Below is an excerpt from a BBC programme called Horizon (link here). It discusses the effect of having all US airplanes (as well as airplanes from other countries as well, I suppose) grounded for three days right after 9/11. They discuss some unquantified temperature measurements from 48 states (about 5,000 measuring stations) from areas "that was[sic] most dominantly affected by the grounding".
Excerpted from the previous link
DR DAVID TRAVIS (University of Wisconsin, Whitewater) We found that the change in temperature range during those three days was just over one degrees C. And you have to realise that from a layman's perspective that doesn't sound like much, but from a climate perspective that is huge.
BCC NARRATOR One degree in just three days no one had ever seen such a big climatic change happen so fast. This was a new kind of climate change. Scientists call it Global Dimming.
Follow the link for a complete context to the comments below.
Hmmmm..... this sounds like a very subjective measurement (areas dominantly affected by the grounding). Though it seems obvious that fewer jets means less pollution and contrails; hence more sunlight reaches the earth's surface; hence more of the sun's energy reaches the earth. Particulates (like coulds) help keep the heat absorbed by the earths surface from radiating back into the sky (this is why, all things being equal, cloudy winter days are usually much warmer that days without clouds). So without the layer of particulates, the earth heats and cools more quickly.
There are several troubling aspects to this. First, its hard to imaging the "global warming" scientists missing out on something this important (see previous posts). Second, what does this do to the "standard" climate models, i.e., is it accounted for?? Third, maybe carbon dioxide is keeping the planet habitable while the climate is really being destroyed (thrown into an ice age) by aircraft contrails.
Another troubling aspect is that somehow these local effects are thought of by scientists as "climatic". What is described in the BBC article is an extremely temporary, local effect which affects the "temperature" of the earth by a degree or more C (temporary because things go back to "normal" when air traffic returns). This effect can be generated by flying a few thousand (not sure on the exact number) airplanes at high altitude for a few hours each day. This means that someone could easily control the amount of energy reaching the earth's surface.
Over all, this seems to create a "hole" in the "climate models".
Apparently its a surprise to scientists that as the amount of pollution (particulates in particular (no pun intended)) increases less sunlight reaches the surface of the earth. This remarkable fact is now called "global dimming". Global dimming means less solar energy is reaching the surface of the earth. Where does it go if not to the surface? Why, its reflected back into space.
Maybe the fact that since the 1970's the amount of particulates has decreased (due, for example, to the clean air act) - hence the dramatic increases "global warming".
Bottom line - no one really knows....