by Judith Curry
. . . suggesting that Dansgaard-Oeschger events resulted from a combination of the effects of sea ice and ice shelves—structures that help define the margins of ice sheets—to account for both the rapid and the slower parts of the cycle.
I searched previous Climate Etc. posts, and it seems that I have not written previously on the topic of abrupt climate change. I guess I’m surprised, since I regard this as a very important topic. Here I am referring to large changes, larger than the magnitude of say the 1976 climate shift which is sometimes referred to as an abrupt climate change.
For background information, I recommend the 2002 NRC report Abrupt Climate Change: Inevitable Surprises. There is a more recent NRC report Abrupt Impacts of Climate Change: Anticipating Surprises. Richard Harris has article on the recent NRC Report. For something that is quicker to read, see the Wikipedia. Jeff Masters has a pretty good overview article The Science of Abrupt Climate Change: Should We Be Worried?
The trigger for this particular post is this brief overview that appeared in Science:
Dansgaard-Oeschger (DO) cycles—rapid warming events with durations of approximately 1000 years, followed by a more gradual return to cold conditions—are some of the most dramatic examples of rapid climate change that occurred in the North Atlantic region over the last glacial period. Although there has been no lack of suggestions about their possible origins, all of the causal mechanisms proposed thus far have run into difficulty explaining one part of the cycle or another. Petersen et al. step into the fray, suggesting that DO events resulted from a combination of the effects of sea ice and ice shelves—structures that help define the margins of ice sheets—to account for both the rapid and the slower parts of the cycle. Their model relies on the ability of thin sea ice to respond quickly to changing environmental conditions and on the more gradual behavior of thick ice shelves to cause cooling. Although more work needs to be done to support this model, it is consistent with existing proxy records, model results, and modern observations.
The paper that is referred was published in Paleoceanography [link to abstract]:
A new mechanism for Dansgaard-Oeschger cycles
S. V. Petersen, D. P. Schrag, and P. U. Clark
Abstract. We present a new hypothesis to explain the millennial-scale temperature variability recorded in ice cores known as Dansgaard-Oeschger (DO) cycles. We propose that an ice shelf acted in concert with sea ice to set the slow and fast timescales of the DO cycle, respectively. The abrupt warming at the onset of a cycle is caused by the rapid retreat of sea ice after the collapse of an ice shelf. The gradual cooling during the subsequent interstadial phase is determined by the timescale of ice-shelf regrowth. Once the ice shelf reaches a critical size, sea ice expands, driving the climate rapidly back into stadial conditions. The stadial phase ends when warm subsurface waters penetrate beneath the ice shelf and cause it to collapse. This hypothesis explains the full shape of the DO cycle, the duration of the different phases, and the transitions between them and is supported by proxy records in the North Atlantic and Nordic Seas.
Some background information from the Introduction:
During the last glacial period, the North Atlantic basin experienced a number of large and abrupt millennial-scale fluctuations in climate referred to as Dansgaard-Oeschger (DO) cycles. Ice cores from Greenland reveal that each cycle began with an abrupt warming from stadial to interstadial conditions. The effects of this warming extended across much of the Northern Hemisphere, while a near-simultaneous cooling occurred in Antarctica. Greenland ice core records then suggest gradual cooling during the initial stages of each interstadial phase, followed by abrupt cooling back to stadial conditions.
A common explanation for these cycles involves changes in the Atlantic meridional overturning circulation (AMOC), perhaps triggered by freshwater forcing, but paleoceanographic evidence for these changes remains elusive.
Description of the new hypothesis:
We propose a conceptual model for DO cycles that explains their characteristic temporal evolution and is supported by existing proxies of ice-sheet, climate, and AMOC variability. In particular, we adopt the sea-ice mechanism of Li et al. [2005, 2010] to explain the fast-changing intervals of the DO cycles. We then invoke an ice shelf to explain the slower-changing phases of the DO cycles. From the perspective of the atmosphere, an ice shelf looks the same as sea ice in terms of its albedo and its insulating effects, which reduce the release of heat from the ocean. However, because ice shelves are much thicker than sea ice (hundreds of meters versus <10 m), they are largely insensitive to small changes in heat transport or wind stress.
We first consider the influence of an ice shelf covering a large region of the ocean east of Greenland in the Nordic Seas. Given the sensitivity analysis by Li et al.  and the number of proxies showing variability of the cryosphere on DO timescales in the Nordic Seas, we focus on an ice shelf along the eastern Greenland margin that could influence sea-ice cover in this region. We propose that the cooling effect of a large ice shelf combined with extensive sea-ice cover would result in regionally cold surface temperatures due to the insulating properties of the ice shelf and sea ice, as well as their effect on local albedo [Li et al., 2005, 2010]. This stadial climate would be maintained for as long as the ice shelf was present.
In the event of the ice shelf’s collapse, potentially caused by warming of subsurface waters, the only remaining ice cover would be sea ice and floating icebergs. A small change in wind stress or heat transport could quickly export or melt this ice, resulting in a large increase in open-ocean area and a corresponding large and abrupt warming over Greenland marking the start of a new DO cycle.
During the interstadial phase of a DO cycle, the near doubling of accumulation over the Greenland Ice Sheet that accompanies the warmer climate would induce a more positive mass balance, causing the ice shelf to begin reforming along the coast. Expansion of the ice shelf to cover increasingly more ocean surface area would cause air temperatures to gradually cool over Greenland. Once the shelf reached a critical size, it would cause sea ice to rapidly expand through the sea-ice-albedo feedback, driving climate back to stadial conditions and completing the DO cycle. The same cycle could not be achieved with multi-year sea ice because its regrowth timescale is inconsistent with the gradual decline of climate over the duration of the interstadial phase. In summary, our hypothesis combines the ability of sea ice in the Nordic Seas to explain the rapid transition into and out of the interstadial phase with a gradually expanding ice shelf derived from eastern Greenland to explain the progressive cooling during the interstadial, provide the mechanism to trigger sea-ice growth to cause the rapid cooling , and sustain the stadial climate once the ice shelf reaches steady state. The duration of the interstadial phase is determined by the time required to regrow the ice shelf to a threshold size, beyond which the local ice-albedo effect causes the rapid expansion of sea ice and the corresponding switch to a stadial climate. After a time, ice-shelf collapse, potentially due to subsurface warming, along with an associated rapid loss of sea ice causes the abrupt warming that starts a new DO cycle.
JC comments: I find the ideas presented by Petersen et al. to be fascinating. I find the topic of abrupt climate change to be more more interesting scientifically than AGW, and potentially of greater societal significance (although one key question is the potential role of AGW in triggering an abrupt event). IMO much more scientific effort should be focused on the topic of abrupt climate change. The recent NRC Report targets some specific observing systems that would help understand and anticipate these changes.