Variation in Earth's surface temperature


We know the Earth's surface temperature for about the past 540 million years ago from a number of sources; the shells of marine organisms buried in marine sediments provide temperature data for 540 million to about a million years ago, marine sediment data has been supplemented with data from Antarctic ice cores to cover the last 800,000 years, and Greenland ice cores provide additional data starting about 123,000 years ago. But the time resolution of this data is typically millennia or centuries at best.
Temperature of the Earth over the last 540 million years
Temperature of the Earth over the last 540 million years
Source

Rapid climate change at the end of the last glacial period


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).

Temperature variation near the Younger Dryas
Temperature variation near the Younger Dryas
Figure Temperature (from GISP/Greenland, Cariaco Basin/Venezuela, and Dome C/Antarctica), annual snow accumulation in Greenland, and fresh water flux near the Younger Dryas cooling period (denoted by the vertical blue bar). The warming period just prior to the Younger Dryas is the Bølling-Allerød period. The warming period after the Younger Dryas is the beginning of the present interglacial or Holocene. Source

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 not able achieve a resolution sufficient to resolve annual layers.

High resolution observations from NGRIP ice cores


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 - proxy for moisture source regions

  • Delta-oxygen-18 - proxy for atmospheric temperature at the time of snow deposition

  • Concentration of insoluble dust, soluble sodium (Na+) and calcium (Ca2+) - Sea salts (sodium (Na+) and calcium (Ca++)) and continental dust (non-sea salt calcium) 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).

Observations of proxies from NGRIP ice core for 11 to 15.5 thousand years ago
Observations of proxies from NGRIP ice core for 11 to 15.5 thousand years ago
Figure Observations from the NGRIP ice core for 11 to 15.5 thousand years ago. Left (A) Deuterium excess (red) and delta-oxygen-18 (dark blue) at 20-year resolution over the entire period. Right (B) Deuterium excess (red), delta-oxygen-18 (dark blue), logarithmic plots of dust content (yellow), calcium concentration (Ca2+ light blue), sodium concentration (Na+ purple), and annual layer thickness (λ green) at annual resolution for transitions at (B) 11,700 years ago, (C) 12,900 years ago, and (D) 14,700 years ago.

High resolution analysis of abrupt climate change from ice cores


The temperature proxy (delta-oxygen-18) record shows that both warming periods involved warming of more than 10 °C. The warming transition 14,700 years ago remarkably occurred within only three years. The other warming transition 11,700 years ago extended over 60 years. The cooling transition 12,900 years ago lasted more than two centuries, significantly longer than the warming transitions.

Of all the variables measured in the ice cores the moisture source proxy (deuterium excess) 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.

Deuterium excess record during rapid climate change
Deuterium excess record during rapid climate change
Figure Moisture source (deuterium excess) record for the three climate transitions to clearly show the rapid change between the climate states.

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 can shift 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.

Editor's comments


In the paleoclimate record warming events during which surface temperatures increase by several degrees Celsius can occur very rapidly. It is not clear what initiates these events and what determines whether the warming is followed by a cooling period or leads to a full interglacial. One proposal is that variations in solar radiation (Milankovitch cycles) are responsible for initiating the warming period, but that ice sheet dynamics determine what follows. In this article the authors analyze Greenland ice cores corresponding two warming events and a cooling event which occurred just before the beginning of the present warm interglacial. The authors use new measurement technology that allows for the first time better than yearly resolution. They measured several variables which allows them to not only determine the variation in temperature during these periods, but also to infer that a sudden change in arctic atmospheric circulation might have contributed to the sudden temperature changes.

Source


Steffensen et al., Science 2008: Vol. 321 no. 5889 pp. 680-684 DOI: 10.1126/science.1157707