Supporting evidence from the EPICA Dronning Maud Land ice core for atmospheric CO2 changes during the past millennium

Urs Siegenthaler,Jakob Schwander,Thomas F Stocker,Jean-Marc Barnola,Kenji Kawamura,Bernhard Stauffer,Eric Monnin,Renato Spahni,Hubertus Fischer

doi:10.1111/j.1600-0889.2005.00131.x

Abstract

The most direct method of investigating past variations of the atmospheric CO2 concentration before 1958, when continuous direct atmospheric CO2 measurements started, is the analysis of air extracted from suitable ice cores. Here we present a new detailed CO2 record from the Dronning Maud Land (DML) ice core, drilled in the framework of the European Project for Ice Coring in Antarctica (EPICA) and some new measurements on a previously drilled ice core from the South Pole. The DML CO2 record shows an increase from about 278 to 282 parts per million by volume (ppmv) between ad 1000 and ad 1200 and a fairly continuous decrease to a mean value of about 277 ppmv around ad 1700. While the new South Pole measurements agree well with DML at the minimum at ad 1700 they are on average about 2 ppmv lower during the period ad 1000–1500. Published measurements from the coastal high-accumulation site Law Dome are considered as very reliable because of the reproducibility of the measurements, high temporal resolution and an accurate time scale. Other Antarctic ice cores could not, or only partly, reproduce the pre-industrial measurements from Law Dome. A comparison of the trends of DML and Law Dome shows a general agreement. However we should be able to rule out co-variations caused by the same artefact. Two possible effects are discussed, first production of CO2 by chemical reactions and second diffusion of dissolved air through the ice matrix into the bubbles. While the first effect cannot be totally excluded, comparison of the Law Dome and DML record shows that dissolved air diffusing to bubbles cannot be responsible for the pre-industrial variation. Therefore, the new record is not a proof of the Law Dome results but the first very strong support from an ice core of the Antarctic plateau.

Highlights

CO2 measurements of recent air extracted from ice cores (Neftel et al, 1985; Etheridge et al, 1996) overlap with direct atmospheric measurements (Keeling and Whorf, 2003) and are in good agreement, clearly confirming the reliability of the recent ice core CO2 record
In this paper we present new CO2 data derived from the European Project for Ice Coring in Antarctica (EPICA) (European Project for Ice coring in Antarctica) ice core currently being drilled in Dronning Maud Land (DML) as well as additional measurements on the South Pole ice core and compare those with the high-resolution Law Dome CO2 record
Variations of the atmospheric CO2 concentrations on a centennial timescale were suggested many years ago based on measurements along the South Pole ice core (Siegenthaler et al, 1988)

Summary

Introduction

CO2 measurements of recent air extracted from ice cores (Neftel et al, 1985; Etheridge et al, 1996) overlap with direct atmospheric measurements (Keeling and Whorf, 2003) and are in good agreement, clearly confirming the reliability of the recent ice core CO2 record. Records from Antarctic ice cores show a mean value of about 280 parts per million by volume (ppmv) for the last millennium before the anthropogenic increase started (Barnola, 1999). Variations between 5 and 10 ppmv are observed in the pre-industrial epoch in Antarctic records from Siegenthaler et al (1988), Barnola et al (1995) and Etheridge et al (1996). These variations, which are important for the understanding of the global carbon cycle, are significant con-.

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