During the 3 days that followed 9/11, all the planes were grounded in this country. During those 3 days, scientists analyzed the effect of the absence of contrails from airplanes in the climate, and realized the effect was not only measurable, but beyond anything expected. The temperature difference between days and nights, a value that is normally fairly stable, increased during those 3 days, and decreased back after.
Pollution particles (such as in contrails) decrease daytime temperature by reflecting the Sun heat, and increase nighttime temperature by reflecting heat loss (infrared) back to Earth.
We can expect that over a longer time, we would also see an increase in Summer temperatures, as well as a decrease in Winter temperatures.
Since September, we are experiencing an unprecedented reduction in World oil consumption (and consequently a reduction in particles emission) due to the financial crisis, as can be seen in the dramatic drop in oil prices (from $140 to $40 a barrel).
Is is a coincidence, then, that while we are experiencing one of our coldest winter, Australia is struggling with an unusually hot summer, experiencing one-in-a-century forest fires?
We can expect hot summers and cold winters in the coming years. Summer heat could become a real problem for many parts of this country, as it was in Europe in 2003 (50,000 deaths). If the electric grid cannot keep up with increased demand from air conditioners, we can expect the worse.
Temperate coastal areas such as the PNW would fare a lot better than the rest of the Nation, and could see an influx in population in the coming years.
It could take up to 20 years to see a reduction of greenhouse gases in the upper atmosphere, finally reducing the greenhouse effect. During those 20 years, a lot of changes may occur that could be irreversible.