Natural Variations of the greenhouse gases nitrous oxide and Methane on Glacial-Interglacial and Millennial Time Scales

Schilt, Adrian (2010). Natural Variations of the greenhouse gases nitrous oxide and Methane on Glacial-Interglacial and Millennial Time Scales (Unpublished). (Dissertation, Universität Bern, Philosophisch–naturwissenschaftliche Fakultät, Physikalisches Institut, Abteilung für Klima– und Umweltphysik)

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The recent increase of the atmospheric abundance of the three most important long–lived greenhouse gases – carbon dioxide, methane, and nitrous oxide – led to a significant disturbance of the radiation balance of the Earth’s climate system. Consequently, global average surface temperature and global average sea level have been increasing since industrialisation began, while northern hemisphere snow cover has been decreasing. The Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) clearly concludes that the warming of the climate system is unequivocal, and that it is very likely that most of the observed increase in global average temperature of the past 50 years is the result of anthropogenic emissions of greenhouse gases. Since future warming may have a severe impact on the economic status of mankind, climate change is a problem of global importance. Although not every region may be affected in the same way and with the same intensity, everyone has to take responsibility: greenhouse gases are well mixed in the atmosphere, wherefore regional emissions influence the whole globe.
Identification and understanding of processes driving the Earth’s climate system are crucial for a reliable projection of, for instance, future temperature, sea level, precipitation patterns, and storm activity. Further, such knowledge is indispensable for national and international political decisions controlling future emissions of greenhouse gases. Because of the long time scales involved in many of the fundamental processes in the climate system, studies of past natural variations are needed in order to obtain insight into the whole range of dynamics. The history of the Earth’s climate is stored in many different archives, such as tree rings, lake and marine sediments, speleothems, corals, peatlands, and historical documents. However, ice cores are the only direct way to reconstruct the past composition of the atmosphere, thanks to air bubbles enclosed in the ice.
Based on reconstructions from ice cores drilled in Greenland and Antarctica, this thesis aims at increasing the knowledge and understanding of past natural variations of the greenhouse gases methane and nitrous oxide on glacial–interglacial and millennial time scales. The ice core records presented here provide indispensable information for computer–based simulations of the Earth’s climate system.
Chapter 1 gives an introduction to the biogeochemical cycles of methane and nitrous oxide. The radiative forcing of these two greenhouse gases, as well as natural sources and sinks are discussed, and the state of knowledge about their past variations is summarised. Further, a short overview of ice cores as a climate archive is given, and the locations of drilling sites are presented.
Chapter 2 extends the history of atmospheric methane back to Marine Isotope Stage 19, revealing that today’s concentrations of this important greenhouse gas have been unprecedented during the last eight glacial–interglacial cycles. Spectral analysis indicates that natural methane variations are dominated by the 100,000 year period of excentricity, but also that the contribution of this orbital component decreases in favour of the 23,000 year period of precession during the last four glacial–interglacial cycles. Throughout the last 800,000 years, methane concentrations clearly respond to every millennial time scale variation observed in Antarctic temperature.
Chapter 3 provides an insight into the nitrous oxide concentrations during the last 800,000 years. While glacial time intervals are partly affected by in situ production of nitrous oxide in the ice, the presented records reveal atmospheric values for all interglacials back to Marine Isotope Stage 19. At the end of most interglacials, nitrous oxide remains substantially longer on interglacial values than methane, but the two greenhouse gases show synchronous millennial time scale variations. Compared to the interglacials of the last 440,000 years, the interglacials between 800,000 and 440,000 years before present are characterised by lower values of Antarctic temperature, carbon dioxide and methane. On the contrary, mean interglacial nitrous oxide concentrations exceed the mean Holocene value also during the latter time interval. However, today’s concentrations of nitrous oxide are unprecedented during all interglacials of the last 800,000 years. Overall, the presented ice core records may indicate that low latitudes dominate emission changes of nitrous oxide during the late Quaternary.
Chapter 4 discusses the evolution of nitrous oxide concentrations during the last 140,000 years, in particular during Marine Isotope Stage 2 (including the Last Glacial Maximum), and during millennial time scale climate variations (Dansgaard/Oeschger events) observed during the last glacial. Further, numerous nitrous oxide, methane and carbon dioxide records from many ice cores are put on a common time scale, allowing for a reconstruction of the radiative forcing in the course of more than a complete glacial–interglacial cycle. Comparing the responses of the three greenhouse gases to Dansgaard/Oeschger events and glacial terminations leads to the hypothesis that processes located in high latitudes determine wether an increasing trend in temperature leads to an interglacial or just to a millennial–scale warming with early return to glacial climates.
While the focus of this thesis mainly lies on the reconstruction of atmospheric methane and nitrous oxide concentrations, collaboration, data exchange and fruitful discussions with colleagues from other institutions further led to interesting scientific results which are presented in Appendix A. These studies cover the development of age scales for different ice cores, as well as the reconstruction of past temperature variations using the isotopic composition of precipitation and atmospheric nitrogen and argon. The results allow for a discussion of climate variations on glacial–interglacial, as well as on millennial to centennial time scales, including the phasing of temperatures observed in Greenland and Antarctica.
Methane and nitrous oxide records presented in the frame of this thesis are tabulated in Appendix C.

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