Jay posted a link to this video: Which suggests that part of the reason that people have been noticing more contrails since the late 1990s is partly due to the introduction of more efficient high-bypass engines with "cooler and wetter" exhaust, which is more likely to produce contrails, as the exhaust is more likely to freeze before it's fully mixed in the ambient air. The video is nicely done, and in large part it follows the classic paper on the subject Experimental Test of the Influence of Propulsion Efficiency on Contrail Formation, Schumann and Busen, 2000. http://elib.dlr.de/9281/1/AIAA-2715-2000.pdf The gist of which is: Now this provides a great answer to "what changed in the late 1990s". However I've never really liked to use it, because I've never been clear as to the exact nature or magnitude of the change. For one thing it's obviously been a very gradual change, over a decade or more, and not the sudden change that people claim. So in that sense it's not really different to the gradual rise in air traffic. For another, the actual amount in increased contrail cover seems to be quite small. The observed difference in formation altitude in the above experiment is just 260 feet, or 650 in ideal conditions. Future increases in engine efficiency might push this to 2,300 feet. But then the paper goes on to say about that: So this very large increase from 0.3 to 0.5 will only increase contrail cover by 20%. This is a lot from a radiative forcing perspective, but does not seem like much in terms of how it might affect the perceptions of individuals. Also it implies that the much smaller increase in efficiencies we've seem with high-bypass engines would result in an even smaller increase in contrail cover. In summary, while changes in engine efficiency have certainly increased the number of contrails, the increase seems to have been pretty small, simply magnifying slightly the gradual increase in air traffic. While it's a nice point to make, I don't think it's very accurate to blame the chemtrailers perceptions of a sudden change in the late 1990s on the introduction of more efficient engines. I think the more accurate reason for this perception is simply an individual tipping point, where either they notice the contrails on a particularly favorable day, and hence pay attention to them, or they hear about the "chemtrail" theory, and start looking for them. Unfortunately this reason is much harder to communicate - so it's tempting to use the "new engines" theory. I just don't think it's entirely accurate. I'll do some more research to try to determine the actual magnitude of the change in contrail cover from engine efficiency, and would appreciate any pointers.