It's been widely known that aircraft contribute significantly to global warming, thanks to their emissions of heat-trapping gases like carbon dioxide. Aircraft currently account for about 3% of the global human-emitted CO2. But a major new study published this April in Nature
found that at any particular moment, the clouds aircraft leave in the sky--contrails--have a warming effect on the climate that is greater than all the CO2 emitted by aircraft since the dawn of aviation 100 years ago. Aircraft condensation trails (contrails) form when hot, moist air coming out of aircraft engines condenses, creating thin line-shaped clouds of ice crystals. Contrails typically last less than five hours, but can exist for up to seventeen hours. These contrails can then gradually spread out and become long-lived high cirrus clouds. These clouds cool the climate by reflecting incoming sunlight back into space. However, contrail clouds also trap heat energy (long-wave infrared radiation emitted by the Earth's surface) that would otherwise escape to space, and the net impact of contrails is to warm the climate. The new study in Nature
used a global climate model to determine that these so-called spreading contrails cause nine times more climate warming than the original line-shaped contrails. Figure 1.
Aircraft contrails over the UK in March 2009 (left image) drifted south and spread out into contrail cirrus clouds (right image) several hours later. Images taken from the 2009 paper, "A case study of the radiative forcing of persistent contrails evolving into contrail-induced cirrus"
, JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, D24201, 17 PP., 2009 doi:10.1029/2009JD012650.
The accompanying news article in Nature
notes that the results of the new study imply that changes in aircraft flight paths and engine design could significantly lower their impact on climate warming by reducing the amount of contrails and cirrus clouds produced. For example, new aircraft engine designs that emit less water vapor would create fewer contrails. This could be accomplished by having a cooling unit that condenses out water vapor in the exhaust before it is emitted into the atmosphere. The condensed water could be vented in the form of large ice crystals or droplets that would fall quickly through the atmosphere.
Jeff Masters (Ricky Rood will be back once the annual end-of-school-year crunch is past.)