However I remain unconviced that human activity is nothing more than a pimple on an elephant's rear when it comes to the global climate. The sun pumps power into the Earth's atmosphere at about 127 million gigawatts. The power of all human industrial activity combined is utterly insignificant compared to ordinary Solar power.

*****

Gary,

You and I had quite a lot of agreement on some earlier threads, but this time I have to disagree -- not a good argument at all.

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.

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.

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?

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.

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.