The ice age that never was
15 December 2006 by Fred Pearce
There was a chill across the world, and it wasn't just the cold war. From the 1940s to the mid-70s, the planet seemed to be in the grip of a global cooling. For a while, almost every outbreak of extreme weather was blamed on it. Some members of a new scientific discipline, climatology, predicted a new ice age. Yet before the 70s were out, temperatures were rising and many of the soothsayers for a new ice age were warning of global warming instead. It is a strange, and now largely forgotten episode. Some say it shows climate scientists are scaremongers and shouldn't be believed, whatever they are predicting. So what happened three decades ago? And why should we believe the climatologists now?
Global cooling was a real phenomenon - and it changed global history. In the winter of 1941, it stopped the German army's advance on Moscow: grease froze in German guns and thousands of soldiers died from cold. Hitler's failure to take Moscow marked a turning point in the second world war. Without the freezing 40s, Hitler might have triumphed. But by the 1970s, no one was giving thanks for global cooling. As snow banks built across the Canadian Arctic and pack ice grew in the North Atlantic, there was concern bordering on panic about where this might be leading.
Without the freezing 40s, Hitler might have triumphed
In July 1971, Stephen Schneider, a young American climate researcher at NASA's Goddard Space Flight Center 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 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.
Many climate scientists, such as Fred Singer, now a well-known contrarian, called for action to halt the cooling. US government advisers proposed putting giant mirrors into orbit to direct more sunlight onto Earth. 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 human-made climate change.
For one thing, the atmosphere was becoming dustier and filling with pollution. Fine, light-scattering particles in the air 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. Meanwhile, Schneider tried to calculate the likely cooling effect of anthropogenic air pollution and compared it with the possible warming effect of carbon dioxide emissions, which it was now clear were accumulating in the atmosphere.
In Schneider's early calculations, published in Science in 1971, the cooling effect was dominant. He said aerosols might have doubled since 1900 and could double again in the coming 50 years. Even allowing for warming from CO2, this could still mean a 3.5 °C drop in global temperatures, 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 made it clear that there had been 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 5000 years. Our present interglacial had already lasted 10,000 years. An ice age was long overdue.
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. The cooling caused by aerosols could kick-start the process, argued Kulka.
It is often claimed 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. It wasn't. There was good science behind the fears of global cooling. So why did the prognosis prove so wrong?
Short memories were partly to blame. A generation of researchers had virtually forgotten the work of Swedish chemist Svante Arrhenius, who predicted at the end of the 19th century that increases in the atmospheric concentration of CO2 would cause significant warming. This meant few took much notice when in the 1960s Charles Keeling began to show that concentrations of the gas had been rising since at least the mid-1950s.
Read full articleContinue reading page |1 |2 Mistakes also played a part. Some of the calculations published with great fanfare were simply wrong. 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 levels of aerosols in the air that he had measured applied globally. They did not; they related only to small areas close to their source. Moreover, much of the aerosols turned out to be natural, so that even if emissions from human sources did quadruple, their effect would be much smaller than he had calculated.
Schneider also realised that he had underestimated the likely warming effect of CO2: it would be three times as great as he first calculated. When he redid his sums, he concluded that 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.
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 one is likely to be unusually long.
Finally, far from cooling, since the middle of the 1970s the planet has been warming exceptionally rapidly. The link between this and the accumulation of greenhouse gases is almost universally accepted.
Most now agree that the cold decades from the 1940s to 1970s had little to do with either anthropogenic pollution or planetary wobbles. The mid-century cooling, Bryson now agrees, was associated with the eruptions of a cluster of medium-sized volcanoes that pumped sunlight-scattering sulphate aerosols into the upper air.
All this raises an alarming question. If climatologists were so wrong then, why should we believe them now? As those who played a part in the cooling scare now readily admit, those early studies were based on flimsy data collected by very few, often young, researchers. In 1971, when Schneider's paper appeared, he was instantly regarded as a world expert. It was his first publication.
Today, vastly more research has been done into how and why climate changes. The consensus on warming is much bigger, much broader, much more sophisticated in its science and much longer-lasting than the spasm of concern about cooling.
Celestial forces may one day have the final say on climate change, eclipsing any warming we have caused. But that is likely to be thousands of years away, and unless we cut our emissions of greenhouse gases soon, the next ice age might never happen at all. Tim Lenton of the Tyndall Centre for Climate Change Research in Norwich, UK, recently calculated that global warming could reach 13 °C in the coming centuries, double the difference between today and the depths of the last ice age.
The influence of aerosols should not be underestimated, however. Most climate modellers agree that aerosols are currently protecting us from some of the impact of greenhouse warming. At the extreme, says Dutch atmospheric chemist Paul Crutzen, they could be offsetting as much as three-quarters of warming. This has led him to suggest that we may one day need a global programme to inject sulphate aerosols into the upper atmosphere to shade the planet from the worst excesses of global warming.
Those sulphates would have the same optical effect as the mid-century volcanic eruptions. Before the 21st century is out, he argues, the forces that shaped global cooling half a century ago will have to be used to rescue us from global warming.
newscientist.com |
