[John H K]

You don't need half the sky to be covered to affect climate with what is effectively artifical cirrus. And given sporadic conditions where contrails will persist for many hours and broaden out, in a high traffic area you could easily get half the sky covered.

[Doctor Stochastic]

The point of the article is that contrails make up about 4-6% of total cloud cover instead of the previously though 1-2%.