The result is best shown on the following graph. The dark line is from the original paper; the red line is from the data revisited by MM03, showing that not only is the 20th century not the warmest on record since 1400, but that global average temperatures spiked much higher than the 20th century on two occasions in the 1400s.
So what's causing the increase in temperature that has been seen? Could be solar activity; German scientists have reported that since 1940 there has been a significant increase in sunspot activity over the normal levels. More sunspot activity means more output from the sun, and even a tiny increase of a fraction of a percent can cause significant changes in the amount of energy the earth receives.
This Canadian National Post article also points out that MBH98 directly contradicted most of the existing scientific literature at the time.
I'm all for cutting back on CO2 emissions, because it means less petroleum use, and less of other pollutants that get out when coal burns. I'm in favor of about $20B a year being spent over the next 25 years on replacing all of the coal- and oil-fired power plants in the US with new nuclear reactors, which offer less pollution and lower radiation output. But let's base it all on real, verifiable science, and not on flawed, singular studies.
Don't forget -- 25 years ago, the worry was that growing pollution was bringing on a new ice age.
