Forty-five years ago a run of cold winters caused a “global cooling” scare.

See The mysterious cycles of ice ages: Orbital wobbles, carbon dioxide and dust all seem to contribute by Matt Ridley. Excerpt:

"Forty-five years ago a run of cold winters caused a “global cooling” scare. “A global deterioration of the climate, by order of magnitude larger than any hitherto experienced by civilised mankind, is a very real possibility and indeed may be due very soon,” read a letter to President Nixon in 1972 from two scientists reporting the views of 42 “top” colleagues. “The cooling has natural causes and falls within the rank of the processes which caused the last ice age.” The administration replied that it was “seized of the matter”.

In the years that followed, newspapers, magazines and television documentaries rushed to sensationalise the coming ice age. The CIA reported a “growing consensus among leading climatologists that the world is undergoing a cooling trend”. The broadcaster Magnus Magnusson pronounced on a BBC Horizon episode that “unless we learn otherwise, it will be prudent to suppose that the next ice age could begin to bite at any time”.

Newsweek ran a cover story that read, in part: “The central fact is that, after three quarters of a century of extraordinarily mild conditions, the Earth seems to be cooling down. Meteorologists disagree about the cause and extent of the cooling trend, as well as over its specific impact on local weather conditions. But they are almost unanimous in the view that the trend will reduce agricultural productivity for the rest of the century.”

This alarm about global cooling has largely been forgotten in the age of global warming, but it has not entirely gone away. Valentina Zharkova of Northumbria University has suggested that a quiescent sun presages another Little Ice Age like that of 1300-1850. I’m not persuaded. Yet the argument that the world is slowly slipping back into a proper ice age after 10,000 years of balmy warmth is in essence true. Most interglacial periods, or times without large ice sheets, last about that long, and ice cores from Greenland show that each of the past three millennia was cooler than the one before.

However, those ice cores, and others from Antarctica, can now put our minds to rest. They reveal that interglacials start abruptly with sudden and rapid warming but end gradually with many thousands of years of slow and erratic cooling. They have also begun to clarify the cause. It is a story that reminds us how vulnerable our civilisation is. If we aspire to keep the show on the road for another 10,000 years, we will have to understand ice ages.The oldest explanation for the coming and going of ice was based on carbon dioxide. In 1895 the Swede Svante Arrhenius, one of the scientists who first championed the greenhouse theory, suggested that the ice retreated because carbon dioxide levels rose, and advanced because they fell. If this were true, he thought, then industrial emissions could head off the next ice age.

Burning coal, Arrhenius said, was therefore a good thing: “By the influence of the increasing percentage of carbonic acid in the atmosphere, we may hope to enjoy ages with more equable and better climates.”

There is indeed a correlation in the ice cores between temperature and carbon dioxide. There is less CO2 in the air when the world is colder and more when it is warmer. An ice core from Vostok in Antarctica found in the late 1990s that CO2 is in lock-step with temperature -- more CO2, warmer; less CO2, colder. As Al Gore put it sarcastically in his 2006 film An Inconvenient Truth, looking at the Vostok graphs: “Did they ever fit together? Most ridiculous thing I ever heard.” So Arrhenius was right? Is CO2 level the driver of ice ages?

Well, not so fast. Inconveniently, the correlation implies causation the wrong way round: at the end of an interglacial, such as the Eemian period, over 100,000 years ago, carbon dioxide levels remain high for many thousands of years while temperature fell steadily. Eventually CO2 followed temperature downward. Here is a chart showing that. If carbon dioxide was a powerful cause, it would not show such a pattern. The world could not cool down while CO2 remained high.

In any case, what causes the carbon dioxide levels to rise and fall? In 1990 the oceanographer John Martin came up with an ingenious explanation. During ice ages, there is lots of dust blowing around the world, because the continents are dry and glaciers are grinding rocks. Some of that dust falls in the ocean, where its iron-rich composition fertilizes plankton blooms, whose increased photosynthesis draws down the carbon dioxide from the air. When the dust stops falling, the plankton blooms fail and the carbon dioxide levels rise, warming the planet again.

Neat. But almost certainly too simplistic. We now know, from Antarctic ice cores, that in each interglacial, rapid warming began when CO2 levels were very low. Temperature and carbon dioxide rise together, and there is no evidence for a pulse of CO2 before any warming starts, if anything the reverse. Well, all right, said scientists, but carbon dioxide is a feedback factor – an amplifier.
Something else starts the warming, but carbon dioxide reinforces it. Yet the ice cores show that in each interglacial cooling returned when CO2 levels were very high and they remained high for tens of thousands of years as the cooling continued. Even as a feedback, carbon dioxide looks feeble."