Persistent contrails have existed and been known about since aircraft could fly high enough.
http://naca.central.cranfield.ac.uk/report...ca-wr-l-474.pdfReasons why contrails in general and persistant contrails specifically are more prevalent.
1. Jet traffic has doubled a few times since the 70's. It is projected to double again in just 10 years.
2. Jet engines today are more powerful than older models. This means they burn more fuel and consequently have much more water vapor in the exhaust. More water means thicker and more persistent contrails.
3. Jets travel higher now (on average) partly due to increased traffic and partly due to increased power allowing higher flight. Higher flight means colder air which means more and thicker contrails. Read that study from 1942 for more explanation on this.
4. An increase in traffic increases the exhaust put into the atmosphere. This exhaust has a cumulative effect and results in conditions more conducive to contrail formation over the long run. I've read some studies about it and will try to find them, though not everything is available online.
5. Evidence that airlines have changed their engines can be found in the noise regulations that the FAA has put out. Around 2000, Stage 3 regs went into effect. This is a regulation governing the noise output of jet engines and required every airline to either replace their engines or install hush kits. These newer engines are not only quieter but are more fuel efficient meaning again, more water vapor. Incidentally, Stage 4 regs were due to take effect in January of this year requiring even more changes.
Proof of persistent contrails from decades past
1981 NOAA photo of contrail at sunset (persistent spreading contrail from 81)
http://goodsky.homestead.com/files/noaacon1981.jpgMany pictures of persistent contrails here taken from space on early shuttle missions. (starts an autmatic slide show after a few seconds)
http://www.astro.ku.dk/~holger/IDA/STSHH.htmlA navigator's log from WWII. Note in particular mission #24 and this quote "The contrails were dense, persistent - really hard to even see our own squadron."
http://www.geocities.com/CapeCanaveral/1393/andy2.htmlWWII pilot's diary Note mission #33 particularly where it says “Heavy dense persistent contrails”
http://www.100thbg.com/mainpages/crews/crews3/jensen.htmAnd of course this site specifically showing evidence of persisting contrails well before the 90's
http://contrailscience.com/persisting-and-...ding-contrails/It has some quotes from newspapers in the 40's, 50's, and 70's as well
The News, Frederick, MD, March 7, 1944
“Contrails frequently have a tendency to cause a complete overcast and cause rain. In Idaho I have seen contrails formed in a perfectly clear sky and four hours later a complete overcast resulted“One of the earliest reference to contrails covering the sky is from the Mansfield News Journal, August 11, 1957, Page 29:
“Within the past few years, the weather bureau has begun to report the trails as actual cloud layers when there are sufficient trails to cover a portion of the sky.”the Arcadia Tribune, April 29, 1970:
Aircraft contrails begin to streak the normally bright Arizona sky at dawn. Through the day, as air traffic peaks, these contrails gradually merge into and almost solid interlaced sheet of cirrus cloud - an artificial cirrus cloud that is frequently as much as 500 meters deep.All those people are just lying though right?