First of all, there doesn't seem to have been a "global cooling trend" among scientists in the 1970s - if there was any kind of consensus, its that warming was more likely than cooling. (or I just misunderstood what you meant by "global cooling trend"?)

It's true that the scientific literature had already started moving in the direction of warming, but my understanding is that the popular/mainstream impression of the science (which was a few years behind) was still centered around global cooling. It was nowhere close to the level of agreement that we see on global warming today, but it was a relatively mainstream and apparently well-founded explanation of events then. The academic balance appears to have started shifting in the 1970s, and the balance in popular circles may have taken a few years to catch up.

Quote:

In July 1971, Stephen Schneider, a young American climate researcher then at NASA’s Goddard Space Flight Centre in New York , made headlines in the New York Times when he warned of a coming cooling that could “trigger an ice age”. Soon after, George Kulka, a respected climatologist from the Czech Academy of Sciences, warned on TV in the US that “the ice age is due now any time”. The US National Academy of Sciences reported “a finite probability that a serious worldwide cooling could befall the Earth within the next 100 years”. As a hint of the horrors in store, weird weather in Africa led to a drought in the Sahel that starved millions. Today we tend to blame global warming for African droughts; back then, many blamed the cold. Climate scientists called for action to halt the cooling. They included Fred Singer, first director of the US National Weather Satellite Service and today a well-known contrarian on global warming. Hubert Lamb, then in the process of establishing the Climatic Research Unit at the University of East Anglia, was of a similar view. The then editor of New Scientist magazine, Nigel Calder, adopted the same cause, making a TV programme called The Weather Machine that featured Lamb saying: “We should be preparing ourselves for a long period of mainly colder seasons… The little ice age lasted more than 300 years .” And Newsweek gave a cover feature spot to an analysis of “the cooling world” written by science journalist, and sometime New Scientist freelance, Peter Gwynne. Advisers to the Nixon administration in Washington DC proposed putting giant mirrors into orbit to direct more sunlight onto Earth. Australians proposed painting their coastline black to raise temperatures. Others suggested sprinkling Himalayan glaciers with soot to absorb heat and maintain the ice-melt that feeds the region’s rivers. What prompted this panic? Three decades of evident, if mild, cooling had set the scene. But there was also genuine concern among climate scientists based on predictions of both natural and man-made climate change. For one thing, the atmosphere was becoming dustier and filling with fine, light-scattering particles that were shading the planet’s surface and, some suspected, causing the cooling. Reid Bryson of the University of Wisconsin-Madison argued that dust storms caused by farms spreading into more arid lands were mostly to blame. Others blamed urban smogs.

Schneider tried to calculate the likely cooling effect of this man-made air pollution. He compared it with the possible warming effect of carbon dioxide emissions, which it was by then clear were also accumulating in the atmosphere. In Schneider’s early calculations, published in Science in 1971, the cooling effect was dominant. He said dust and sulphurous smog particles might have doubled since 1900 and could double again in the coming 50 years. Even allowing for warming from carbon dioxide, this could still mean a drop in global temperatures of 3.5C, which, “if sustained over a period of several years… is believed to be sufficient to trigger an ice age”. At the same time, research into the history and timing of past ice ages had found that there were many more than the four originally guessed at, their appearance driven by regular planetary wobbles. Worse, it was now clear that ice ages were the norm rather than the exception. According to Kulka, the most recent interval between ice ages appeared to have lasted only 5,000 years. Our present interglacial had already lasted 10,000 years. An ice age was long overdue. Perhaps pollution was already triggering its onset. The early 1970s also saw the first analysis of Greenland ice cores, and with it the suggestion that climate could change very fast. The last ice age may have taken hold within as little as a century. So the cooling in the mid-20th century might not have been a short-term blip but the start of a rapid slide into the next global freeze. Some climate scientists say today that the fad for cooling was a brief interlude propagated by a few renegade researchers, or even that the story is a myth invented by today’s climate sceptics. Not so. There was , as Thomas Petersen of the National Oceanic and Atmospheric Administration (NOAA) showed in a detailed analysis in 2008, no consensus on global cooling. But equally, there was good science behind the fears. So why did Schneider and his fellow ice warriors get the prognosis so wrong? One reason is that some of the calculations published with great fanfare were simply incorrect. Soon after his 1971 paper came out, Schneider realised he had greatly overestimated the future cooling effect from human-made aerosols. He had assumed that the increased concentrations of particles that he had measured in the air applied globally. They did not; they related only to small areas close to their source. Moreover, much of the particle load in the atmosphere turned out to be natural, so that even if emissions from human sources doubled in the coming 50 years, their effect would be much smaller than he had calculated. Schneider also realised that he had underestimated the likely warming effect of carbon dioxide: it would be three times as great as he first calculated. When he redid the maths, the balance between warming and cooling now tipped strongly towards warming. In 1974, he published a retraction of his earlier prognosis – “just like honest scientists are supposed to do”, he says today. The science of ice ages has also advanced since. The planetary wobbles that periodically tip the world into ice ages are not identical, so some interglacial periods last longer than others. Good theoretical work now shows that the current interglacial is likely to be unusually long. Most climate scientists now agree that the cold decades from the 1940s to 1970s had little to do with either man-made pollution or planetary wobbles. The mid-century cooling was mostly associated with two natural phenomena: first the eruption of a cluster of medium-sized volcanoes that pumped sunlight-scattering sulphate particles into the upper air , and second ocean oscillations such as the Pacific Decadal Oscillation, a kind of slow-motion El Niño that moved heat out of the atmosphere and into the oceans.

Pearce, Fred (2012-10-14). The Climate Files: The battle for the truth about global warming (Kindle Locations 428-472). . Kindle Edition.