Contrails contribute to climate change by interfering with long wave (infrared) radiation escape into space. Contrail coverage can reach 6% in eastern North America, and up to 10% in central Europe.
In a 1999 report, Aviation and the Global Atmosphere, Intergovernmental Panel on Climate Change suggests that the net effect of flying on climate change would be 2 – 4 times that from the carbon dioxide emissions alone. One of the larger contributors was the addition of water vapor high enough in the atmosphere that quick turnover could not occur. The effects of nitrogen oxides and aerosols was also important. Uncertainties were enormous.
Now a new analysis suggests that the average contribution of contrails is 31 milliWatts (1 mW = 0.001 W) per square meter, compared to 28 mW for the total contribution of CO2 from airplanes from the beginning of the jet age.
The original article, Global radiative forcing from contrail cirrus, has added information and some figures.
Neither article suggests if the estimates on aerosols or nitrogen oxides has changed. The IPCC report suggests that there is little net effect from aerosols, as black carbon aerosols and sulfate aerosols produce effects of similar magnitude but opposite sign.
Context: The 1997 IPCC Report from Working Group 1 gives a total radiative forcing of 1.6 W/m2 (big error bars). Forcings are human changes that change the net radiation balance of the Earth, and include positive forcings such as carbon dioxide, methane, ground layer ozone, and black carbon on snow, and negative forcings such as land use change (deserts reflect better than forests) and destruction of stratospheric ozone.
The contributions of flying to date from contrails and carbon dioxide add 59 mW/m2, or about 4% of net forcings*.
* Sometimes people compare to net forcings, sometimes to positive forcings or the largest positive forcing, carbon dioxide.