17 December 2009
Trucks belching emissions are obnoxious, but I don’t feel the same disgust when I see fluffy white contrails in the sky. Maybe I should.
Contrails are the artificial clouds formed by condensation of water and emissions from plane engines. New insights into their effects on climate were discussed during session A42A: Climate and Chemistry Impacts of Aviation and Aerospace Emissions I. Contrails form in ice supersaturation regions in the sky, where the relative humidity is over 100 percent. Their presence changes the sky’s radiative forcing, the energy balance between incoming and outgoing radiation, measured in Watts per square meter. Positive forcing warms the system, while a negative numbers tends to cool it.
Dr. David W Fahey, of NOAA’s Earth Systems Research Laboratory, introduced the session by highlighting the holes remaining in our understanding of contrail’s climate impact. Airplane engine emissions and combustion products react with the local atmosphere, causing changes to the local atmosphere’s radiative forcing. These contrails last between six and eight hours before dissipating into clouds.
In 2005, aviation represented 3.5 percent of anthropogenic radiative forcing, up to 4.9 percent if you include cloudiness caused by the contrails. Future contrail impacts could be two to three times higher by 2050, a 20 percent increase per decade.
Global models show that contrails can create up to 10 percent of total cloud cover in Europe, comparable to natural cirrus cloud coverage, says Ulrike Burkhardt, from the DLR Institute for Atmospheric Physics in Germany. Natural coverage reacts to the extra humidity by reducing their radiative forcing up to two percent in these high traffic areas.
Preliminary model data from Marc Jacobson of Stanford University indicates a temperature increase between .03 and .06 K in the next decade due airplane emissions. This means that aircraft emissions could be responsible for up to 8 percent of surface warming and 14-20 percent of Arctic warming each decade.
There are still several holes in the investigation of contrail effects on climate, but I will never look at them the same again.
–Jennifer Welsh, UC Santa Cruz Science Communications Graduate Student