According to the NYT, in the First Assessment Report (AR1) of the Intergovernmental Panel on Climate Change published in 1990:
The unequivocal detection of the enhanced greenhouse effect from observations is not likely for a decade or more.
In the AR2 of 1995:
The balance of evidence suggests a discernable human influence on global climate.
AR3 of 2001:
There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities.
Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.