Using a new analytical technique, temperature and other key variables have been measured in Greenland ice cores at better than yearly resolution for the first time. The study analyzed three abrupt climate events (two warming and one cooling) in the period from 15.5 to 11 thousand years ago. The high-resolution records reveal that the two warming events involved a warming from glacial to warm interglacial of more than 10 °C. The warming transition beginning 14,700 years ago occurred within only three years. The other warming transition 11,700 years ago extended over 60 years. The high-resolution records demonstrate that the two warming events followed the same general pattern involving a sudden shift over one to three years in polar atmospheric circulation.
In the Northern Hemisphere, the last glacial period ended with two abrupt warmings which were interrupted by a cooling event. The temperature changed rapidly from glacial to mild conditions beginning 14,700 years ago (Bølling/Allerød period), from warm to glacial beginning 12,900 years ago (Younger Dryas period) and again from cold to mild conditions beginning 11,700 years ago (beginning of the present era or Holocene).
The shape and duration of the abrupt climate changes at the termination of the last glacial period have previously been analyzed from Greenland ice core records, but the sampling of these cores was on the order of decades and centuries.
Using new continuous flow analysis (CFA) techniques, Greenland ice cores have been analyzed for the first time at better than yearly resolution. The analysis focuses on two abrupt warming events 14,700 years ago (Bølling/Allerød) and 11,700 years ago (beginning of the Holocene) and the intervening cooling period 12,900 years ago (Younger Dryas). The quantities measured from the ice cores are;
- Annual snow layer thickness – important for determining dating
- Deuterium excess (delta-D) – proxy for source regions of precipitation
- Delta-18oxygen – proxy for atmospheric temperature at the time of snow deposition
- Concentrations of insoluble dust and sea salts sodium (Na+) and calcium (Ca++)
Sodium and calcium sea salt and continental dust concentrations strongly correlate to regional wind strengths (the westerlies) and to semi-permanent weather pressure centers.
High resolution measurements corresponding to one to three samples per year have made it possible to reliably estimate the thickness of annual snow layers. This is turn has enabled the construction of a new Greenland time scale, called the Greenland Ice Core Chronology 2005 (GICC05).
The temperature proxy (delta-18oxygen) record shows that both warming periods involved warming of more than 10 °C. Remarkably the analysis reveals that the warming transition 14,700 years ago occurred within only three years. The second warming transition 11,700 years ago was longer but also rapid extending over 60 years. In contrast the cooling transition 12,900 years ago lasted more than two centuries, significantly longer than the warming transitions, but still very rapid compared to the timescale of the glacial deglacial cycles.
Of all the variables measured in the ice cores the proxy for the source of precipitation the deuterium excess (delta-D) record exhibits the most abrupt change during the three transition periods. Deuterium excess is interpreted as a proxy of past ocean surface temperatures in the regions of the ocean where the moisture deposited in Greenland originated from. Evaporation conditions at the source ocean region change either because a shift in atmospheric circulation brings moisture from a different part of the ocean or because of changing sea surface temperature, humidity, or wind conditions in the ocean region from which the moisture originates.
During the warming periods the deuterium excess record shows a rapid decrease over one to three years, corresponding to a cooling by 2 to 4 °C at the ocean moisture-source region. For the cooling transition, a rapid increase was observed. The authors interpret the extremely rapid shifts in both the warmings and the cooling to be more likely a consequence of changed source regions of the water vapor reaching Greenland, which suggests a change in atmospheric circulation (large-scale movement of air and the means by which thermal energy is distributed on the surface of the Earth) from one year to the next. They argue that if the variation in the deuterium excess was due only to changes in weather pattern trajectories with respect to the NGRIP observation point, then simultaneous changes in other parameters, such as dust content, calcium concentration, sodium concentration, and annual layer thickness would be expected, but this was not observed.
The high-resolution records from the Greenland NGRIP ice core project reveal that the two major warming events followed the same general pattern. They provide evidence that polar atmospheric circulation shifted rapidly, over a period of one to three years, resulting in rapid changes from glacial to warm interglacial temperatures with warming of more than 10 °C over a matter of years.