Ongoing D-O event in North Atlantic

[Vice-Regent]

There has been a rapid change in SST anomalies south of Greenland especially along the Labrador Sea. This is a significant risk factor for ice cliff failure and bottom melt along the Jakobshavn Glacier. As a result there are novel indicators of an ongoing Dansgaard–Oeschger oscillation and urgent action is needed to study the present and future effects of this particular episode of Greenland Meltwater injection. There is a larger than average risk of rapid Sea Level Rise especially along the US East Coast and Equatorial Pacific region for the period 2019-2030 (followed by a Heinrich Meltwater Event and subsequent cooling south of Greenland).

Effect

In the Northern Hemisphere, they take the form of rapid warming episodes, typically in a matter of decades, each followed by gradual cooling over a longer period. For example, about 11,500 years ago, averaged annual temperatures on the Greenland ice sheet warmed by around 8 °C over 40 years, in three steps of five years (see,^[3]Stewart, chapter 13), where a 5 °C change over 30–40 years is more common.

Heinrich events only occur in the cold spells immediately preceding D-O warmings, leading some to suggest that D-O cycles may cause the events, or at least constrain their timing.^[4]

The course of a D-O event sees a rapid warming of temperature, followed by a cool period lasting a few hundred years.^[5] This cold period sees an expansion of the polar front, with ice floating further south across the North Atlantic Ocean.^[5]

Causes

The processes behind the timing and amplitude of these events (as recorded in ice cores) are still unclear. The pattern in the Southern Hemisphere is different, with slow warming and much smaller temperature fluctuations. Indeed, the Vostok ice core was drilled before the Greenland cores, and the existence of Dansgaard–Oeschger events was not widely recognised until the Greenland (GRIP/GISP2) cores were done; after which there was some reexamination of the Vostok core to see if these events had somehow been "missed".^{[verification needed]}

 

A closeup near 40 kyr BP, showing reproducibility between cores

The events appear to reflect changes in the North Atlantic Ocean circulation, perhaps triggered by an influx of fresh water.^[5]

The events may be caused by an amplification of solar forcings, or by a cause internal to the earth system – either a "binge-purge" cycle of ice sheets accumulating so much mass they become unstable, as postulated for Heinrich events, or an oscillation in deep ocean currents (Maslin et al.. 2001, p25).

More recently, these events have been attributed to changes in the size of the ice sheets ^[6] and atmospheric carbon dioxide ^[7]. The former determines the strength of the Atlantic Ocean circulation via altering the northern hemisphere westerly winds, gulf stream, and sea-ice systems. The latter modulates atmospheric inter-basin freshwater transport across Central America, which changes the freshwater budget in the North Atlantic and thus the circulation. They further suggest the existence of a window of AMOC bistability ('sweet spot' for abrupt climate changes) associated with ice volume and atmospheric CO2, accounting for the occurrences of D-O type events under intermediate glacial conditions in the late Pleistocene.

Timing

Although the effects of the Dansgaard–Oeschger events are largely constrained to ice cores taken from Greenland,^[8] there is evidence to suggest that D-O events have been globally synchronous.^[9] A spectral analysis of the American GISP2 isotope record^[10] showed a peak of [¹⁸O:¹⁶O] abundance around 1500 years. This was proposed by Schulz (2002)^[11] to be a regular periodicity of 1470 years. This finding was supported by Rahmstorf (2003);^[12] if only the most recent 50,000 years from the GISP2 core are examined, the variation of the trigger is ±12% (±2% in the 5 most recent events, whose dates are probably most precise). However the older parts of the GISP2 core do not show this regularity, nor do the same events in the GRIP core. This may be because the first 50 kyr of the GISP2 core are most accurately dated, by layer counting. The climate system response to the trigger is varying within 8% of the period. Oscillations within the Earth system can be expected to be far more irregular in period. Rahmstorf suggests that the highly regular pattern would point more to an orbital cycle. Such a source has not been identified. The closest orbital cycle, a Lunar cycle of 1,800 years, cannot be reconciled with this pattern.^[12] The dating between the European GRIP ice core and the American GISP2 ice core differs by about 5000 years at 50,000 years BP. It was noted by Ditlevsen et al. (2005)^[13] that the spectral peak found in the GISP2 ice core was not present in the GRIP core, and thus depended critically on the accuracy of the dating. The dating issue was largely solved by the accurate dating of the NGRIP core.^[14] Using this dating the recurrence of Dansgaard–Oeschger events is random consistent with a noise induced Poisson process.^[15]

D-O cycles may set their own timescale. Maslin et al.. (2001) suggested that each ice sheet had its own conditions of stability, but that on melting, the influx of freshwater was enough to reconfigure ocean currents – causing melting elsewhere. More specifically, D-O cold events, and their associated influx of meltwater, reduce the strength of the North Atlantic Deep Water current (NADW), weakening the northern hemisphere circulation and therefore resulting in an increased transfer of heat polewards in the southern hemisphere. This warmer water results in melting of Antarctic ice, thereby reducing density stratification and the strength of the Antarctic Bottom Water current (AABW). This allows the NADW to return to its previous strength, driving northern hemisphere melting – and another D-O cold event. This theory may also explain Heinrich events' apparent connection to the D-O cycle; when the accumulation of meltwater in the oceans reaches a threshold, it may have raised sea level enough to undercut the Laurentide ice sheet – causing a Heinrich event and resetting the cycle.

The little ice age of ~400 to 200 years ago has been interpreted by some as the cold part of a D-O cycle.^[5]

 

 

[etudiant]

Apart from the warmer than usual water around Greenland, is there any evidence that this kind of post ice age event is possible during inter glacial periods such as the one we are now in?

[Vice-Regent]

1 hour ago, etudiant said:

Apart from the warmer than usual water around Greenland, is there any evidence that this kind of post ice age event is possible during inter glacial periods such as the one we are now in?

Put simply. There are no analogs. There is a high level of risk as a result of the high level of uncertainty. It would require a large CO2 forcing to cause a meltwater injection cooling in the Holocene/Anthropocene because our ice sheets are much more reduced already versus the Younger Dryas.

 

[Drz1111]

Jesus Christ.  I spent 5 years studying D-O events.   Please stop spraying bullshit everywhere.   There is no D-O event ongoing, forthcoming, incoming or anything of the sort.  PS I wrote some of the wiki article you plagiarized.

[Vice-Regent]

1 minute ago, Drz1111 said:

Jesus Christ.  I spent 5 years studying D-O events.   Please stop spraying bullshit everywhere.   There is no D-O event ongoing, forthcoming, incoming or anything of the sort.  PS I wrote some of the wiki article you plagiarized.

Incredible thanks for clarifying that. It is well-written. As of this other non-sense. We must be cautious going forward. Uncharted territory.

[Vice-Regent]

This is what a D-O event looks like in 2019. No doubt about it.

[Drz1111]

No, that is not.  You're just ignorant.  Stop posting.

[Vice-Regent]

4 minutes ago, Drz1111 said:

No, that is not.  You're just ignorant.  Stop posting.

Okay so why are D-O events not possible in the Holocene and what would prevent them from concurring in the future? You need to be more articulate.

[Drz1111]

Read the wiki article you plagiarized.  It’s all in there, including an explanation responding to exactly your question.