This is not new...any earth scientist knows that there have been past ice ages...this was recognized in, I think, the 19th century if not earlier. But, one thing that is relatively new, and is often measured from ice cores, is that the CO2 and methane concentrations change with the global coolings and warmings....naturally before, of course. What is also new is that there have been sudden, drastic warmings...many deg C in a decade or few decades...during deglaciations. The earth's population has increased just a bit since about 10,000 years ago....what were we..a few millions or tens of millions back then?...now we are approaching 7 billion. Not a lot of leeway if there are abrupt (or even gradual) climate changes. Also, the levels of CO2 that are seen in ice cores have never been (never!!!) as high as they are going to be in 50 or 100 years. They can measure or infer CO2 farther back in time other ways than bubbles in ice...and the worst case prediction for CO2 levels would be the highest that they have been in 50 million years. Mammals were pretty strange looking back then..there were no ice sheets...let's see. no ice sheets, thermal expansion of water....I think this about 300 feet of sea level rise...although it would probably take thousands of years to make a large dent in the East Antarctic ice sheet, you could get 10s of feet of rise from West Antarctica and Greenland (although the consensus is that there will be 2 or 3 feet of sea level rise this century).

In summary, man's effects are added to natural variability...but, as Wally Broeker (sp?) says...we are "poking the angry beast with a stick"...and we are easily capable of kicking earth's climate into a different state.
Chris