Alarmist UCSD professor Jeff Severinghaus faces some questioning

Here.

Note his non-answer on a key question:

tassili(Q) While I believe in glaobal warming, and recycle and all that good stuff, I run into people who say, "No such thing. The earth has cycles of warming and cooling. Look at the ice age." What is good proof that this is a man made change?

Dr_Jeff_Severinghaus(A) Dear Tassili, This is an excellent question - I'm glad you asked it. The best proof that this is man-made comes from the isotopes (different flavors of an element, if you will) of carbon in atmospheric carbon dioxide. We've been measuring these for 41 years now at the Scripps Institution of Oceanography, continuing the pioneering work that Charles David Keeling began. These isotopes are like the "smoking gun" that shows that the CO2 rise is human-caused. Natural CO2 is rich in these isotopes (carbon-13 and carbon-14), but fossil fuel CO2 is depleted in them. So if you measure the atmospheric abundances of these, you can tell where the CO2 is coming from - natural or fossil. Indeed, the isotopes have been taking a nose dive over the past 150 years of the industrial revolution. So there is absolutely no doubt that humans have done it - in this "whodunit". Then you add in the basic physics, known from the laboratory for 140 years, that carbon dioxide traps heat. That is really an airtight case at that point - the evidence goes on and on but that is the key part.