* I don't believe contrails last more than a few minutes. I think the ones that linger for hours are chemtrails that are said to contain aluminum, barium salts, lead, mercury, mycoplasma, mold spores and other toxins, like ethylene dibromide. Will Thomas says the military now also is spraying out nano-size radio-controlled chips in chemtrails. See http://www.helpfreetheearth.com/news34_chemtrails.html.
* This site http://www.lightwatcher.com/chemtrails/smoking_gun.html mentions an online book on global warming, called Policy Implications of Greenhouse Warming: Mitigation, Adaptation, and the Science Base (1992), by Committee on Science, Engineering, and Public Policy at http://books.nap.edu/books/0309043867/html/index.html, which says on Page 447:
Screening Doesn't WorkScreening Out Some Sunlight
Another option for mitigating a global warming would be to try to control the global radiation balance by limiting the amount of incoming radiation from the sun. This could be done by increasing the reflectivity of the earth, i.e., the albedo. Proposals for increasing the whiteness of roofs and surface features would have some effect, but only a fraction of incident solar radiation reaches the earth's surface and a purposeful change in albedo would have more impact if done high in the atmosphere. According to Ramanathan (1988), an increase in planetary albedo of just 0.5 percent is sufficient to halve the effect of a CO2 doubling. Placing a screen in the atmosphere or low earth orbit could take several forms: it could involve changing the quantity or character of cloud cover, it could take the form of a continuous sheet, or it could be divided into many ''mirrors" or a cloud of dust. Preliminary characterizations of some of the possibilities that might be considered are provided below.
* Since that was written in 1992, it's plausible that part of the purpose of chemtrails has been to screen out solar radiation. But, as seen on Mars in 2001 with its global dust storm, screening can actually increase warming as it did on Mars at that time. And I think the way it happens is that the atmosphere becomes more conductive, so electrical energy is conducted more easily to the planet where it heats it up somewhat. So, if chemtrails were meant to reduce global warming by screening out sunlight, the effort may have backfired, although I don't know that the chemtrail effect has actually been all that significant, compared to the increase in solar energy during the 90s. But it's worth considering.