertainties in their predictions. Additionally, they want other researchers to devise different methods to supply independent estimates, so that they can check their results against each other. Kärcher also hopes that their findings will help climate considerations be included in planning for the future of aviation. “Engineers improve their engines, air traffic management people improve routing and flight planning but climate scientists are often not involved in these activities,” Kärcher said. “Hopefully these findings provide a scientific basis to bring these fields together. As we know that human-induced climate change is happening, every sector, no matter how small it is, should attempt to reduce its pressure on the climate.”