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) Beyond such fascinating hints, however, the Greenland ice cores could say little about long-term cycles. They were too short to reach past a single glacial cycle. And the ice flowed like tar at great depths, confusing the record. In the 1970s, despite the arduous efforts of the ice drillers, the most reliable data were still coming from deep-sea cores.

That work too was strenuous and hazardous, manhandling long wet pipes on a heaving deck. Oceanographers (like ice drillers) lived close together for weeks or months at a time under Spartan conditions, far from their families. The teams might function smoothly — or not. Either way, the scientists labored long hours, for the problems were stimulating, the results could be exciting, and dedication to work seemed normal with everyone around them doing the same.

Deep-sea drilling
To make it worthwhile, scientists had to draw on all their knowledge and luck to find the right places to drill on the ocean bed. In these few places, layers of silt had built up unusually swiftly and steadily and without disturbance. Meanwhile, drilling techniques were finally worked out that could extract the continuous hundred-meter cores of clay that Emiliani had been asking for since the 1950s. Improved techniques for measuring the layers gave data good enough for thorough analysis.
The most prominent feature turned out to be a 100,000-year cycle — evidently a key to the entire climate puzzle. Several earlier studies had tentatively identified this long-term cycle. Corroboration was in hand from Kukla's loess layers in Czechoslovakia, at the opposite end of the world from some of the deep-sea cores. Here too the 100,000-year cycle stood out.(37)

[snip]

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).

[snip]

During the 1980s, the work advanced steadily with few surprises. Ocean drilling in particular, pursued on an international scale, produced ever better cores. A costly project dedicated to "spectral mapping" (SPECMAP) yielded a spectrum of cycles that matched the astronomical calculations with gratifying precision going back hundreds of millennia.
Five separate cores confirmed that variations in the Earth's orbit drove the coming and going of ice ages.(46) But an unexpected finding brought in a new complication. The prominent 100,000-year cycle (due to changes in the orbit's eccentricity) had dominated climate change only during the most recent million years. During a long earlier phase of the Pleistocene epoch, the rise and decay of ice sheets had followed the 41,000-year cycle (due to shifts in the inclination of the Earth's axis).(47)