The ice record of greenhouse gases: a view in the context of future changes

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Abstract

Analysis of air trapped in polar ice provides the most direct information on the natural variability of Greenhouse Trace Gases (GTG). It gives the context for the dramatic change in their atmospheric concentrations induced by anthropogenic activities over the last 200 yr, leading to present-day levels which have been unprecedented over the last 400,000 yr. The GTG ice record also provides insight into the processes generally involved in the interplay between these trace gases and the climate and in particular those which are likely to take place in the next centuries in terms of climate changes and climate feedbacks on ecosystems. The paper gives selected examples of the GTG record, taken during different climatic periods in the past, and illustrating what we can learn in terms of processes.

Introduction

The experience of the past has widely influenced the course of societies in many fields like economics or social and political sciences. Similarly paleo-climatologists and environmentalists are looking to the past for the key to understanding the future.

As far as greenhouse gases are concerned, anthropogenic activities release different greenhouse trace-gases into the atmosphere which are affecting our climate. We know that their accumulation in the atmosphere most likely leads to an overall increase of temperature, but large uncertainties still remain regarding the climatic sensitivity to greenhouse gases (what is the temperature increase expected for 2×CO2?), the carbon cycle budget (where is the unidentified CO2 sink?), and the feedbacks on the ecosystems involved (how will ecosystems react to climate and atmospheric composition changes?).

The predictions for the future are made by using general circulation models when simulating the climate of the 2×CO2 world, or carbon cycle models when developing scenarios of future CO2 emissions. These models are based on our understanding of the processes involved. Because most of the main anthropogenic greenhouse trace-gases (CO2, CH4, N2O) were present in the atmosphere and contributing to the radiative budget of the Earth's surface prior to human activities, the evidence for past greenhouse trace-gas changes currently offers a unique tool for investigating what are the active processes occurring under different climatic conditions in the interplay between greenhouse gases and climate.

In this paper we do not intend to give a complete overview of the greenhouse gas record from ice cores. We will focus on some of the recent results obtained on the record to place the current anthropogenic greenhouse-gas perturbation into the context of natural variability, and to address the insight provided by the ice core record of greenhouse trace-gases into the processes which could be involved in the next centuries as a consequence of the anthropogenic release of greenhouse gases. The potential of the method, consisting of looking at the past for (i) exploring the processes involved in changing the climate and (ii) a better understanding of the interplay between climate changes and biogeochemical cycles, is investigated here under different types of global climate:

  • the Holocene conditions which provide the natural background context for the anthropogenic changes;

  • the abrupt climatic changes during the last glacial–interglacial transition (Bolling/Alleröd, Younger Dryas) and during the last ice age (Dansgaard–Oeschger events);

  • the glacial–interglacial cycles, which provide the opportunity to investigate the sensitivity of the climate to different forcings.

Section snippets

The ice record of greenhouse trace-gases: anthropogenic and natural changes

Analysis of the air trapped in ice cores provides the most direct evidences of past changes in atmospheric trace-gases. A synthesis review of the works performed before 1993 on the reliability and the interpretation of the ice record of greenhouse gases can be found in Raynaud et al. (1993). Since then, new sets of measurements, new ice cores (GISP and GRIP cores in Greenland; Law Dome and Taylor Dome cores and the extension of the Vostok core in Antarctica) and firn air sampling in Antarctica

Methane and CO2 changes during the holocene

Studying the Holocene is of special interest for understanding interactions between climate and biogeochemical cycles under climatic conditions close to those prevailing today. We may thereby provide information related to posssible feedback processes in the projection of future global warming.

High-resolution methane records covering the whole of the Holocene (starting 11,500 years ago, at the end of the Younger Dryas) are now available from Antarctic and Greenland ice cores (Chappellaz et al.,

Greenhouse gases and abrupt climatic changes during the last glacial–interglacial transition and the last ice age.

Abrupt climatic changes took place during the past over a few decades and on a hemispheric scale, with wide amplitudes in the North Atlantic region (of the order of 10°C over Greenland). The relatively recent discovery of these past changes was a real surprise and is of importance for exploring the variability of our future climate. We will here assess the relationships between variations in CO2 and CH4 and changes in the isotopic composition of the ice (proxy for paleo-temperature). We first

Climate sensitivity of greenhouse trace gases and the glacial–interglacial cycles

The record of the Vostok ice core now covers about the last 400,000 yr (Petit et al., 1999; Fig. 4). The highest CO2 and CH4 mixing ratios are found during the interglacials and the lowest during the glacial maxima. The overall remarkable correlation of CO2 and CH4 with Antarctic temperature suggests that the GTG (essentially CO2) are important as amplifiers of the initial orbital forcing. It also reflects the dynamics of the oceanic and the continental biospheric carbon reservoirs in relation

Conclusion

We are well aware that the past will not provide a precise analogue of the future, but it provides lessons to be learned from real experiments undergone by the earth–climate system and the paleo-record also shows how the system reacts under different climatic conditions.

The GTG ice record provides the context for the dramatic change in their atmospheric concentrations induced by the growing anthropogenic perturbation over the last 200 yr, leading to present-day levels which have been

Acknowledgements

This work has been supported by the EC programme “Environment and Climate 1994-1998”. We thank very much R.S. Bradley for his stimulating and useful comments.

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