Chapter 15 Millennial Oscillations in Climate This chapter includes millennial oscillations during glaciations, millennial oscillations during the last 8000 years, causes of millennial-scale oscillations, and implications of millennial oscillations for future climate. To begin with, we must know how the oxygen isotope is represented in the ice core. The mean 18 O value of the ocean changes in response to the amount of 16 O-rich water vapor extracted from seawater and stored in ice sheets on the land. The ocean is left with 18 O-rich water from which planktic foraminifera extract oxygen for their shells. A second kind of 18 O signal important to studies of climate is changes in the 18 O value of ice layers. These values can vary by 5-10 over time: more negative values are typical of colder climates, distant proximity of source region, high 18 O composition of source, and high elevation of ice. Millennial Oscillations During Glaciations Early ice core studies in the 1970s concentrated on climate signals from 18 O and dust. The ice core reveals distinct changes in the past climate, and records spanning much of the last 100,000-year glacial cycle. The gradual underlying trends defining these larger-scale divisions in the ice core sequence were interpreted as the result of orbital-scale changes in Earth s climate between colder, drier, dustier glaciations and warmer, wetter, less dusty interglaciations, including the modern interglacial climate. The Greenland Ice Sheet Project sites were carefully chosen in areas of smooth underlying bedrock to minimize the impact of changes in ice flow that can disturb the deeper ice layer, and they were drilled within 30 kilometers of each other to see whether they would reveal similar climate histories. Because the two cores recorded the same signal over the last 110,000 years, scientists had no doubt results of local complications in ice flow. These millennial-scale cycles oscillations are often informally referred to as Dansgaard-Oeschger, who found and studied them in the earlier ice sheet core records. Recent ice core studies at higher resolution and with greater analytical precision have also detected fairly regular-looking oscillations close to 1500 years in length. Two questions are recurred from that they proving a connection to the oscillations documented in Greenland ice: (1) Is the climatic archive being examined capable of recording such brief oscillations? (2) How accurately can the oscillations that are detected be dated? Unfortunately, few archives combine annual resolution with long records (see Fig.3-14). Fortunately, some archives that that record climate changes lasting tens or hundreds of years do exist, allowing resolution of millennial-scale changes. The second problem is dating any oscillations that are detected accurately enough to determine their correlation with those found in Greenland ice. Within a dating uncertainty of 1000 years, singnals from two regions
could be (1) changing synchronously, (2) leading or lagging each other, or(3) responding in a totally opposite sense. In the studies of North Atlantic sediments near Greenland, two climatic proxies were found: (1) the relative amounts of polar foraminifera shells versus grains of ice-rafted sand and (2) the percentage of polar species of foraminifera out of the total population. As in the case of orbital-scale changes, more ice-rafted debris and higher percentage of polar foraminifera indicated colder water with more icebergs present. Changes in the percentage of polar foraminifera in North Atlantic cores closely match the pattern of 18 O changes in Greenland ice. This new evidence shows that millennial-oscillations were not restricted just to changes in air temperature and circulation over the Greenland ice sheet. They also involved other important parts of climate system, including the surface waters of the North Atlantic, where planktic foraminifera lived, and the Atlantic margins of the ice sheets, which supplied the icebergs that carried the course debris. Radiocarbon dating of the CaCO 3 shells of foraminifera in the younger ice-rafting events showed up to tenfold increase in the rate of deposition of ice-rafted debris, so that can trace the source regions of the bedrock with the character. The distinctive geochemical isotopic tracers point to limestone fragments from north of Hudson Bay, volcanic rocks from Iceland, and red sandstones from eastern North America. Some scientists infer that the record shows true cycle 1500 years that gradually build toward cooler conditions and end in major ice-rafting episodes, both other scientists do not believe that records provided by ice cores and ocean sediments can be dated accurately enough to prove that the oscillations are truly cyclic, and some even cite evidence from sediments in regions outside the area of ice-rafted deposition from Scandinavia rather than North America. Millennial-scale climate changes in Europe The same kind of short-term oscillations that appear in 18 O changes in Greenland ice also appear in changes in the character of European soil. These soils were richer in organic material during warmer episodes. The younger pollen fluctuations dated by 14 C appear to match the age of the ice core changes. Because the size of northern hemisphere ice sheets varied only a little during most millennial oscillations, some scientists have turned to a second explanation that draws on knowledge gained from study of orbital changes. Could these millennial oscillations be produced by some kind of interaction between the North Atlantic and the ice sheet margins, with the changes in the North Atlantic than propagated through the atmosphere to other regions? At orbital time scale, changes in the temperature of the North Atlantic s surface can influence temperature and precipitation in Europe and Asia. Millennial-scale climate changes in Florida and the Santa Barbara Basin In Florida, although the dating is not accurate enough to specify the relative timing.
Some scientists infer that melting snow flow via Mississippi River to the Gulf, and temperature changes in the Gulf of Mexico. The explanation for the short-term climate changes in Florida could still lie in the changes occurring at high latitudes. But many scientists infer that climate changes in Greenland, the North Atlantic, and farther south may be separate regional responses to a more pervasive cause of climate change acting at hemispheric and possibly even global scales. In the evidence from the Santa Barbara Basin sediments, the pattern of 18 O changes in planktic foraminifera closely matches 18 O changes in Greenland ice. Millennial-scale climate changes in Antarctic The Antarctic ice cores contain short-term 18 O oscillations with some resemblance to the Greenland ice record, but the amplitude of the Antarctic 18 O changes is smaller. Because the temperature oscillations over Antarctic were at times opposite to those over Greenland with smaller amplitude. Furthermore, marine geologists Tom Crowley and Tom Stocker proposed that the northward conveyor-belt flow was vigorous to remove heat from Southern Ocean and cool it. When the northward conveyor-belt flow is weaker, the Antarctic is warmer. Simulations with several oceanic GCMs suggest that such a hemispheric seesaw effect is likely. This short-term fluctuations discovered in the North Atlantic match that in the loess deposits of China. These are evidences from that millennial oscillations were all over the world. The relationship between greenhouse gases and millennial oscillations Greenland ice cores show definite millennial-scale changes in methane. CH 4 concentrations were about half of the difference between fully glacial and fully interglacial values. Scientists concluded that the CH 4 fluctuations lag about three decades behind the 18 O changes. This relationship implies that methane values were driven by millennial oscillations, and infers cold air temperatures suppressed the production and release of methane from boggy regions in Asia. It is difficult to know the relationship between the CO 2 of ice core and millennial oscillations. In Greenland, CO 2 values are contaminated by interacting with CaCO 3 dust in the ice. Any CO 2 oscillations lasting 1000 to 3000 years would be obliterated, so millennial-scale events were too brief to be detected. Millennial Oscillations During the Last 8000 Years Millennial-scale 18 O oscillations are not obvious in Greenland ice core during the last 8000 years, but small fluctuations do occur in sea salt (Na + and Cl - )from the ocean and in the amount of dust from the continents. These trends are interpreted as indicating changes in wind strength with a weak 2600-year cycle. Similarly, sediments from the North Atlantic indicate no major episodes of ice rafting or plankton reduction during the last 8000 years. The only large northern hemisphere ice sheet left in existence during the last 6000 years is the one on Greenland. Close
examination of North Atlantic sediments has recently detected intervals with slight increases in concentrations of sand-sized mineral grains. Even though these grains are 1000 to 100,000 times less abundant than the concentrations reached during glacial intervals, they show that small pulse of ice rafting did exist, apparently spaced at an interval of about 1500 years. An earlier study of this time interval found a 2500-year interval between advances, but one compilation of glacial advances can be interpreted as showing a 1500-year cyclicity. Causes of Millennial-Scale Oscillations Three hypothesis view millennial-scale changes are: 1. The natural oscillations inherent in the internal behavior of northern hemisphere ice sheet. 2. The result of internal interactions among several parts of the climate system. 3. A response to solar variations external to the climate system. The reason led us to the first hypothesis is that in general ice sheet response to the climate very slowly, except the margins of ice sheet. The cycle contains the growth of ice sheet and the melting process into the sea-water. This will raise some questions: Why would coastal ice margins cycle so regularly? Would the ice margins grow to just the size that made them to collapse in exactly 1500 years? Would a rise in sea level of a single meter have caused other ice margins to collapse? The second hypothesis states that single climate factor can t sustain the millennial-scale climate and one must pay attention to the interactions among several components of Earth s climate system. Wally Broecker hypothesized that a natural salt oscillator operates in the North Atlantic when large ice sheets are present. This cycle is found via the salt concentration changes in the ocean, in the change of the circulations of atmosphere and ice sheet. A mechanism through the atmosphere, not only changes temperature and precipitation pattern in Europe and Asia but also the character of lower-level circulation. Another mechanism is the changes in the surface-ocean conveyor flow that will cause an opposite-phase seesaw in the southern hemisphere surface ocean or even the global-wide. But evidence shows events spaced at intervals of 5000 years or more, and it is still unclear whether or not millennial oscillations lasting less than 4000 years are linked. The last hypothesis is for the difference existence between ages derived by counting tree rings and those derived by 14 C dating of those same rings, which difference about a few hundred or thousand years. Some studies suggest that 14 C of atmosphere changes was come from other galaxies, and might explain the observed short-term differences in the ages derived by the two dating methods. More 14 C production due to less deflection of cosmic particles by a strong sun, can make the air temperature cool. In fact, there are evidences confirming the idea that these differences in 14 C and tree ring age have an origin external to the climate system comes from an isotope of the element beryllium. But the major cycle of 14 C production is at 420 years, with much
weaker cycles at 2000-2500 years. In summary, the origin of millennial-scale oscillations in climate is still unknown. Implications of Millennial oscillations for Future Climate The implication for our future, leaving aside for now the next few centuries of greenhouse-gas alteration of climate, is that a slow natural orbital-scale cooling of the northern hemisphere will be interrupted by millennial-scale cool oscillations. Unfortunately, the irregular nature of these oscillations during the last 8000 years makes it hard to predict exactly when such oscillation will occur in the future.