Nitrous oxide and methane variations covering the last 100,000 years: Insight into climatic and environmental processes

Flückiger, Jacqueline (2003). Nitrous oxide and methane variations covering the last 100,000 years: Insight into climatic and environmental processes (Unpublished). (Dissertation, Universität Bern, Philosophisch–naturwissenschaftliche Fakultät, Physikalisches Institut, Abteilung für Klima– und Umweltphysik)

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This year, another record-breaking hot and dry summer in large parts of Europe has once more caught the attention of the public and raised questions about climate change and global warming. The evidence is growing that human activities affect the climate system, primarily through the emission of greenhouse gases and aerosols, resulting in significant warming observed over the last century [IPCC, 2001].
In the discussion of climate change, records of climate variations in the past play a crucial role for several reasons. First, paleodata are used to study mechanisms and feedbacks operating in the climate system. Second, attribution of climate change to anthropogenic causes relies on the central finding that the observed trends are unusual. Here, reconstructions of the past climate provide insight into the range of natural climate variability. Third, the simulation of climate variations in the past and the validation with data remain one of the harshest test for the climate models that are used to predict future climate. And fourth, quantifying the effect of anthropogenic emissions of greenhouse gases on the climate requires precise knowledge of the global biogeochemical cycles. The reconstruction of atmospheric greenhouse gas variations in the past offers a unique opportunity to study the key biogeochemical processes.
Climate change leaves its traces in a variety of archives, among them marine and lake sediments, tree rings, stalagmites, corals, and others. But many indicators record climate indirectly and/or reflect local as well as large-scale changes. Greenhouse gases are well-mixed over the Earth, reflect large-scale or global changes, and thus are exceptional in this respect. The large ice sheets provide an archive of ancient air trapped in small air bubbles that can be measured directly. Deep ice core drillings in Greenland and Antarctica have provided an often undisturbed record of greenhouse gas concentrations in the past. While atmospheric carbon dioxide (CO2) and methane (CH4) have been reconstructed from ice cores over four glacial cycles, the nitrogen cycle has received little attention, and measurements of nitrous oxide (N2O) have been very sparse until recently. In this thesis, several studies are presented that provide new high resolution records of the evolution of the three greenhouse gases over the last 100,000 years and investigate the processes and changes in the respective ecosystems and biogeochemical cycles.
The first introductory chapter presents an overview on atmospheric N2O, CH4 and CO2, their anthropogenic increase over the last 250 years as well as the last glacial-interglacial transition, a time period of increasing greenhouse gas concentrations in parallel to natural climatic changes. In distinct depth intervals of Greenland and Antarctic ice cores, atmospheric N2O records are disturbed by artifacts. The second part of chapter 1 discusses these N2O artifacts and gives an overview of their occurrence, their possible origin and methods to distinguish them from the atmospheric trend.
The climate over the last few glacial cycles was dominated by relatively short interglacial warm phases and long ice-ages. Each cycle was about 100,000 years long. The study in chapter 2 gives an overview of the greenhouse gas variations over the last 60,000 years covering part of the last glacial epoch, the last glacial-interglacial transition and the present warm period, the Holocene. The agreement of different records and cores and the reliability of smaller variations found in individual records are discussed.
The climate over the last 10,000 years has been exceptionally stable, providing an ideal time period to study the natural variability of the current interglacial warm phase. In chapter 3, high-resolution measurements of N2O and CH4 are presented and compared with CO2 regarding potential processes and changes in sources and sinks of both the ocean and the terrestrial biosphere.
The end of last glacial period was characterized by a rather gradual warming in the south. In contrast, a more abrupt warming is seen in the northern hemisphere, interrupted by a return to cold conditions for several centuries. High resolution records of N2O, CH4 and CO2 covering this time period are presented in the two studies of chapters 4 and 5. The different characteristics of the three greenhouse gases reveal important insight and unprecedented details into how the biogeochemical cycles have responded and thereby driven climate change in the two hemispheres.
The climate of the last glacial was punctuated by abrupt and large-scale warm events in the northern hemisphere, with local temperature changing by 10◦C or more within a few decades. These so-called Dansgaard-Oeschger (DO) events occurred with remarkable regularity, and their trigger remains unknown. The high-resolution N2O and CH4 records measured over several of these events are presented in chapter 6. Both N2O and CH4 show changes in parallel to DO events, but with different characteristics. The study discusses the amplitudes, timing and duration of these events, as well as different hypotheses of terrestrial and oceanic changes in sources and sinks that might have caused the observed changes.
Atmospheric CH4 shows an interhemispheric gradient, caused by the short lifetime of about 10 years and the sources that are located mainly in the northern hemisphere. From the reconstruction of the past CH4 concentration from both Greenland and Antarctic ice cores, the interhemispheric gradient, and thus the source distribution among the tropics and the mid to high northern latitudes can be calculated. In the study presented in chapter 7, CH4 measurements over the last glacial period and the transition to the Holocene are used to reconstruct CH4 source changes in different latitudes for various time intervals as well as for glacial cold and warm events.
New ice cores drilled in both Greenland and Antarctica, combined with improved analytical techniques will soon lead to continuous greenhouse gas reconstructions over the last 800,000 years or more, providing insight into how climate has changed in the past. However, it will be a long and challenging task to understand why greenhouse gases changed the way we observe them, and how these changes are related to the large-scale or global environmental and climatic processes. This thesis is one step into this direction.

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