One of the undying, zombie-like arguments against climate change is that you can't trust climate scientists because they started out making doom and gloom claims about global cooling in the 1970s. But this, along with many other things comedian Dennis Miller has said on late night talk shows, needn't be taken seriously.

By the time fears of an ice age reached the public's attention, there was a long history of concerns about warming. The idea that burning fossil fuels would warm the planet can be traced back to an 1896 paper by Swedish scientist Svante Arrhenius. In the 1930s, Britain's Guy Callendar concluded that global warming was already underway. So it seems a bit odd that anyone worried about cooling. What was really going on back in the '70s—both in science and in the media?

Reaching maturity

For climate science, the 1970s were a pivotal era. Even though the discipline was born much earlier, it's probably fair to say that climate science grew up in that decade.

This was partly the result of some new fields that were opening to concerted exploration for the first time. Up-to-date estimates of global temperature were finally being maintained, and they were accompanied by ongoing atmospheric CO­₂ measurements, which had only started in 1958. The first punch-card-driven computer models of Earth's climate system were being developed, giving researchers new tools to analytically attack questions about the physics of Earth's atmosphere. And we were just getting a handle on how sunlight-reflecting aerosol particles behaved in the atmosphere.

As all of this was happening, fossil fuel use (and pollution) grew rapidly. Not entirely coincidentally, there was a lull in 20^th century warming between the mid-1940s and mid-1970s. That dip is now understood as being the result of two factors: a post-World War II surge in the emissions of aerosols from dirty fossil fuel burning and the cool phase of a Pacific Ocean cycle related to the strength of the trade winds. (That same Pacific cycle suppressed global surface temperatures a bit over the past two decades.) But at the time, the causes of the dip were far from clear.

As it happens, these slightly cooler temperatures were accompanied by the first ice core and sediment records of the glacial cycles. The records were a revelation, displaying a rhythm that validated decades-old predictions that the cycles were driven by variations in Earth's orbit. With this new-found context, it became natural to ask where we were in the orbital ice age cycle.

All of this data made for several disparate threads of research that were in the process of being woven together. We had to figure out what the return to the cold part of the glacial cycles really looked like and when it was due to begin. We had to work out how sensitive Earth's climate is to increasing CO₂. There were also unsettled questions about the strength of competing warming and cooling effects of different types of aerosols—as fossil fuel use continued, which would be more important, the aerosols or the CO₂? Understanding this would help us figure out whether human activity was responsible for the cooling over the previous couple decades or if that was just the part of the natural variability we should expect.

Straight to the source(s)

The scientific literature at the time shows a field in flux. Taking the time to actually read those papers is illuminating.

Consider a 1971 paper published in Science by Stephen Schneider and Ichtiaque Rasool. The paper used a very simple mathematical model of Earth's climate to weigh the impact of CO₂ and aerosols. The authors got a very small value for CO₂ warming and an aerosol cooling effect that grew much faster than CO₂'s warming. That led to a conditional statement about future fossil fuel use:

[I]t is projected that man's potential to pollute will increase six- to eightfold in the next 50 years. If this increased rate of injection of particulate matter in the atmosphere should raise the present global background opacity by a factor of 4, our calculations suggest a decrease in global temperature by as much as 3.5°C. Such a large decrease in the average surface temperature of Earth, sustained over a period of [a] few years, is believed to be sufficient to trigger an ice age. However, by that time, nuclear power may have largely replaced fossil fuels as a means of energy production.

Nuclear power may not have replaced fossil fuel use, but pollution controls helped us avoid a world in which aerosols increased so drastically. The paper wasn't influential, anyway, as further research confirmed that CO₂ was not nearly so weak and aerosols not so strong. In fact, a 1974 review paper by William Kellogg and the same Stephen Schneider described aerosols only as poorly understood, while (pretty accurately) estimating 0.5 degrees Celsius warming by 2000 as a result of CO₂ emissions.

The real booster of the cooling predictions was Reid Bryson, who later rejected anthropogenic warming even as global temperatures climbed. In 1975 and 1976 papers, Bryson concluded that aerosol cooling would dominate over CO₂ warming—a fact he felt was demonstrated by recent temperatures. As one of his papers put it, "Since 1940, the effect of the rapid rise of atmospheric turbidity appears to have exceeded the effect of rising carbon dioxide, resulting in a rapid downward trend of temperature. There is no indication that these trends will be reversed, and there is some reason to believe that man-made pollution will have an increased effect in the future."

But other studies in that same special volume of papers projected warming. One by climate modeling pioneer Syukuro Manabe projected a total of 0.8 degrees Celsius warming over the 20th century (for one estimate of the increase in CO₂). Another by J. Murray Mitchell concluded, "Of the two forms of pollution, it appears that the carbon dioxide increase is more influential in raising planetary temperatures than the anthropogenic particle increase is in lowering planetary temperatures."

Another paper on the cooling list was published by NOAA's Earl Barrett in 1971. Barrett starts by dismissing a rapid warming trend on the grounds that the concentration of CO₂ in the atmosphere was on track to take 340 years to double. (We will actually hit that doubling point well before the end of this century if we don't cut emissions.)

Like many of these papers, Barrett was trying to build a mathematical model to quantify the cooling influence of aerosols. Using that model, he concluded, "To produce the drastic change of 10°C in mean surface temperature it would thus be necessary to increase the present rate of [aerosol] particle emissions by a factor of 60-100. It therefore appears that the initiation of a glacial epoch by man-made pollutants, while not imminent, is not entirely outside the realm of possibility." It may be a forecast of cooling, but that's not exactly a portentous Game of Thrones-like alarum of impending wintry doom.

Things came to a head in 1975, when the US National Academy of Sciences published a report that prompted a number of news stories. The thrust of the report was that climate prediction was not yet possible, and it outlined a plan of action for fostering a research program to change that. But it also summarized the state of scientific knowledge at the time.

The report highlighted that we still had much to learn about aerosols, but it concluded, "Of the two forms of pollution, the carbon dioxide increase is probably the more influential at the present time in changing temperatures near the Earth's surface." Future pollution trends were unclear, though, so the report added, "If both the CO₂ and particulate inputs to the atmosphere grow at equal rates in the future[…] the particulate effect will grow in importance relative to that of CO₂."

But it is part of a sentence from a paleoclimate appendix that gets quoted today by those who seek to re-write the history of climate science: "[…]there is a finite probability that a serious worldwide cooling could befall the Earth within the next hundred years." Given the full context, however it's hard to make this out as some kind of forecast.

There seems little doubt that the present period of unusual warmth will eventually give way to a time of colder climate, but there is no consensus with regard to either the magnitude or rapidity of the transition. The onset of this climatic decline could be several thousand years in the future, although there is a finite probability that a serious worldwide cooling could befall the Earth within the next hundred years.

The phrase "there is a finite probability that" could just as well be written "it's not impossible that." As a prediction of looming catastrophe, that would be some pretty weak tea.