Temperature, CO2 and CH4 from Dome C ice core

Rising CO2 lagged Southern Hemisphere warming at the beginning of the last deglaciation

In this study a new chronology has been applied to EPICA Dome C (Antarctica) ice core temperature, atmospheric CO2 and methane concentrations over the last deglaciation about 19,000 to 11,000 years ago. Comparing the CO2 record to the Antarctic surface air temperature reveals a close correlation, but the resolution of the record is not sufficient to determine whether there is a lag between temperature and CO2. However, the times at which temperature and CO2 began to rise can be distinguished and reveal that the start of increasing CO2 lagged the beginning of rising temperature by about 800 years. An uncertainty analysis suggests that the lag could have been as low as 200 or as much as 1400 years. This result is consistent with the Southern Hemisphere playing a dominant role in the rise in atmospheric CO2. In contrast the rise in methane appears to have been determined by Northern Hemisphere processes.

Introduction

The Vostok (Antarctica) ice core record, which extends over past 420,000 years, has revealed the major features of the last four deglaciations. The record shows increases of the CO2 concentration between 80 and 100 ppmv for each of the glacial terminations. During the last deglaciation the concentration of CO2 in the atmosphere increased by 40%.  The Vostok data also revealed a close correlation between atmospheric CO2 concentration and surface temperature, but the resolution was insufficient to determine whether there was a lag between the two.  Since then the EPICA Dome C ice core has carried the record back to about 800,000 years ago and has also found a close correlation between temperature and CO2 concentration through this period.

There are two important challenges in measuring and dating concentrations in ice.  An accurate chronology is difficult to establish making it challenging to see the detailed progression in the rise of CO2 at glacial terminations.   Recent research by Eric Monnin et al. has made it possible to establish a chronology reliable to about 200 years which has been used to compare CO2 and methane concentrations with surface temperature over the last deglaciation for Antarctica.  Lemieux-Dudona et al. have gone one step further and created a common chronology for Antarctic and Greenland ice cores making it possible to compare  temperature and atmospheric concentration of CO2 and methane in the Northern and Southern Hemispheres over the deglacial period.

Observations

Temperature, CO2 and CH4 from Dome C ice core
Antarctica surface temperature (delta-deuterium) – solid curve
Atmospheric CO2 concentration – solid circles
Atmospheric methane concentration – diamonds
Younger Dryas (YD) cold and Bølling-Allerød (B/A) warm events – shaded bars

The EPICA Dome C (Antarctica) ice core has been used to measure a high resolution record of CO2 and methane concentrations over the last deglaciation. The ice core was sampled for the period from 22,000 to 9,000 years before the present.  The biggest challenge was determining chronologies for ice and for the air trapped in the ice.  This  requires a complex calculation because air can diffuse in the surface snow/ice mixture called the firn.   In this research chronologies have been established for the ice (which applies to the temperature proxies found in the ice) and for the age of the enclosed air.  It is estimated to be reliable to within 200 years back to 10,000 years ago and to within 2000 years back to 41,000 years ago.

In addition the possibility of CO2 enrichment by chemical reactions between impurities in the Dome C ice core was carefully investigated. Because of the physical characteristics of the Dome C ice it was concluded that this was not an important factor and a correction for this effect was not required.

Lemieux-Dudona et al. have applied a new dating method based on inverse techniques to consistently date ice cores in Antarctica and Greenland. This method uses regional or global markers such as methane spikes, volcanic ash, and other global markers to link chronologies on as common time scale. The method has been applied to one Greenland (NGRIP) and three Antarctic (EPICA Dome C, EPICA Dronning Maud Land, and Vostok) ices cores to produce consistent ice and gas chronologies over the last deglaciation.

Comparison of increasing temperature and CO2 in Antarctica

Deuterium abundance (delta-deuterium) is a proxy for Antarctic surface air temperature. Comparing the CO2 record to the surface air temperature reveals a close correlation.  The analysis reveals that atmospheric CO2 concentration increased from 189 ppmv between 18,100 and 17,000 years ago to 265 ppmv between 11,100 and 10,500 years ago. The total increase was found to be 76 ppmv.  The rise in CO2 concentration was found to progress in four steps.

Interval Age (years before the present) CO2 at start(ppmv) CO2 at end(ppmv) CO2 rate of increase (ppmv/1000 years) Period – North Period – South
I 17,000 to 15,400 189 219 20
II 15,400 to 13,800 219 239 8
III 13,800 to 12,300 239 237 –1 Bølling-Allerød(B/A) warming Antarctic Cold Reversal(ACR) cooling
IV 12,300 to 11,200 237 259 20 Younger Dryas(YD) cooling Antarctica warming

Within this framework there are two periods of very rapid CO2 increase. These periods could have been shorter, even less than a few centuries, because of the uncertainty in the chronology of the enclosed air.

Age (years before the present) Duration(years) Increase(ppmv)
13,800 300 or less 8
11,200 200 or less 6

The resolution of the ice core record is insufficient to determine whether there is a lag between the temperature and CO2 concentration.  However, it is possible estimate the times at which temperature and CO2 began to rise. Surface air temperature began to rise about 17,800 years ago. Increasing CO2 began to rise 17,000 years ago lagging the beginning of rising temperature by 800 years. An uncertainty analysis reveals that the lag could be as low as 200 years or as much as 1400 years.

Comparison of increasing temperature, CO2 and methane in Antarctica and Greenland

Comparing CO2 and methane concentrations over the deglaciation is illuminating because it is believed that the sources of the increasing CO2 and methane are different.

The rise of CO2 and methane during the deglacial period are similar in some respects. They both start to rise at the same time. During the transition between intervals II and III there is a fast rise in both CO2 and methane. But there are also important differences. During Interval III, which corresponds to the Bølling/Allerød (B/A) warm phase in the North Atlantic region and to the Antarctic Cold Reversal (ACR), there is a small, slow decrease in the CO2 concentration, whereas the methane rises dramatically to a level near the concentration in the holocene after the deglaciation. During Interval IV, which corresponds to the cooling Younger Dryas (YD) epoch in the North Atlantic region and to the warming interval after the ACR in Antarctica, there is a continuous CO2 increase terminated by a fast CO2 rise at the transition to the holocene, the current warm period. During this period methane concentration drops ~200 parts per billion by volume (ppbv), returning to concentrations comparable to Interval II.

The parallel rise in methane and CO2 in interval I is interesting because the sources of CO2 and methane are different. The methane increase in Interval I is also seen in the Greenland ice core record (GRIP). The methane rise is generally ascribed to changes in the extent and activity of wetlands in northern latitudes and the tropics.

The fast increases of CO2 and methane concentrations between intervals II and III, about 13,800 years ago on the Antarctica time scale, correspond to the fast warming in the Northern Hemisphere observed at 14,500 years ago using the the Greenland chronology. This warming is thought to be caused by enhanced formation of North Atlantic Deep Water and it has been suggested that the sudden CO2 increase could have been caused by a reduction in stratification in the Southern Ocean which resulted in upwelling and CO2 ventilation. The methane increase, on the other hand, is thought to have been caused by an expansion of wetlands in the tropics and northern latitudes.

CO2 decreased slightly during interval III and then increased during interval IV. Methane concentration follows the temperature evolution of the Northern Hemisphere in intervals III and IV. The accelerated CO2 increase at the end of interval IV is thought to be connected to the fast warming in the Northern Hemisphere rather a climate change in the Southern Hemisphere.

It is important to note that the CO2 increase in interval I, which occurred before any substantial warming in the Northern Hemisphere, supports the view that the Southern Hemisphere was the source of the CO2 increase.

The results of these studies support the hypothesis that the Southern Ocean was the most important factor in regulating CO2 concentration during the last deglaciation. However, the fast increases between intervals II and III and at the end of interval IV show that additional mechanisms in the Northern Hemisphere influenced CO2, presumably through changes in North Atlantic Deep Water formation. The record shows that methane was regulated by a completely different mechanism probably related to the expansion/contraction of wetlands in the northern and tropical regions.

Atmospheric CO2 Concentrations over the Last Glacial Termination, Eric Monnin, Andreas Indermühle, André Dällenbach, Jacqueline Flückiger, Bernhard Stauffer, Thomas F. Stocker, Dominique Raynaud, Jean-Marc Barnola, Science 05 Jan 2001: Vol. 291, Issue 5501, pp. 112-114 DOI: 10.1126/science.291.5501.112

Consistent dating for Antarctic and Greenland ice cores, Bénédicte Lemieux-Dudon, Eric Blayo, Jean-Robert Petit, Claire Waelbroeck, Anders Svensson, Catherine Ritz, Jean-Marc Barnola, Bianca Maria Narcisi, Frédéric Parrenin, Quaternary Science Reviews, 29, Issues 1–2, January 2010, Pages 8–20.