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 agreed upon by all scientists. During the Holocene, similar repeating climate patterns are called 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 go back to the end of the last warm period, the Eemian interglacial (about 115,000 years ago). Ice core data from Antarctica suggest that these events are connected to the Antarctic Isotope Maxima through a climate relationship between the two hemispheres, called the polar see-saw. If this connection also applies to earlier ice ages, Antarctic records indicate that Dansgaard–Oeschger events likely occurred during those 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 shows that Greenland’s ice core record can be recreated using Antarctic ice core data. This allows scientists to rebuild an older Greenland record by studying the nearly million-year-old Antarctic ice core record.
Effect
In the Northern Hemisphere, these events cause quick warming periods that happen over decades, followed by slower cooling over longer times. For example, about 11,500 years ago, average yearly temperatures on the Greenland ice sheet rose by about 8°C over 40 years, in three steps of five years. A 5°C change over 30–40 years is more typical. 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 how sediments build up, like in the Tisza River.
Heinrich events only happen during the cold periods right before these warming events, leading some scientists to suggest that these cycles might cause the events or at least control when they occur.
Each event begins with a rapid warming, followed by a cold period lasting a few hundred years. During this cold time, the polar front expands, and ice spreads farther south across the North Atlantic Ocean.
These events are also thought to cause small increases in atmospheric carbon dioxide levels, 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 these events have also been found in the Northern Andes, where they matched changes in plant life during the Penultimate Glacial Period.
Causes
Scientists are still trying to understand why certain climate events, as shown in ice cores, happen at specific times and with certain intensities. In the Southern Hemisphere, warming occurs more slowly, and temperature changes are much smaller. The Vostok ice core in Antarctica was studied before ice cores from Greenland, and the existence of Dansgaard–Oeschger events was not widely known until Greenland cores (GRIP/GISP2) were analyzed. After this discovery, scientists revisited the Vostok core to check if these events might have been overlooked.
These events seem to be linked to changes in the North Atlantic Ocean's circulation, possibly caused by large amounts of fresh water or heavy rainfall. Possible causes include stronger effects of solar changes or natural cycles within Earth's systems. These could involve ice sheets growing so large they become unstable, as seen in Heinrich events, or shifts in deep ocean currents, as suggested by some studies.
These events have also been connected to changes in ice sheet size and levels of atmospheric carbon dioxide. Larger ice sheets affect ocean circulation by changing wind patterns, the Gulf Stream, and sea ice in the Northern Hemisphere. Higher carbon dioxide levels influence how freshwater moves between ocean basies near Central America, altering the North Atlantic's freshwater balance and circulation. These findings support the idea of a "D-O window" — a period when ocean circulation is especially sensitive to changes in ice volume and carbon dioxide — which explains why Dansgaard–Oeschger-type events occurred during certain times in Earth's history.
Timing
The effects of the Dansgaard–Oeschger (D-O) events are mainly found in ice cores from Greenland. However, evidence suggests these events may have occurred at the same time globally. A study of the American GISP2 ice core found a pattern in oxygen isotope levels that repeats about every 1,470 years. This was proposed by Schulz (2002) as a regular cycle. Rahmstorf (2003) supported this, noting that the most recent 50,000 years of the GISP2 core show a variation of about ±12% in the timing of these events (±2% for the five most recent, which are likely the most accurate).
However, older parts of the GISP2 core and the GRIP core (from Greenland) do not show this regular cycle. This may be because the first 50,000 years of the GISP2 core are dated more precisely through counting layers. The climate system’s response to the trigger for these events varies by about 8% of the cycle’s length. Natural changes in Earth’s systems are expected to be less regular. Rahmstorf suggested that a highly regular pattern might point to an orbital cycle, but no such cycle has been identified. A lunar cycle of 1,800 years does not match this pattern. The dates from the European GRIP core and the American GISP2 core differ by about 5,000 years for events 50,000 years ago. Ditlevsen et al. (2005) noted that the 1,470-year pattern in the GISP2 core was not found in the GRIP core, showing the importance of accurate dating. This issue was resolved by the precise dating of the NGRIP core, which showed that D-O events occur randomly, like a process called a Poisson process.
D-O cycles may set their own timing. Maslin et al. (2001) suggested that each ice sheet has unique stability conditions. However, when ice melts, the added freshwater can change 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 circulation in the northern hemisphere. This shifts heat toward the southern hemisphere, warming Antarctic waters and melting ice there. This weakens the Antarctic Bottom Water current (AABW), allowing the NADW to strengthen again, which causes more melting in the northern hemisphere and another D-O cold event.
This theory also explains the connection between Heinrich events and D-O cycles. When meltwater accumulates in the ocean and reaches a certain level, it may raise sea levels enough to destabilize the Laurentide ice sheet, triggering a Heinrich event and restarting the cycle.
Some scientists have suggested that the Little Ice Age, which occurred between 400 and 200 years ago, may have been the 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 widely understood. Dansgaard et al. (AGU geophysical monograph 33, 1985) described these events in the GRIP core as "violent oscillations" in the oxygen isotope measurements. They observed that these patterns matched similar changes in the Camp Century core, which was located 1,400 km (870 mi) away. This matching provided evidence that the events were linked to large-scale climate changes, rather than local conditions. Dansgaard et al. suggested these events might be connected to slow-moving 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.
This pattern of repeated cycles is also seen during the Holocene period, where the events are called Bond events.