How to warm the cold heart of Antarctica | Carlsbergfondet
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How to warm the cold heart of Antarctica

Repeated abrupt climate changes dominated the last glacial period in the North Atlantic region. When cold conditions prevailed in Greenland, Antarctica slowly warmed, but the mechanisms behind this warming was not understood until our research combined insights from state-of-the-art climate models and ice-core research.

By Associate Professor Sune Olander Rasmussen, Section for the Physics of Ice, Climate and Earth, Niels Bohr Institute, University of Copenhagen, Denmark

The variable glacial climate

During the last glacial period (approx. 119,000 – 11,700 years ago), Greenland was about 25°C colder than present, which is about 5 times the global average cooling. However, the climate was very variable in the North Atlantic region: during the so-called Dansgaard-Oeschger events, abrupt climate changes of up to 16°C occurred repeatedly. Read more about the sequence of cold phases (Greenland Stadials, GS) and warmer periods (Greenland Interstadials, GI) here.

Ice-core records reveal that greenhouse gas concentrations in the atmosphere are changing faster now than at any time in the past 800,000 years. The resulting climate changes influence the entire Earth, but the changes are not evenly distributed, and future changes may be abrupt and irreversible. 

The ChronoClimate project seeks to increase our understanding of the mechanisms by which abrupt changes in certain elements of the climate system propagate over vast distances. 

We do this by studying climate changes of the glacial period, where Earth underwent dramatic natural climate change related to variations in how the atmosphere and ocean move heat between regions.

The Climate Laboratory

The study of past climate change is a virtual laboratory where we can study climate changes on many scales and hope to gain new insights into the physics of the climate system. Insights that are likely also applicable to present and future climate scenarios. This is particularly important because it allows us to study the mechanisms of abrupt changes that may be triggered during the next centuries if climate system tipping points are reached.

The study of past climate change is a virtual laboratory where we can study climate changes on many scales and hope to gain new insights into the physics of the climate system.

One specific problem tackled by the ChronoClimate project is how the climate of the north and south polar regions were connected during the so-called Dansgaard-Oeschger events, which punctuated the glacial climate. These events are characterised by incredibly abrupt temperature changes in Greenland — up to 16°C within the space of a few decades.

Figure 1. The map shows the surface temperature change as Greenland goes from a relatively mild interstadial period into a cold stadial period. The graph shows how especially the North Atlantic surface waters cool, while the southern hemisphere warms in most places, including Antarctica.

It has been known for about 20 years that Antarctica slowly warms during the cold periods prior to these Greenland warmings (see figure 1). Similarly, Antarctica starts cooling when Greenland abruptly warms, but the relationship had only been described in general terms without analysis of the actual physical mechanisms at play.

The bipolar seesaw

The bipolar seesaw hypothesis describes how abrupt climate changes can have huge and disparate regional impacts by changing the heat distribution within the climate system (even while the total amount of energy in the climate system remains relatively constant). Using a climate model, a highly advanced computer simulation of the coupled elements of the climate system, we can trace how and where heat is redistributed during abrupt climate change events.

Figure 2. This graph shows the water temperature changes at various depths in the 30°W-15°E belt of the Atlantic Ocean 200-300 years after the onset of the abrupt change shown in Figure 1. While North Atlantic surface waters cool, heat builds up in the ocean essentially everywhere north of the Antarctic Circumpolar Current (the ACC, dashed line). How the heats gets to Antarctica is the tricky part that our research addresses.

Our first result was that heat is building up not mainly in the South Atlantic and Southern Ocean as previously thought, but in all oceans north of the zone of strong winds and ocean currents that encircle Antarctica (the so-called the Antarctic Circumpolar Current, see figure 2).

The Land Far Far Away: Antarctica

The theory of the bipolar seesaw

The bipolar seesaw concept, in which Antarctica warms when Greenland is cold (and vice versa) was inspired by ice-core data from Greenland and Antarctica. Critical to development of the hypothesis was the ability to align ice-core records from Antarctica and Greenland using the fact that bubbles of past atmospheric air in all ice-cores shown a common signal of methane concentration variability. Read more about how ice-cores record past atmospheric composition and how it is used for synchronization of polar climate records.

But if the heat only builds up north of the Antarctic Circumpolar Current (ACC), how does it then propagate to Antarctica? The ACC is so strong, that it acts as a barrier that does not easily let any warming signal through. 

The model allowed us to trace the energy and showed that the Antarctic warming is the result of a complex interplay between ocean and atmosphere processes, including slow but persistent turbulent mixing in the ocean, leading to reduced sea-ice cover around Antarctica, which in turn results in increased release of heat from the ocean to the atmosphere. 

This was amplified by the so-called ice-albedo feedback, which describes that the change from bright sea ice to darker ocean leads to an increased absorption of incoming radiation from the sun.

We can only resolve where past climate changes started and how they propagate when the climate records are very accurately dated

Thanks to this work, we now know that turbulent mixing and sea ice melt are critical processes in the observed warming of the Antarctic atmosphere during the Dansgaard-Oeschger abrupt climate changes.

What is next?

The importance of reliable dating

An ongoing part of the ChronoClimate project is to improve dating and synchronization methods for climate data from ice cores, stalactites/stalagmites, and marine sediments. We can only resolve where past climate changes started and how they propagate when the climate records are very accurately dated. Read more here.

The analysis is in full agreement with ice-core observations, and we are now refining the study using a more complex climate model — the same class of model used to make projections of future climate. 

In addition, we are working in parallel on tightening the data constraints by improving the precision of the synchronization of the ice-core records from Greenland and Antarctica. The aim is to confirm the first findings and further resolve the governing physical mechanisms.

Although our results focus on the mechanisms of past abrupt climate change, they reveal fundamental information on climate physics, in particular on the global response to crossing climate system tipping points. 

The increased process understanding is relevant to our present and future climate, which is being forced by greenhouse gas emissions, and could be approaching a tipping point due to increased melting of ice from the ice sheets. The study will thus contribute to improve the predictive power of climate models.