The Old Ice Age Myth Put to Rest

by Bill Chameides | February 10th, 2009
posted by Erica Rowell (Editor)

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A new paper shows that air pollution helped slow global warming in the 1970s until it was cleaned up.

Global warming skeptics often point to an “imminent ice age” touted by the popular press in the 1970s to dissuade the public from believing today’s very real threat of global warming. The argument was always a red herring, and now a new paper resolves the scientific issues raised by the supposed warnings.

As early as the nineteenth century, scientists recognized that greenhouse gases warm the planet, and that increases in atmospheric carbon dioxide (CO2) could lead to global warming on much shorter time scales than the natural, cyclic fluctuations related to ice ages and interglacials. Around the same time, global temperatures began to rise, and scientists became increasingly concerned that humans were interfering with the climate.

What Happened in the Mid-Twentieth Century

In the 1950s the upward trend in global temperatures unexpectedly halted and temperatures declined somewhat. This led some to become concerned about global cooling, and, in turn, newspaper headlines proclaimed an imminent ice age. Climate skeptics often point to that period as evidence that climate scientists are not to be trusted – warnings of global cooling back then, warnings of global warming now. (It is ironic that many climate skeptics are now beating the warning drums of an imminent new ice age.)

What the skeptics fail to admit is that within the scientific literature — as opposed to the mainstream press — there was no consensus that global cooling would be a long-term trend and global warming remained a serious concern. A favorite news article cited by climate skeptics is from Newsweek in April 1975: “The Cooling World [pdf].” And yet even this piece made abundantly clear that, as far as the scientific community was concerned, there was little certainty as to what was occurring. Here‘s an excerpt:

“‘Our knowledge of the mechanisms of climatic change is at least as fragmentary as our data,’ concedes the National Academy of Sciences report. ‘Not only are the basic scientific questions largely unanswered, but in many cases we do not yet know enough to pose the key questions.'”

Global Warming Was a Persistent Concern

In fact in 1979, just four years after that article, the National Academy of Sciences warned that a doubling of CO2 would increase global temperatures by 1.5 to 4.5 degrees Celsius (“Carbon Dioxide and Climate: A Scientific Assessment,” NAS Press, 1979), and shortly thereafter a resumption of the upward trend in temperatures was detected.

Over the past quarter century, scientific research on global climate change has intensified; international programs have been organized. We now have detailed, global data sets from satellites and ever-more sophisticated instruments. Computer models, incorporating more and more data and capable of recreating past trends, can more precisely predict future scenarios. As a result, our understanding of the climate system is immeasurably stronger than in the 1970s. The National Academy of Sciences, which admitted we did not know enough to “pose the key questions” in 1975, now says:

“Greenhouse gases are accumulating in the Earth’s atmosphere as a result of human activities, causing surface air temperatures … to rise.” (NAS, 2001)

“The scientific understanding of climate change is now sufficiently clear to justify nations taking prompt action. … We urge all nations … to take prompt action to reduce the causes of climate change.” (NAS [pdf], 2005)

Now that’s a consensus.

Particulate Pollution from Burning Coal Kept Warming Temporarily at Bay

So what about that mid-twentieth century cooling? A comprehensive understanding of the climate requires an understanding of that event. Here’s what we’ve figured out.

Shortly after World War II, America’s and Europe’s industrial capacity skyrocketed. Much of that industrial expansion was fueled by coal – the dirtiest fossil fuel. Burning coal produces a range of air pollutants. Among these are aerosols, tiny particles that produce haze and prevent sunlight from penetrating the Earth’s surface, effectively cooling the planet.

This pollution from burning coal is nasty stuff. At different points in history, it has been so bad that literally hundreds of people have been  hospitalized or perished (see here and here). And so in the late 1970s and 1980s we began to burn less coal and mandate pollution-control devices. As a result, particulate pollution declined, the cooling effect of these aerosols was overwhelmed by the greenhouse gases, and global warming resumed. (See related post.)

A picture began to emerge that linked the dip in mid-twentieth century temperatures to increased coal-burning.

New Recreation Confirms Theory of Short Cooling Period

Great theory, but how do we know it is correct? Enter Rolf Philipona of the Swiss federal agency MeteoSwiss and his colleagues. In their study, published in Geophysical Research Letters, they recreated the warming/cooling trends caused by changes in particulate pollution and greenhouse gases. To do this, they analyzed measurements of surface sunlight, temperatures, and humidity collected from 1980 to the present at 25 stations in Switzerland and eight in Germany.

Their analysis shows that increases in the solar radiation reaching the surface at these sites – almost certainly due to declining particulate pollution – led to the reversal in the cooling and the rapid increase in temperature over the region of study. (Note: satellite observation shows that the increased solar radiation reaching the surface cannot be explained by an increase in the output of energy from the sun.

Slow but steady and firmly based on real data – that’s how science advances. This study adds yet another detail to our understanding of climate change and confirms that we are on the right track. In this case, the data collected at surface sites in Europe give us more evidence for what caused the anomalous cooling period in the 1970s – and why warming kicked in again as soon as the particulate pollution was dramatically reduced.

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