I’ve been hearing climate deniers talking about a global cooling scare in the nineteen seventies for a while now, and I finally got curious about where this narrative had come from–I didn’t think it had been made up whole cloth, but I hadn’t heard word one about it from any credible source, either. You’d think that if climate scientists had thought an ice age was imminent as recently as the seventies, at least some of the scientists I know would mention it occasionally?
So, I looked into it. And I found not one but two explanations.
In one story, Peter Gwynne, a science writer for Newsweek, wrote a short article on an idea some scientists were kicking around at the time–that a thirty-year cooling trend might continue and develop into a real ice age. The article was published on April 28, 1975, and attracted enough attention that other publications picked up the story with their own articles. Books and TV shows followed.
Scientific American, my source for this particular story, explains that the cooling trend is
now believed to be a consequence of soot and aerosols that offered a partial shield to the earth as well as the gradual retreat of an abnormally warm interlude.
there also was a small but growing counter-theory that carbon dioxide and other pollutants accompanying the Industrial Age were creating a warming belt in the atmosphere, and by about 1980 it was clear that the earth’s average temperature was headed upward.
Scientific American acknowledges that the global cooling thing has no legitimate place in the climate discussion today, and reports that Mr. Gwynne himself is somewhat embarrassed by the anti-scientific uses to which his writing is being put. He does stand by what he wrote, given the limits of available knowledge at the time.
Ok, but there are a couple of problems with that story, starting with the fact that the greenhouse effect was not a “small but growing counter-theory” in the 1970’s–the effect of carbon dioxide on the climate has been known since 1859. The first calculations of the human role in climate change were made in 1896.
And it’s not like global warming was some far-out thing nobody was paying attention to back in the 1970’s, either. No less a person than the fiction writer, Ursula K. LeGuin had started making oblique references to climate change as early as 1969 (her novel, The Left Hand of Darkness, includes a flawless description of the natural greenhouse effect, as well as a reference to an alien planet that is hot because “an exploitive civilization wrecked its natural balances, burned up the forests for kindling, as it were.” Several of her later books also refer to the Earth itself getting warmer, too). Perhaps more starkly, the 1970’s were when Exxon was busy figuring out what it was going to do about global warming, of which its internal documents prove it was well aware.
Beyond all that, there wasn’t a 30-year cooling trend, except perhaps in a mathematical sense. According to the National Oceanic and Atmospheric Administration (NOAA), the period from the mid-1940’s to the mid-1970’s was cooler than previous years had been, but there was a lot of minor temperature fluctuation, not a consistent cooling. A cool period of relative stability is not the same thing as an oncoming ice age.
So, I did some more poking and found the second story.
Apparently, in the 1970’s, the greenhouse effect was well-known, but the cooling effect of sulfate emissions (“aerosols”) had just been discovered and it wasn’t clear yet which would prove dominant. A few climate scientists thought the aerosols might win out–between 1965 and 1979, 10% of the scientific papers on the subject predicted cooling, but 28% could make no prediction and 62% predicted warming. In other words, the coming ice age was a legitimate scientific idea for a while, but only a small minority of studies ever supported it.
I’m not actually sure, based on what I’ve read, whether anybody ever proved that sulfate emissions couldn’t have counteracted carbon emissions under some scenarios that were plausible back then. As history has actually played out, sulfate emissions have been dramatically reduced (they also cause acid rain), while carbon emissions have continued to climb. Aerosols still complicate climate predictions, but no one thinks they’re going to cause an ice age anymore.
There’s no cooling trend mentioned in there.
The way I see these two stories blending, I suspect that what really happened was that the end of the warming trend of the first third of the 20th century was taken (maybe correctly) as evidence of the cooling power of aerosols. Some climate scientists thought the aerosols could go on to trigger a cooling trend, but most did not. Peter Gwynne, being a writer who cared about science and about getting his writing published, chose to focus on the minority opinion, since that seemed more sensational at the time. He has admitted that the story “pushed the envelope a little bit,” in deference to Newsweek’s penchant for what Scientific American called an “over-ventilated style.”
The ventilation would have seemed harmless at the time, if the article was fundamentally accurate, as I’m willing to buy that it was. Nobody can represent the entire breadth of the scientific conversation on any one topic in just nine paragraphs. You have to choose which of all possible stories you’re going to tell, in order to tell any story at all.
That deniers have since pounced on his article for political and anti-scientific purposes is not Mr. Gwynne’s doing. Being co-opted is a risk all published writers run–it’s the Scylla to the charybdis of being utterly ignored.
Curiously, the one detail I thought would enter the discussion apparently didn’t, except as a note of context written long after the fact by one or another of my sources–astronomically speaking, we’re supposed to be in an ice age already.
The primary factor that dictated the glacial/interglacial cycle through recent geological history was the Milankovitch Cycle, an interaction between three separate variations in Earth’s orbit that together dramatically how much solar radiation we get at different times of the year. We’re at a point in the cycle where we should be heading into a new ice age, but aren’t because our carbon dioxide levels are too high.
The connection between that cycle and climate was confirmed in 1976, so it may be another thread of the “global cooling” story that none of my sources happened to tease out–but if not, there may have been good reason to ignore it.
The onset of ice ages is very slow. I have to cite one of my grad school classes here (Tom Wessels was the teacher–I’ve cited him as a source here before) as I haven’t been able to lay my hands on an appropriate link, but ice ages melt quickly (as in many hundreds of years) and grow slowly (as in many thousands of years). In fact, the warmest point of our current interglacial (before now, anyway) was thousands of years ago. No, the cooling was never enough to initiate continental glaciation on North America or Asia, but cooling was in progress.There is an excellent illustration of this cooling, and how long and consistent it was, here (yes, that is a web comic, but this one’s not a joke).
Sliding towards an ice age doesn’t look like anything special, it turns out. More or less, it looks like all of human history.