The 1990 IPCC Report contains an explanation as to why they knew the effect of greenhouse gases on climate:
"measurements from ice cores going back 160,000 years shows the earth’s temperature closely parallels the amount of carbon dioxide and methane in the atmosphere (see Figure 2). Although we do not know the details of cause and eﬀect, calculations indicate that changes in these greenhouse gases were part, but not all, of the large (5-7 °C) global temperature swings between ice ages and interglacial periods."
(emphasis added) Thus, the IPCC argued in 1990 that, although the cause and eﬀect was not known, calculations can be made that are in accordance with what is expected! We now know (as they did not know in 1990) that the temperature changes manifest in the Vostok ice cores preceded the changes in CO2 by approximately 800 years, i.e. that temperature drives CO2 levels, not the other way around. This is exactly as expected from solubility data of CO2 in water v. temperature. Hmmm, no mention of this embarrassing error in subsequent IPCC reports, but never mind, they made their calculations before the high resolution ice core data on CO2 was available, and they're stickin' to 'em!