Sunday, February 10, 2008

Transcript of chat on global warming with UCSD professor Jeff Severinghaus

"'s guest speaker is Dr. Jeff Severinghaus, who is a professor of geosciences in the Geosciences Research Division at Scripps Institution of Oceanography, University of California, San Diego."

Read these excerpts on man-made global warming and natural solar warming:

(Q) While I believe in global 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?

(A) 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.

(Q) What is the best evidence that atmospheric CO2 rather than some other factor has caused the increase in temperature over the past 100 years? The two are correlated but this correlation does not prove causation, right?

(A) The best evidence is what I alluded to in a recent answer - basic physics. It has been possible for more than 100 years to calculate how much heat gets trapped by CO2. You can do this about as well as you can calculate how fast an apple will fall under Newton's Laws of Gravity. But we also know that nature on her own causes climate change, and that some of the 20th century warming was natural in origin. The warming of the 1920s and 1930s, that was associated with the famous "dust bowl", was caused by the sun. Solar activity peaked then, as we can easily tell from records of sunspots and cosmic ray fluxes. But all the solar indicators have been constant in the past 30 years, leaving no doubt that the current warming cannot be attributed to the Sun.

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