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Offline Elderberry

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The Discovery of Global Warming
« on: February 10, 2019, 04:47:51 PM » February 2018

 Past Climate Cycles: Ice Age Speculations

To understand climate change, the obvious first step would be to explain the colossal coming and going of ice ages. Scientists devised ingenious techniques to recover evidence from the distant past, first from deposits left on land, then also from sea floor sediments, and then still better by drilling deep into ice. These paleoclimatologists succeeded brilliantly, discovering a strangely regular pattern of glacial cycles. The pattern pointed to a surprising answer, so precise that some ventured to predict future changes. The timing of the cycles was apparently set by minor changes in sunlight caused by slow variations of the Earth's orbit. Just how that could regulate the ice ages remained uncertain, for the climate system turned out to be dauntingly complex. In particular, it turned out that"greenhouse" gases like carbon dioxide played a surprisingly powerful role in governing global climate. One lesson was clear: the system is delicately poised, so that a little stimulus might drive a great change. (There is a separate essay on shorter-term climate fluctuations, lasting a few years to a century or so, possibly related to Variations of the Sun.)

"The origin of these climatic trends... is a difficult subject: by long tradition the happy hunting ground for robust speculation, it suffers because so few can separate fact from fancy." — G.S. Callendar(1)

What could explain the change from a warm to a glacial epoch, and the cycling of ice ages within a glacial epoch? A solution to the puzzle would bring deep satisfaction and eternal fame to whoever solved it. Perhaps the solution would also tell when the next ice age might descend upon humanity — and would reveal mechanisms that could produce other kinds of climate change..

Ice core studies also confirmed a feature that researchers had already noticed in deep-sea cores: the glacial cycle followed a sawtooth curve. In each cycle, a spurt of rapid warming was followed by a more gradual, irregular descent back into the cold over tens of thousands of years. A closer look showed that temperatures tended to cluster at the two ends of the curve. It seemed that the climate system had two fairly stable modes, brief warmth and more enduring cold, with relatively rapid shifts between them. Warm intervals like the past few thousand years normally did not last long. (36)

In 1973, Nicholas (Nick) Shackleton nailed it all down for certain. What made it possible was the new magnetic-reversal dates established by radioactive potassium, plus Shackleton's uncommon combination of technical expertise in different fields. A splendid deep-sea core had been pulled — "one of the best and most complete records of the entire Pleistocene that is known" — the famous core Vema 28-238 (named after the Lamont Observatory's oceanographic research vessel, a converted luxury yacht). It reached back over a million years, and included the most recent reversal of the Earth's magnetic field, which geologists dated at a bit over 700,000 years ago. This calibrated the chronology for the entire core. As a further benefit, Shackleton managed to extract and analyze the rare shells of foraminifera plankton that lived in the deep sea, and which reflected basic oceanic changes independent of the fluctuating sea-surface temperatures. The deep-sea foraminifera showed the same isotopic variations as surface ones, confirming that the variations gave a record of the withdrawal of water to form ice sheets. When Shackleton showed his graph of long-term change to a roomful of climate scientists, a spontaneous cheer went up.(39*)

The core Vema 28-238 and a few others contained such a long run of consistent data that it was possible to analyze the numbers with a mathematically sophisticated "frequency-domain" calculation, a well-established technique for picking out the lengths of cycles in a set of data.(40) Detailed measurements and numerical calculations found a set of favored frequencies, a spectrum of regular cycles visible amid the noise of random fluctuations. The first unimpeachable results (well, almost unimpeachable) were achieved in 1976 by James Hays, John Imbrie and Shackleton. The trio not only analyzed the oxygen-isotope record in selected cores from the Indian Ocean, but checked their curves against temperatures deduced from the assemblage of foraminifera species found in each layer.

The long cores proved beyond doubt what Emiliani had stoutly maintained — there had been not four major ice ages, but dozens. The analysis showed cycles with lengths roughly 20,000 and 40,000 years, and especially the very strong cycle around 100,000 years, all in agreement with Milankovitch calculations.(41) Extrapolating the curves ahead, the group predicted cooling for the next 20,000 years. As Emiliani, Kukla, and other specialists had already concluded several years earlier, the Earth was gradually — indeed, perhaps quite soon as geologists reckoned time — heading into a new ice age (see above).

These results, like so many in paleoclimatology, were promptly called into question.(42) For one thing, there was no solid reason to suppose that our current interglacial period would be of average length and was therefore nearing its end. But the main results withstood all criticism. Confirmation came from other scientists who likewise found cycles near twenty and forty thousand years, give or take a few thousand. The most impressive analysis remained the pioneering work of Hays, Imbrie, and Shackleton. They could even split the 20,000 year cycle into a pair of cycles with lengths of 19,000 and 23,000 years — exactly what the best new astronomical calculations predicted. By the late 1970s, most scientists were convinced that orbital variations acted as a "pacemaker" to set the timing of ice ages.(43) Science magazine reported in 1978 that the evidence for the Milankovitch theory was now "convincing," and the theory "has recently gained widespread acceptance as a factor" in climate change.(44)

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Offline Fishrrman

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Re: The Discovery of Global Warming
« Reply #1 on: February 10, 2019, 06:39:50 PM »
"The Discovery of Global Warming..."

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