Researchers explain the physics of Northern Hemisphere warming and cooling cycles, called Dansgaard-Oeschger Oscillations, that occurred during the last ice age in a new study published in Geophysical Research Letters. Complex computer modelling study agrees well with measurements obtained from Greenland ice core records of the Earth’s past temperature variations providing an insight into the physics driving Dansgaard-Oeschger Oscillations, a remarkable glacial climate phenomenon.
From the University of Toronto
The evolving Earth’s climate has undergone many rapid shifts in temperature in the geologic past. The increases in global temperatures that have occurred since the industrial revolution (0.8 °C or 1.4 °F) pale in comparison with some of the rapid temperature changes that have been inferred to have occurred from isotopic composition records in 3 km deep ice cores drilled at summit Greenland.
A modelling study that uses one of the recent climate models used to predict the role of human induced future climate change, has been modified to simulate the climate that occurred between 50,000 and 30,000 years ago. The study, whose first author is Guido Vettoretti, has been published in Geophysical Research Letters, and demonstrates that these models are capable of simulating violent changes in climate that have occurred in the recent geologic past. The study simulated the climate of the Earth over many thousands of years, requiring a massive amount of computational resources, sometimes taking almost a year to complete a single simulation on the equivalent of roughly 1,000 desktop computers.
Using models to understanding how the Earth has undergone large rapid climate shocks is an important tool in assessing how sensitive the Earth may be to the impact of humans on our climate system. The last 10,000 years, referred to as the Holocene is the most recent warm period that separates a series of ice age cycles lasting roughly 100,000 years. There have been about 10 of these ice age cycles in the last million years. The warm periods, or interglacials are relatively calm compared with the climate during an ice age cycle.
Throughout the course of an ice age cycle, the Earth’s climate quickly jumps into a warm period reminiscent of these interglacials but lasting only several hundred years and then gradually returns to a very cold climate over several more hundred years. This saw-toothed process usually lasts about one or two millennia and is referred to as “Dansgaard-Oeschger Oscillations” after the initial scientists who investigated the phenomenon, Willi Dansgaard and Hans Oeschger.
To understand how the climate flips into a warm state, the model simulation was analysed to understand the relevant mechanisms describing the atmosphere and ocean physics of the warming process. The model shows that during the cold period before the warming event (called a stadial), the North Atlantic is covered by sea ice expanding all the way down to almost 40 °N latitude across the Atlantic. Heat then travels in the ocean from the warm tropics to the cold North Atlantic, which is carried mostly by a weak Gulf Stream and the North Atlantic current, much weaker than today.
The heat then collects under the massive glacial sea ice lid southeast of Greenland, which is inhibited from rising to the surface because of the stabilizing saline properties of the ocean in this region. After a period of time the ocean’s surface layer under the sea ice becomes salty enough that the ocean layers in this region becomes unstable and a large amount of heat rises to the surface and is then subsequently released to the atmosphere and recorded in the chemical makeup of summit Greenland ice. These events create super polynya measuring more than 1 million square kilometers, a large region of ocean that is completely surrounded by sea ice with surface water mixing to great depth.
Glacial super polynyas and Dansgaard-Oeschger Oscillations
To understand how these “glacial super polynyas” form required an investigation of the details ocean physics involved. An interesting characteristic of the ocean circulation is that the density (mass/unit volume) of seawater depends on both how warm (temperature) and salty (salinity) the water is. In cold regions of the ocean such as in the Arctic, the salinity dominates density changes, while in warm regions such as the tropics, temperature dominates the density. Another unique property of seawater is that the density varies linearly with the amount of salt concentration, while the temperature dependence on the density varies both linearly and quadratically (the temperature times itself).
The temperature influence on density also changes with depth in the ocean. These non-linear properties allow for a unique physical behaviour when determining the density of seawater. One example is the mixing of two parcels of seawater both having different temperatures and salinity, but both having the same density. You would expect them to remain at the same density when combined, but they must increase in density when combined due to these non-linear properties of seawater density.
In the ice age climate, just before the onset of the formation of these glacial super polynyas, the high latitude North Atlantic is covered by a thick layer of sea ice, with cold fresh water just below the surface. A few hundred meters further down the temperature increases substantially and becomes more saline. This interesting non- linear behavior of seawater density is observed in the modern world oceans with this type of water structure and the authors hypothesise that this would have occur during the formation of these super polynyas during an ice age. When the cold fresh water is mixed downward with the warm saline water the density increases and an instability accelerates. This is hypothesised to aid in the massive ocean instability that occurs during the polynya formation and the mechanism by which the ocean circulation starts to increase towards more a more modern vigorous strength for several hundred years increasing the annual average temperature of the high latitudes by 10 or 15 °C. The authors also hypothesis that other non-linear properties of seawater are also important in the instability. These rapid increases in in temperature, which are simulated to occur over a few years and corroborated by the ice core records demonstrate that the Earth is capable of violent change.
The work was conducted in the Department of Physics at the University of Toronto.
Late Quaternary rapid warming events inferred on the basis of oxygen isotopic data from Greenland ice cores are the most prominent characteristic of millennial scale Dansgaard-Oeschger oscillations. In a coupled climate model simulation which has accurately reproduced this oscillatory behavior for the first time, we show that formation of a glacial North Atlantic super polynya characterizes the initial stage of transition from cold stadial to warm interstadial conditions. The winter polynya forms within the otherwise sea ice covered North Atlantic as a consequence of the onset of a thermohaline convective instability beneath an extensive stadial sea ice lid. Early in the stadial period, the tendency for thermal convective instability of an extensive warm pool beneath the sea ice lid is strongly inhibited by a stabilizing vertical salinity gradient, which gradually diminishes until a thermohaline convective instability occurs that leads to polynya formation and the rapid retreat of North Atlantic sea ice cover.
Vettoretti, G., and W. R. Peltier (2016); Thermohaline instability and the formation of glacial North Atlantic super polynyas at the onset of Dansgaard-Oeschger warming events; Geophysical Research Letters, 43, doi:10.1002/2016GL068891.
University of Toronto news release.