An Antarctic ice core reveals atmospheric CO₂ variations over the past few centuries

Abstract

When snow is transformed into ice (firnification process) by sedimentation near the surface of an ice sheet, some of the atmospheric air is trapped in the inter-grain spaces which are progressively isolated from the surrounding atmosphere. The resulting material is an air-tight bubbly glacial ice. By analysing the air extracted from the bubbles found in the ice, it is possible to determine the air composition and thus its CO₂ content for the period during which the air was trapped. We aim here to provide the most direct evidence obtained so far for the background atmospheric CO₂ concentrations over the centuries preceding the recent anthropogenic perturbation due to the industrial revolution of the past century. This background level is important for assessing both the origin and the climatic response of the anthropogenic perturbation to the atmospheric CO₂ (see ref. 1). Our results, obtained from the analysis of the air trapped in the bubbles of an Antarctic ice core, indicate that the background level could have been as low as 260 p.p.m.v. (parts per million by volume) before the major anthropogenic influence and suggest that the so-called ‘pre-industrial’ CO₂ level was not constant over the few hundred years preceding the nineteenth century.