Guest essay by Dr. Don J. Easterbrook
Dept. of Geology, Western Washington University, Bellingham, WA
The results of oxygen isotope measurements from ice cores in the Greenland and Antarctic ice sheets several decades ago stunned the scientific world. Among the surprises from the cores was the recognition of multiple, late Pleistocene, extraordinarily abrupt, intense periods of warming and cooling. The most precise records of late Pleistocene climate changes are the ice cores of the Greenland Ice Sheet Project (GISP) and the Greenland Ice Core Project (GRIP). These cores are especially important because the ages of the ice at various levels in the core have been measured by counting annual layers in the ice, giving a very accurate chronology of climatic fluctuations determined by measurement of annual layers.
Figure 1. Oxygen isotope fluctuations in the GISP2 Greenland ice core. Red = warm periods, blue = cold periods. Up on the vertical axis is warmer (Plotted from data in Stuiver and Grootes, 1997)
Figure 2. Temperature fluctuations in the GISP2 Greenland ice core. Red = warm periods, blue = cold periods. (Plotted from data in Cuffy and Clow, 1997 and Alley, 2000)
Figure 1 is the oxygen isotope record from the GISP2 Greenland ice core from 11,000 to 15,000 years before present (BP). Although it is not a direct measure of temperature, the 18O/16O ratio is a proxy for temperature. Figure 2 is the reconstructed temperature record for the same time interval, based on data from Cuffy and Clow (1997), modified by Alley (2000). Comparison of the two different approaches in Figures 1 and 2 shows essential agreement.
The temperature variations shown in Figures 1, 2, and 3 correlate very well with other geologic data that has led to recognition of several named periods of warming and cooling. The named periods of warming and cooling discussed below were established long before the same climatic events were discovered in the ice cores.
 Bølling warm period: Abrupt, intense warming 14,500 years ago resulted in sudden wholesale melting of the huge continental ice sheets that occupied vast areas in North America, Europe, and Russia and extensive alpine glaciers in mountainous areas. What made this warming so remarkable was not only its abrupt onset, but also its intensity. Temperatures in Greenland rose 20°F (~12° C), about equal to the total cooling of the late Pleistocene glaciation, to near present-day levels in about one century (Figs. 1, 2). Although this temperature change is for Greenland, simultaneous glacial retreat all over the world indicates that this was a global event. Prior to the ice core analyses, such large swings in temperature were believed to have taken a thousand years or more. This warming event, known as the Bølling (Figs. 1, 2) lasted only a few hundred years, but temperatures rose to near-modern levels.
 Older Dryas cold period: At the end of Bølling, temperatures suddenly plummeted about 20° F (~11°C) from the Bølling maximum in a few hundred years (Figs. 1, 2), initiating the Older Dryas cold period, which lasted from about 14,300 to 14,000 years before present (BP). Temperatures returned to near full glacial levels and glaciers halted their rapid retreat.
 Allerød warm period: About 14,000 years BP, temperatures once again rose abruptly and the Allerød warm period began. It lasted until 12,800 years BP, but was not as warm as the present or the Bølling. However, the rate of warming was very intense ~8° F (~4.5°C) accomplished in as little as a single century.
 Inter-Allerød cold period: Near the end of the Allerød warm period (13-14,000 years BP), temperatures dropped precipitously, ~14° F (~8°C) in about a century (Figs. 1, 2) during a time known as the Inter-Allerød cold period (IACP). Temperatures returned to near full Ice Age levels but persisted for only a few hundred years, so glaciers halted their retreat but did not rebuild to former extents. Just as suddenly as it had cooled, the IACP warmed abruptly 9° F (~5°C) and temperatures returned to Allerød levels.
 Younger Dryas cold period: 12,800 years ago, temperatures plunged ~14° F (~8°C) to full glacial levels where they remained for 1300 years during the Younger Dryas (YD). Because of the abruptness, intensity, and duration of the cooling, the YD is the best known of the Dansgaard/Oerscher events. Glaciers, including remnants of the huge ice sheets and alpine glaciers, re-advanced, leaving moraines as footprints of their former presence. Temperatures rose sharply, about 21° F (~12° C) 11,500 years ago, marking the end of the Younger Dryas cold period and the end of the Pleistocene Ice Age. Additional details of the YD may be found at http://wattsupwiththat.com/2012/06/19/the-intriguing-problem-of-the-younger-dryaswhat-does-it-mean-and-what-caused-it/
Radiocarbon and isotope dating of glacial moraines in regions all over the world and abrupt changes in oxygen isotope ratios in Greenland and Antarctic ice cores indicate that the Younger Dryas cooling was globally synchronous. Evidence of Younger Dryas advance of continental ice sheets is reported from the Scandinavian ice sheet, the North American Laurentide and Cordilleran ice sheets, and the Russian ice sheet. Alpine and ice cap glaciers also advanced during Younger Dryas cooling in both the Northern and Southern hemispheres, e.g., many places in the Rocky Mts. of the U.S. and Canada, the Cascade Mts. of Washington, the European Alps, the Southern Alps of New Zealand, and the Patagonian Andes Mts. of South America.
Multiple Climatic events within the Younger Dryas
The Younger Dryas cooling was not just a single climatic event. Climatic warming and cooling occurred not only before and after the YD, but significant climate fluctuations also occurred within the YD. That these were global events that occurred in both hemispheres is shown not only by the ice cores of Greenland and Antarctica, but also in glacial deposits of the major, late Pleistocene ice sheets of the world, all of which experienced multiple moraine-building episodes as did alpine glaciers.
Figure 3. Oxygen isotope record from the Greenland ice core showing an abrupt temperature drop 12,800 years ago, 1,300 years of cool climate, and sudden warming 11,500 years ago. Up on the vertical axis is warmer. (Plotted from data in Cuffy and Clow, 1997 and Alley, 2000)
Figure 3 shows a plot of oxygen isotope variation within the YD. Temperatures fluctuated up and down at least a dozen times, some brief warming periods reaching near-Allerød levels. That these climatic fluctuations were real and global in extent is shown by multiple YD and IACP moraines in the Puget Lowland of Washington, Loch Lomond in the Scottish Highlands, European Alps, Rocky Mts., Alaska, Cascade Range, Andes, New Zealand Alps, and elsewhere.
Magnitude and rate of abrupt climate changes
How do past temperature oscillations compare with recent global warming (1978-1998) or with warming periods over the past millennia? The answer to the question of magnitude and rates of climate change can be found in the δ18O and ice core temperature data (http://wattsupwiththat.com/2011/01/24/easterbrook-on-the-magnitude-of-greenland-gisp2-ice-core-data/).
We can compare the warming and cooling in the past century to approximate 100 year periods in the past 25,000 years (Fig. 4). Not all of the periods noted here are exactly 100 years̶̶—some are slightly more, some are slightly less, but they are close enough to allow comparison of magnitude and rates with the past century.
Figure 4. Magnitudes of the largest warming/cooling events over the past 25,000 years. Temperatures on the vertical axis are rise or fall of temperatures in about a century. Event number 1 is about 24,000years ago and event number 15 is about 11,000 years old. At least three warming events were 20 to 24 times the magnitude of warming over the past century and four were 6 to 9 times the magnitude of warming over the past century. The magnitude of the only modern warming which might possibly have been caused by CO2. (1978-1998) is insignificant compared to the earlier periods of warming. (Plotted from data in Cuffy and Clow, 1997 and Alley, 2000)
Implications of multiple Younger Dryas and Inter-Allerød climatic fluctuations
The multiple nature of YD moraines in widely separated areas of the world and in both hemispheres indicates that the YD consisted of more than a single climatic event and these occurred virtually simultaneously worldwide. Ice sheets and alpine glaciers were sensitive to the multiple YD phases.
What can we learn from all of this?
(1) The ice core isotope data were hugely significant because they showed that the Younger Dryas, as well as the other late Pleistocene warming and cooling events, could not possibly be caused by human emissions of CO2 because they occurred thousands of years before such emissions had any effect on atmospheric CO2.
(2) The magnitude and intensity of multiple climatic fluctuations has been up to 20 times larger than warming during the past century.
(3) Single events, i.e., volcanic activity or cosmic impacts, cannot have caused the abrupt Dansgaard/Oerscher warming and cooling events because of the multiplicity of warm/cold events over periods of thousands of years.
(4) The absence of a time lag between the N and S Hemisphere glacial fluctuations precludes an oceanic cause and is not consistent with the North Atlantic Deep Ocean Water hypothesis for the cause of the Younger Dryas.
(5) The abruptness of the climate changes and their multiplicity could not have been caused by slow, Croll-Milankovitch orbital forcing, which occurs over many tens of thousands of years. Since fluctuations to and from full glacial climates occurred over short periods of time, clearly a cause other than the Croll-Milankovitch theory is capable of causing the Ice Ages .