Search

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) betweenad1000 andad1200 and a fairly continuous decrease to a mean value of about 277 ppmv aroundad1700. While the new South Pole measurements agree well with DML at the minimum atad1700 they are on average about 2 ppmv lower during the periodad1000–1500. 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. [ABSTRACT FROM AUTHOR]

Abstract: Reconstructions of past atmospheric concentrations of greenhouse gases provide unique insight into the biogeochemical cycles and the past radiative forcing in the Earth''s climate system. We present new measurements of atmospheric nitrous oxide along the ice cores of the North Greenland Ice Core Project and Talos Dome sites. Using records of several other ice cores, we are now able to establish the first complete composite nitrous oxide record reaching back to the beginning of the previous interglacial about 140,000yr ago. On the basis of such composite ice core records, we further calculate the radiative forcing of the three most important greenhouse gases carbon dioxide, methane and nitrous oxide during more than a full glacial–interglacial cycle. Nitrous oxide varies in line with climate, reaching very low concentrations of about 200 parts per billion by volume during Marine Isotope Stages 4 and 2, and showing substantial responses to millennial time scale climate variations during the last glacial. A large part of these millennial time scale variations can be explained by parallel changes in the sources of methane and nitrous oxide. However, as revealed by high-resolution measurements covering the Dansgaard/Oeschger events 17 to 15, the evolution of these two greenhouse gases may be decoupled on the centennial time scale. Carbon dioxide and methane concentrations do not reach interglacial levels in the course of millennial time scale climate variations during the last glacial. In contrast, nitrous oxide often reaches interglacial concentrations in response to both, glacial terminations and Dansgaard/Oeschger events. This indicates, from a biogeochemical point of view, similar drivers in both temporal cases. While carbon dioxide and methane concentrations are more strongly controlled by climate changes in high latitudes, nitrous oxide emissions changes may mainly stem from the ocean and/or from soils located at low latitudes. Accordingly, we speculate that high latitudes could play the leading role to trigger glacial terminations. [Copyright &y& Elsevier]