"The issue isn't whether our total power output compares to solar input, but whether something we are doing is changing the value of one the feedback terms in the climate equation. The global warming argument is that what we are doing has a lot of leverage."

It's possible, but very doubtful that CO2 levels associated with this lever.

"Take a silly example -- if we put our minds to it, I'm sure that in the next fifty or so years we could place in orbit huge sheets of mylar that would reflect almost all incoming solar radiation. That'd be a big project, but trivial in energy use compared to incoming solar energy. Even so, it would turn the planet into a giant Sno-Cone."

Well, that's bit simplistic, but generally true. But it shows how spending hundreds of billions to trillions of dollars on curbing CO2 level is pretty silly, when for a much smaller amount money on space related activity could have far more affect on global temperature. Of course, once you actually have the power to really affect global temperature, then you would need to make choices about whether you actually want to cool the planet.

"Ditto, if we put 1% of world GNP into producing super-greenhouse gasses -- some of the exotic CFCs that have been talked about for Mars terraforming -- I think even you would agree that we'd eventually warm the climate for a relatively small energy input.

A little less silly: if we turned the world's reserves of coal, oil shales and sands, oil & gas, methane clathrates, etc all into atmospheric CO2 all at once, would you agree that level of increase would have an effect on climate?"

Methane is a far more effective green house gas than CO2, and the quantity of Methane in the form of methane clathrates in the oceans completely dwarf the total oil reserves in terms energy. If you were to burn this methane instead of merely releasing into the atmosphere, you discussing to different issues- Methane is in terms of energy is cleaner than oil or coal and releasing less CO2 per unit of energy generated.

"Our industrial activity is a couple orders of magnitude lower than that scenario, but the question is just how much CO2 increase will it take to cause a noticable change in climate? Somewhere above a few cavemen with a campfire, and somewhere below turning all the limestone back into CO2 (a la Venus), there's a point where our current climate equilbrium will be disturbed."

Well one problem here is that CO2 is almost treated as if it were posion or some runaway substance. CO2 is food for plant life, as far as plants are concerned there is a shortage of CO2 in our atmosphere.

"I'll grant that it's not conclusive that current concentrations have done anything, but neither is the idea crazy, and it's not conclusive that nothing has happened either. When you start talking about triple or more pre-industrial CO2 levels as is likely in the next century, I think there's plenty of reason to be concerned."

But you are doubling a trace substance. And in the past CO2 level have been much higher. We also know that when global temperatures rises it is "natural"/normal for CO2 levels to rise. In recent times, geologically speaking, we have been in a warming tread, since last ice age 10,000 years age. And very recently we are recovering from the Little Ice Age.

Now recently in terms of human time scales, the rapid rise in global temperature, described as the famous "hockey stick", has considerable doubt attached to this data. Whether you want call it a fraud, hoax, mistake, or lacking some data, this could be a personal choice, but not completely dependable or accurate is fairly indisputable.
And the slowest to correct this data has strong odor of political moviations driving scientific results.