RealClimate: Quite a bit has already been written on the ice core feat (including Richard Alley’s commendable inside story “The Two Mile Time Machine”), and no doubt much more will be.

It was the early, pioneering ice coring efforts in Greenland in the 1980s and 90s that first revealed the abrupt climate shifts called “Dansgaard-Oeschger events” (or simply DO events), which have fascinated and vexed climatologists ever since. Temperatures in Greenland jumped up by more than 10 ºC within a few decades at the beginning of DO events, typically remaining warm for several centuries after. This happened over twenty times during the last great Ice Age, between about 100,000 and 10,000 years before present.

…nailing down the mechanism of the mysterious abrupt climate jumps in Greenland and their reverberations around the world, which can be identified in places as diverse as Chinese caves, Caribbean seafloor sediments and many others. So what are the new data telling us?

These data connect the Antarctic ups and downs of climate to the much greater ones of Greenland.

It is (at least in the model) a result of a big change in northward heat transport in the Atlantic. If the heat transport by the Atlantic thermohaline circulation suddenly increases for some reason (we’ll come to that), Greenland suddenly gets warm (an effect amplified by receding sea ice cover of the seas near Greenland) and Antarctica starts to cool.

But irrespective of the details: the new data from Antarctica(the ice-core date) clearly point to ocean heat transport changes as the explanation for the abrupt climate changes found in Greenland. RealClimate

Today our understanding of what have might caused past DO events got better, though today the reason for the mechanism in play is different. But it appears to lead into the same direction. Joe Romm interviewed Jennifer Francis (Rutger University) and examined a recent (May 2013) study which was included in the Geophysical Research Letter:

Exceptional 2012 Greenland Ice Melt Caused By Jet Stream Changes That May Be Driven By Global Warming

New research finds that “unusual changes in atmospheric jet stream circulation caused the exceptional surface melt of the Greenland Ice Sheet (GrIS) in summer 2012.” Prof. Jennifer Francis tells me these changes are consistent with those caused by warming-driven “Arctic Amplification.” And that means GrIS may melt faster than climate models have projected.

Back in May, a study found that by 2025, there is a “50-50 chance” of this unprecedented ice melt happening annually simply based on the continued rapid warming of GrIS.

As the news release explains, an international team used a computer model and satellite data “to confirm a record surface melting of the GrIS for at least the last 50 years – when on 11 July 2012, more than 90 percent of the ice-sheet surface melted. This far exceeded the previous surface melt extent record of 52 percent in 2010.” Weather station data “showed that several new high Greenland temperature records were set in summer 2012.”

The research “clearly demonstrates that the record surface melting of the GrIS was mainly caused by highly unusual atmospheric circulation and jet stream changes, which were also responsible for last summer’s unusually wet weather in England.”

Asked by Joe Romm, Jennifer Francis, one of the leading Scientist for Arctic Amplification commented on this study:

Sea ice loss is only one factor driving Arctic Amplification. In spring and summer, the Arctic Amplification appears to be caused mainly by 2 other factors: 1) the decline of snow cover on high-latitude land areas creates an albedo feedback similar to sea ice but is instead involves the earlier drying and heating of the soil under the snow, which promotes an earlier warm season on the continents and contributes to enhanced Arctic warming, and 2) increasing water vapor transport into the Arctic.

As you know, water vapor is a powerful greenhouse gas, it releases heat into the atmosphere when it condenses into clouds, and extra water vapor promotes additional clouds, which are also effective trappers of heat below them. The water vapor effect may be the most important for the summer ridging over Greenland, as it causes warming through a deeper layer of the atmosphere than the snow/ice effects (see Alexeev et al, Climate Dynamics, 2005; Porter et al, JGR, 2012), and thus has a more direct impact on weakening the poleward temperature gradient and consequently the jet-stream zonal winds. There was also a recent study by Von Walden and coworkers showing that cirrus clouds also caused additional surface warming over Greenland last summer, and these are just the type of clouds you’d expect to see under an upper-level ridge of high pressure.

If this ridging pattern continues to be as persistent as it has been since 2007 (see Hanna et al, 2012), then it could be that the climate models are underestimating the amount of surface melt from the Greenland ice sheet.

To further understand implications and feedbacks, we need to ask question’s such as: “How does the Greenland ice melt affect the environment, in particular the ocean waters, the ocean currents?” or “What role, does fresh and or melt water influx play?”

A new NASA and University of Washington study allays concerns that melting Arctic sea ice could be increasing the amount of freshwater in the Arctic enough to have an impact on the global “ocean conveyor belt” that redistributes heat around our planet. [..] “Knowing the pathways of freshwater is important to understanding global climate because freshwater protects sea ice by helping create a strongly stratified cold layer between the ice and warmer, saltier water below that comes into the Arctic from the Atlantic Ocean,” said Morison. “The reduction in freshwater entering the Eurasian Basin resulting from the Arctic Oscillation change could contribute to sea ice declines in that part of the Arctic.”

Between 2003 and 2008, the resulting redistribution of freshwater was equivalent to adding 10 feet (3 meters) of freshwater over the central Beaufort Sea. NASA Arctic ocean currents changed

What is the THC / Thermohaline circulation and what has it in common with Greenland melt and a DO (Dansgaard Oeschger) event?

From a fact sheet about the THC from 2006 – by Stefan Rahmstorf: The most dramatic climate events recorded in Greenland, the Dansgaard-Oeschger (DO) events, could be explained by north-south shifts in convection location, i.e. transitions between warm and cold modes of the Atlantic THC. The coincidence of changes in ocean circulation and in surface climate, as well as the climate effects and stability properties of the THC discussed above, suggest that ocean circulation changes can play a key dynamical role in abrupt glacial climate change. Proxy data show that the South Atlantic cooled when the north warmed and vice versa, a see-saw of northern and southern hemisphere temperatures which is indicative of an ocean heat transport change.

Heinrich events are thought to result from icesheet instability: an episodic iceberg discharge would have provided a large freshwater addition of the order of 0.1 Sv to the Atlantic for several centuries [Roche, et al., 2004]. Models suggest this could have been enough to stop all NADW formation due to the reduced density of ocean surface waters; this in turn would explain the cooling found in proxy data, especially around the mid-latitude Atlantic. In Greenland, Heinrich events appear to have had little effect, presumably as they occurred during cold phases (cold mode, Fig. 14) when the THC already did not reach far enough north to warm Greenland climate.

DO events are dramatic warm events with a large amplitude in Greenland (8 to 16 ºC within a decade or so, [Severinghaus, et al., 2003]) and areas surrounding the northern Atlantic. A large coinciding increase in salinity in the Irminger Sea suggests a northward push of warm, salty Atlantic waters into the Nordic Seas [Kreveld, et al., 2000], which could explain the large warming. What triggered these ocean circulation changes is unknown.

During deglaciation, melting ice sheets surrounding the North Atlantic have apparently lead to episodic influx of meltwater into the ocean. The effect of this meltwater on the THC could explain the oscillations and abrupt cold reversals seen e.g. in Greenland ice cores during this time, including the Younger Dryas and the 8k cold events. This contrasts with the much smoother history of deglaciation found in Antarctic ice cores. The future of the THC Global warming can affect the THC in two ways: surface warming and surface freshening, both reducing the density of high-latitude surface waters and thus inhibiting deep water formation. Most models predict a significant weakening of NADW formation (by 20-50%) in response to anthropogenic global warming during the 21st century [IPCC, 2001]. Some also find a reduction in AABW formation.

These models did not include meltwater runoff from the Greenland ice sheet, which is difficult to predict. A melting of the ice sheet during 1,000 years would imply an average meltwater influx of ~0.1 Sv, similar to that estimated for Heinrich events. Hence, the future evolution of the Atlantic THC is probably closely tied to the fate of the Greenland ice sheet. Model simulations – even those that lead to a complete shutdown in future – find that the influence of anthropogenic warming on the THC until today should be smaller than the natural variability. Therefore, any variations observed to this date are likely related to natural oscillations.

However, observations indicate a widespread freshening trend in the northern Atlantic, which, should it continue, could contribute to a future weakening of the THC in the Atlantic [Curry and Mauritzen, 2005]. A major weakening or shut-down of NADW formation could have serious impacts on marine ecosystems, sea level and surface climate, including a shift in ITCZ and tropical rainfall belts. A THC collapse is widely discussed as one of a number of “low probability – high impact” risks associated with global warming. Source / The Thermohaline Ocean Circulation A Brief Fact Sheet

The latest science suggests that past abrupt Greenland warming (so called DO events) may be triggered by the Jet Stream – induced from what we call arctic amplification. Since the planet’s cryosphere setup is much different to the past, it might mean that we have to face simultaneous melt – in the northern and southern hemisphere. Nevertheless there will be brief periods with cooling, induced by ocean current changes and ice melt (freshwater influx and ice berg break off).

From the Dansgaard Oeschger Wiki:

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.

Related

Video: Lake El’gygytgyn, Pleistocene super-Interglacials and Arctic warmth

• Results show that during the Pleistocene (2.588 million – 11.7 thousand years ago), there were a number of super-interglacials – like the present period but much wetter and several degrees warmer in the Arctic, during which the Greenland and West Antarctic ice-sheets didn’t just melt a bit. They disappeared.

Increasing fresh water runoff from rivers in Russia and now possibly a higher influx of Greenland melt water make the scenario of a DO event – a THC/MOC alteration, weakening or shut down more likely. Further, because of the unprecedented rate of emissions today, we can expect changes might be faster too. And this faster melting also means that we possibly have to adjust our current understanding of future sea level rise. If the ocean currents change, the weather changes (different evaporation pattern) which in turn would change rainfall pattern, weather extremes, among other things. Another big question is how plants could sustain an abrupt climate shift.

Further reading

• Arctic Amplification
• Antarctic Ice Shelves Melt Mostly From Below
• Sea-ice switches and abrupt climate change
• Abrupt Climate Change During the Last Ice Age
• Arctic Ocean freshwater will cause ‘unpredictable changes on climate’
• The Dark Snow Project
• Dansgaard Oeschger