Célia J. Sapart, 22 June 2012, 2:30 pm, University Hall, Domplein 29, Utrecht
Promotor: Prof. dr. T. Röckmann
Co-promotores: Dr. W. Sturges, University of East Anglia, Dr. R.S.W. van de Wal
Methane (CH4) is a strong greenhouse gas and even though its atmospheric abundance is lower than carbon dioxide (CO2), CH4 has a global warming potential twenty-five times larger than CO2 and its atmospheric abundance has drastically increased since 1800. Understanding the evolution of the CH4 atmospheric abundance is complex, because it is controlled by multiple sources (e.g. wetlands, biomass burning, ruminants, rice paddies and fossil fuel) and sinks, and large uncertainties exist on how sensitive those sources and sinks are to climate variability.
The aim of our research was to understand the influence of climate variability and anthropogenic activity on the CH4 budget, i.e. the balance between the different sources and sinks, during the last two millennia. For this purpose a technique was developed to analyze the CH4 isotopic composition of air in ice cores.
Analysis of the isotopic composition of CH4 preserved in ice cores provides evidence for the environmental drivers of variations in CH4 mixing ratios, because different sources and sinks affect the isotopic composition of CH4 uniquely.
Our main results from air trapped in Greenland ice cores shows that the carbon isotopic composition (d13C) of CH4 underwent pronounced centennial-scale variations between 200 BC and 1600 AD without clear corresponding changes in CH4 mixing ratios. Two-box model calculations suggest that those centennial-scale variations in isotope ratios are due to changes in biomass burning and biogenic sources (e.g. wetlands, agriculture), which are correlated with both natural climate variability, including the Medieval Climate Anomaly and with changes in human population, land-use and important events in history as the expansion of the Roman Empire, the fall of the Han dynasty and the Medieval period.