A Dansgaard–Oeschger event (often called a D–O event) is a sudden change in Earth's climate. These events happened 25 times during the last glacial period. Some scientists believe these events occur in a pattern that repeats roughly every 1,470 years or multiples of that time, but this idea is not fully agreed upon. Similar climate patterns during the Holocene are known as Bond events. The term "Dansgaard–Oeschger" is named after scientists Willi Dansgaard and Hans Oeschger.
Evidence
The strongest evidence for Dansgaard–Oeschger events is found in Greenland ice cores, which only provide data from the end of the last warm period, the Eemian interglacial (about 115,000 years ago). Ice cores from Antarctica suggest that these events are connected to changes in Antarctica, called Antarctic Isotope Maxima, through a climate pattern known as the polar see-saw. If this connection is true for earlier ice ages, Antarctic data indicate that Dansgaard–Oeschger events likely occurred during previous glacial periods as well. However, Greenland ice cores only cover the most recent glacial period, so direct evidence of these events in older glacial periods from Greenland is not available. Research by Stephen Barker and others has shown that the Greenland ice core record can be recreated using data from Antarctic ice cores. This method allows scientists to build a much older Greenland record based on the nearly million-year-long Antarctic ice core record.
Effect
In the Northern Hemisphere, these events cause quick temperature increases that usually happen over decades, followed by slower cooling that lasts much longer. For example, about 11,500 years ago, average yearly temperatures on the Greenland ice sheet rose by about 8 degrees Celsius over 40 years, in three steps of five years. A 5-degree Celsius change over 30–40 years is more common. The warming from these events also reached farther south into central North America, as shown by changes in oxygen levels in cave formations that match the timing of these events in Greenland ice cores. In Europe, these events are also seen in changes in how rivers flow and deposit sediment, such as in the Tisza River.
Heinrich events only happen during the cold periods right before these warming events, leading some scientists to think these warming cycles might cause the Heinrich events or at least control when they occur.
Each D-O event begins with a rapid warming, followed by a cooler period lasting a few hundred years. During this cold time, the boundary of the polar region moves farther south, with ice extending further into the North Atlantic Ocean.
D-O events are also believed to cause small increases in the amount of carbon dioxide in the atmosphere, about 5 parts per million.
During these events, changes in oxygen levels in cave formations in Floresia show that the Indonesian-Australian Monsoon weakened.
The effects of D-O events have also been found in the Northern Andes, where these events during the Penultimate Glacial Period matched changes in plant life patterns.
Causes
The reasons for when and how strong these events are (as shown in ice cores) are not fully understood. In the Southern Hemisphere, warming happens more slowly, and temperature changes are much smaller. The Vostok ice core was studied before the Greenland cores, and the presence of Dansgaard–Oeschger events was not widely known until the Greenland (GRIP/GISP 2) cores were analyzed. After this, scientists reviewed the Vostok core again to determine if these events had been overlooked.
These events seem to be linked to changes in the North Atlantic Ocean's movement, possibly caused by an increase in fresh water or rainfall.
Possible causes include stronger effects of solar changes or factors within Earth's systems, such as a cycle where ice sheets grow so large they become unstable (as suggested for Heinrich events) or shifts in deep ocean currents (Maslin et al., 2001, p25).
These events have been connected to changes in ice sheet size and atmospheric carbon dioxide. Ice sheet size affects the strength of the Atlantic Ocean's circulation by altering northern hemisphere winds, the Gulf Stream, and sea ice. Atmospheric carbon dioxide influences how freshwater moves between ocean basins across Central America, changing the freshwater balance in the North Atlantic and affecting ocean circulation. These findings support the idea of a "D-O window" of AMOC bistability (a period when ocean currents are more likely to change abruptly) linked to ice volume and atmospheric CO₂, explaining why D-O type events occurred during moderate glacial conditions in the late Pleistocene.
Timing
The effects of Dansgaard–Oeschger (D-O) events are mainly seen in ice cores from Greenland, but evidence suggests these events happened at the same time across the globe. Scientists studied the GISP2 ice core from America and found a pattern in oxygen levels that repeats about every 1500 years. Schulz (2002) suggested this pattern repeats every 1470 years. Rahmstorf (2003) supported this, noting that the variation in the timing of these events was about ±12% over the past 50,000 years, with smaller variations in the most recent events.
However, older parts of the GISP2 core and the GRIP core from Greenland do not show this regular pattern. This may be because the first 50,000 years of the GISP2 core were dated more accurately by counting layers. The climate system’s response to the trigger of these events varies by about 8% of the cycle’s length. Natural changes in Earth’s systems are expected to be less regular. Rahmstorf proposed that the highly regular pattern might be linked to an orbital cycle, but no such cycle has been found. A Lunar cycle of 1,800 years does not match this pattern. Dating differences between the GRIP and GISP2 cores were about 5,000 years at 50,000 years before present. Ditlevsen et al. (2005) noted that the GISP2 core’s pattern was not present in the GRIP core, showing how accurate dating affects results. The NGRIP core’s dating helped resolve this issue, revealing that D-O events occur randomly, like a process driven by noise.
D-O cycles may set their own timing. Maslin et al. (2001) suggested that each ice sheet has unique stability conditions, but when ice melts, the added freshwater changes ocean currents, causing melting in other areas. Specifically, D-O cold events and the meltwater they produce weaken the North Atlantic Deep Water current (NADW), reducing northern hemisphere circulation and increasing heat transfer to the southern hemisphere. Warmer water in the south melts Antarctic ice, reducing density differences and weakening the Antarctic Bottom Water current (AABW). This allows the NADW to strengthen again, causing more melting in the north and starting another D-O cold event.
This theory may also explain the connection between Heinrich events and D-O cycles. When meltwater accumulates enough to raise sea levels, it could have undercut the Laurentide ice sheet, causing a Heinrich event and restarting the cycle.
Some scientists believe the Little Ice Age, which occurred between 400 and 200 years ago, was a cold phase of a D-O cycle.
History
The signals in ice cores now known as Dansgaard–Oeschger events were visible in the original GISP core and the Camp Century Greenland core. However, when these ice cores were first studied, their importance was noticed but not fully understood. Dansgaard et al. (AGU geophysical monograph 33, 1985) described these signals in the GRIP core as "violent oscillations" in the δO signal. They observed that these patterns matched similar events in the Camp Century core, which was located 1,400 km (870 mi) away. This match suggested that these events were not limited to one area but were linked to large-scale climate changes. Without the Camp Century core, these patterns might have been thought to be local changes. Dansgaard et al. suggested that these events might be connected to certain patterns in the atmosphere and ocean systems. These Dansgaard–Oeschger events are believed to influence the "Sahara pump," a process that has affected human evolution and movement. These repeating patterns are also seen during the Holocene period and are called Bond events.