Detecting the greenhouse effect... or not

IPCC 1990 report Detection of the Greenhouse Effect in the Observations:

"Given our rudimentary understanding of the magnitude and causes of low frequency natural variablity, it is virtually impossible to demonstrate a cause-effect relationship with high confidence from studies of a single variable (However if the global warming becomes sufficiently large, we will eventually be able to claim detection simply because there will be no other explanation)"

Dr. James Hansen in 1988 testimony before the US Senate:

"...the global warming is now large enough that we can ascribe with a high degree of confidence a cause and effect relationship to the greenhouse effect."

Anthony Watts has the full story.