Chaos Theory by Chromasia.com. click to enlargeAn excellent web site titled The Discovery of Global Warming is provided by the Center for History of Physics of the American Institute of Physics.
Created by Spencer Weart, the Director of the Center, this is the best site I have seen on climate modeling and climate change by far. Fully hypertextual, you'll find everything from the first mention of greenhouse gases to political ramifications and chicanery, to a very detailed description of General circulation models.
But the best is the section on Chaos in the Atmosphere , which provides an enormous amount of detail on atmospheric modeling well before Lorenz, and well beyond. From the intro:
...in the 1950s, work with slightly more complex physical and computer models turned up hints that even quite simple systems could lurch in unexpected ways. During the 1960s, computer experts working on weather prediction realized that such surprises were common in systems with realistic feedbacks. The climate system in particular might wobble all on its own without any external push, in a "chaotic" fashion that by its very nature was unforeseeable.
Skeptics of global warming usually fixate on the idea behind the statement "the climate system in...might wobble all on its own without any external push, in a "chaotic" fashion that by its very nature was unforeseeable." They'll use this to discount any effect of carbon emissions, i.e. human-produced climate change.
The situation is tricky. One can't accuse climate-change deniers of not accepting a scientific model of climate change. Instead, they are using the results of accepted scientific modeling, which appears to show that chaotic variations in climate are to be expected without any human influence.
Weart does a great job of addressing this issue. I'll try to summarize here, but there's no substitute for reading the full article (which I will definitely include as a reading in my upcoming Fall 2007 version of Chaos and Fractals.
Until the future actually came, there would be no way to prove that the modelers understood all the essential forces. If an unlucky combination sent the real climate temporarily into one of the unusual states found in some model runs, that could confuse people about what was happening. But it was not likely to change the eventual outcome. What was no longer in doubt was the most important insight produced by the half-century of computer experiments. Under some circumstances a fairly small change in conditions, even something that seemed so slight as an increase of greenhouse gases, could nudge climate into a severely different state. The climate looked less like a simple predictable system than like a confused beast, which a dozen different forces were prodding in different directions. It responded sluggishly, but once it began to move it would be hard to stop.
The most important point Weart makes comes from one of the basic tenets of chaos theory: even a little bit of greenhouse gas emissions could be enough to put the climate system into a chaotic state. If the climate system is indeed chaos, it won't take much to cause it to cycle wildly. there is no disagreement that the models themselves are chaotic. There is no disagreement that our climate is chaotic (and, as mentioned above, the climate-change deniers implicitly treat our climate as chaotic - prone to erratic behavior based on its internal dynamics alone. So why should anyone disagree that reducing greenhouse gases is a wise thing to do? Because they implicitly accept the fact that the climate is chaotic, they also must accept that small perturbations can make things much worse.
Only a cynic would point out that deniers often have a financial stake in the continued use of certain fossil fuels.