This has come up again with the discovery of a (purported) CIA report on the subject. As William Connelley says, CIA reports aren't scientific research and do no represent an objective look at the status of science.
Nor would a consensus in the popular media of that time mean much either, though I think that notion was well refuted by the 2008 Peterson, Connelley and Fleck paper in BAMS.
Whenever I see someone advancing the idea of 1970's global cooling, it tells me they don't really understand how science works and what it means -- and so they are grasping at straws.
Were some people in the 1970s thinking about global cooling? Yes -- given the temperature data, anyone would have. But "thinking about" something and publishing claims that it's happening are two very different things, and people like Callendar, Plass, Gilbert, had already been thinking about an enhanced greenhouse effect for decades. [Warning note added: Tom Nelson purposely distorts what I'm saying here by quoting only part of what I wrote.] And there were reports on that as well.
- The 1965 report to the Johnson Administration has a chapter on CO2’s potential to cause warming: “Restoring the Quality of Our Environment,” Report of the Environmental Pollution Panel, President’s Science Advisory Committee (1965), pp 111-133
- A 1969 memo from President Nixon’s Democratic adviser, Daniel Patrick Moynihan, wrote about concerns over CO2’s impact.
- In 1967 Syukuro Manabe and Richard Wetherald had a model that found a climate sensitivity of 2.3 C.
- Wallace Broecker's 1975 article in Science was directly worried about CO2: "Climatic Change: Are We on the Brink of a Pronounced Global Warming?", Wallace S. Broecker, Science Vol. 189 no. 4201 pp. 460-463, August 8, 1975
Besides, science advances. New discoveries are made and new ideas come into prominence(*). Here is a partial list of fundamental properties about the universe that weren't known in 1970:
And this is just physics. Biology and genetics have changed even more, as has medicine. Computers (or models) aren't in the same universe....
The state of scientific knowledge in 2012 is far better than it was in the 1970s. What happened then is no more relevant than if someone said quantum chromodynamics (the theory of the strong force) is in doubt today because in the 1970s science wasn't sure how many quarks there were.
(*) Note that these discoveries are extensions of existing knowledge, not replacements for them. (Yes, they replaced ideas -- but those ideas weren't knowledge, but hypotheses.) At this point, the scientific idea of an enhanced greenhouse effect from fossil fuel emissions will not be overthrown, ever. It is established fact, with long line of theoretical and experimental evidence that goes back to Fourier. It's here to stay, and its baseline sensitivity of ~1.2°C for a CO2e-doubling will not change much. (The magnitude of eedbacks, though, are a different matter -- it's a difficult calculation.)
There may be discoveries about properties of clouds in a warming world, or perhaps a closing of the open (viz. unproven) steps in the Svensmark Hypothesis, or some other complicating factors discovered. Better models may find a slightly lower climate sensitivity (a la Schmitter et al last year). But the enhanced greenhouse effect is here to stay -- that's never going away as a proven idea or as an observation, and all the CIA reports ever written won't change that.
(And if the enhanced GHE ever did cease to exist, I would, literally, turn off all my electronics, lock my doors, and probably sit quaking in a dark corner of the room, because it means some fundamental alteration of natural laws has taken place, or been caused to take place, that is so far outside our ken that our very existence is in immediate danger.)