So fly the jets during the day and there's no more global warming </sarcasm>
Seriously, this is more envirowacko tripe. Unless every plane in United's, American's, Northwest's and Delta's inventories were to fly over the same patch of earth continuously, there is no way that half the sky over any patch of earth can possibly be half-filled with contrails.
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.