One of the most interesting parts of the paleoclimate record over the last 100,000 years, is the series of abrupt climate changes prior to the Holocene that have occurred on very rapid timescales, ranging from years to decades (Alley et al, 2003). These changes were large, fast, and occurred when the climate was pushed across certain thresholds.
Of particular note, is the well over 20 Dansgaard-Oeschger events since the last interglacial. Typically, a rapid warming on timescales of decades was followed by slower cooling, rapid cooling, and then a brief period of little temperature change. A value near 1500 years between these events is common, although sometimes there are skips and so the spacing could be a scalar multiple of near 1500 years. Successive D-O oscillations become progressively cooler as the cold-based ice sheet grows in Hudson Bay, and when the base of the ice thaws, you get a Heinrich event surge that dumps large number of icebergs that calved from the Laurentide ice sheet into the North Atlantic, via the Hudson Straight (or perhaps other sources such as the Icelandic and British Isles ice sheet). This succession of progressively cooler D-O events, punctuated by a Heinrich event (until the next cycle begins or the climate becomes too warm for an ice sheet to grow) is a Bond cycle. These events are common before the Holocene which led to a climate punctuated by high-frequency variations and a much more variable situation than that which humans have enjoyed over the past 10,000 years.
Heinrich event 1 (which is actually the youngest Heinrich event, not the oldest) occurred approximately 17,000 years ago was followed by an abrupt warming around 14,500 years ago, something known as the Bolling-Allerod interval (an event interrupted by the Older Dryas and the Inter-Alleroid cold periods). This warming is seen in a wide variety of proxy records including the Greenland summit. This was then followed by the cooling into the Younger Dryas, and then finally a warming into our current Holocene interglacial.
The physical mechanisms responsible for the Heinrich event 1- Bolling-Allerod transitions have been controversial. In the recent issue of Science, a study by Liu et al., 2009 uses advanced modeling to tackle this particular question in more detail. One thing is clear: These abrupt changes are related to the Atlantic Meridional Ocean Circulation (AMOC)(loosely, the conveyor-belt or thermohaline circulation).
It has been known for some time that a reduction North Atlantic sinking would warm the south while cooling the north in a bipolar see-saw (Discussed previously at RealClimate), and so understanding how the AMOC, freshwater flux into the ocean, and other atmospheric changes are related is crucial. Attributing causes to various abrupt shifts is very important for our understanding of the physical climate system and possible tipping points in the future. Models of intermediate complexity (e.g., Ganopolski and Rahmstorf 2002) model D-O like events as a threshold process involving stochastic resonance. This is one possible mechanism in which “noise” and a very weak “signal” (a weak but true 1500 year periodicity in forcing) could combine (Alley et al., 2001) although dating issues are such that placing very high confidence on a true periodicity at 1500 years is difficult. Transitions like that of the Bolling-Allerod (which is in some ways similar to a D-O event) could involve surface warming of the North Atlantic or reduced melt water influx, but Liu et al. use the first synchronously coupled atmosphere-ocean general circulation model that goes from the Last Glacial Maximum to the Bolling-Allerod, in a rather unique way to investigate that topic- a transient modeling approach that prescribes the time evolution of external boundary condition changes. They force their model changes in insolation from Milankovitch, atmospheric greenhouse gas concentrations, continental ice sheets and coastlines, and meltwater flux over the North Atlantic and Gulf of Mexico.
The authors also get a bipolar seesaw response characterized by a cooling over the Northern Hemisphere and a warming over the Southern Hemisphere into Heinrich event 1 that is caused by decreased poleward heat transport from the AMOC. They demonstrate warming that is global in spatial extent from Heinrich event 1 to the Bolling-Allerod, showing signs of polar amplification and maximum warming in the Arctic and North Atlantic. Here is a global illustration of temperature anomalies of Heirich event 1 relative to the LGM, the Bolling-Allerod relative to Heinrich 1, and then the Bolling-Allerod relative to the LGM.
Liu et al 2009
At the Bolling-Allerod onset, Greenland warms tremendously, largely due to reduced melt water influx, and Antarctica continues to warm as a result of large increases in greenhouse gas concentration. The large increase in methane is mostly caused by an increase in wetland extent and temperature, as wetlands are the primary source of methane in the pre-industrial time period. Perhaps tropical wetlands were a major contributor since ice sheets covered the primary extratropical methane sources during this time (Chappellaz et al 1993). Changes in the position of the Intertropical Convergence Zone (ITCZ) cause rainfall suppression at Heinrich event 1 and enhancement at the Bolling-Allerod.
The unique aspect of this paper is that many studies in the past, using models of lesser complexity, show that abrupt warming from Heinrich event 1 to the Bolling-Allerod was caused by a sudden resumption of the AMOC in response to a gradual perturbation. However, Liu et al. simulates the Bolling-Allerod warming largely as a linear response to Melt water flux. When the discharge of meltwater from the retreating glacial ice sheets during Heinrich Event 1 stops suddenly, this is where there is a transition to a new state. As the Meltwater flux increases, the AMOC diminishes nearly linearly, in contrast to many intermediate climate models.
Much of the warming into the Bolling-Allerod is thus caused by the AMOC, and also by the increase of both methane and CO2 (about 40 ppmv for CO2) as well as an “overshoot” (by overshoot, they mean this is recovery beyond the glacial-state transport) of the AMOC due to convection in the Nordic sea. Whether this overshoot exists in observational records is actually unclear.
Graphic from the non-technical article in Science by Axel Timmermann and Laurie Menviel
As another note, this kind of modeling needs to be performed in the future by different groups to check the robustness of Liu et al and to provide further perspective on the mechanisms and spatio-temporal extent of abrupt climate change. Unfortunately, as pointed out in the accompanying perspective piece by Timmerman and Menviel , this is a very computationally demanding task, and already involved one and a half years of model number-crunching to get initial results. They close with an insightful line,
“Ultimately, breakthroughs in our understanding of Earth’s climate evolution will come from close interactions between paleoproxy experts, paleoclimate modelers, and climate dynamicists. It is time to train a new generation of scientists familiar with all these fields.”
Alley, R.B., S. Anandakrishnan, and P. Jung. 2001: Stochastic resonance in the North Atlantic. Paleoceanography, 16(2):190-198
Alley, R.B., et al, 2003: Abrupt climate change. Science, 299, 2005-2010
Chappellaz J., et al., 1993: Synchronous changes in atmospheric CH4 and Greenland climate between 40 and 8 kyr BP, Nature 366, 443-445
Ganopolski, A. and S. Rahmstorf, 2002: Abrupt glacial climate changes due to stochastic resonance, Phys. Rev. Let. 88(3), 038501
Liu Z., et al., 2009: Transient Simulation of Last Deglaciation with a New Mechanism for Bølling-Allerød Warming, Science 325: 310-314
Timmerman, A. and L. Menviel 2009: What Drives Climate Flip-Flops? Science 325: 273-274