One of the hot topics, so to speak, in the global warming debate is allocating responsibility for 20th century warming between natural and man-made effects.  This is harder than one might imagine -- after all, no one's thermometer has two readings, one for "natural" and one for "man-made."  This week, from CERN in Geneva, comes an important new study in this debate.

Global warming skeptics argue that only a portion, possibly a small portion, of recent warming is due to man-made CO2 and greenhouse gasses.  Climate alarmists have, in turn, argued that all of 20th century warming, and more, was due to anthropogenic effects  (if the "and more" is confusing, it means that some scientists believe that certain man-made and natural cooling effects actually reduced man-made warming below what it might have been.)

It is only in this context that Michael Mann's famous hockey stick studies make even a bit of sense.  After all, what do pre-industrial world temperature trends have anything to tell us about the effect of man-made Co2 on 20th century temperatures?

But Mann's work had a very specific purpose -- to make the case that the natural variability of temperatures, at least on a millennial scale, is very small.  Though considered by many to be deeply flawed, Mann's work seemed to say that the anecdotal historical record was wrong, that there was not a Medieval warm period or very cold period during the solar minima of the 17th and 18th centuries.  In his hockey stick, the only significant trend in temperatures began with the industrial age, and man's production of CO2.



