Learning Math: Extrapolation

The Dallas Morning News published a story about climate change titled "State climatologist predicts rising temperatures over coming decades in Texas." In it was this alarming prediction:

"Texas A&M University atmospheric sciences professor John Nielsen-Gammon said recently that models he's analyzed show temperatures rising as much as 1 degree each decade, meaning that by 2060, temperatures across the state would be 5 degrees hotter than now."

That prompted one reader to reply:

"Of course it's warming. It's been warming for the past 20,000 years. That's what happens when you come out of an ice age."

Let's do the math. One degree rise in temperature per decade for the last 20,000 years and the global temperatures today would be in the vicinity of 2000 degrees. Obviously, it hasn't been warming for the past 20,000 years, at least nowhere near the rate it's rising today. Something more than just natural ice age cyclical behavior is pushing temperatures higher at a much faster rate. The simple-minded rebuttal that the earth has been warming ever since the last ice age just doesn't hold up to basic math.