Chemtrail believers frequently question why contrails are becoming more prevalent today compared to previous decades. The answer of course is the almost exponential growth in world air traffic since 1970, a period just before deregulation began in the USA and elsewhere. Here is an ICAO news release from 1970 regarding traffic figures. http://legacy.icao.int/icao/en/nr/1970/pio197017_e.pdf While the figure of 386 million passengers is significant, the real measure is the RPK's or Revenue passenger kilometres carried out which takes into account both aircraft capacity and the distances flown, to give the true indication, or volume of air traffic in a certain year. In 1970 the RPK figure was 465,000 million RPK's. In 2011 that had increased to 5000,000 million, including a 53% since 2000. This graphic was drawn from ICAO's forecast document. The RPK figure is forecast to more than double in the next 20 years. This is the main reason contrails are becoming more prevalent and will continue to do so in the future.