Investigating observational constraints on the contemporary methane budget

Abstract

Methane (CH4) is an important greenhouse gas, naturally produced by bio-degradation of organic material (mainly in wetlands), by continuous and eruptive releases from mud volcanoes, and by combustion of organic material in forest and peat fires. Large quantities of methane are also emitted by human activities, related to agriculture (cattle farming, rice cultivation), waste management (landfills, water treatment plants), and energy production and use (extraction of fossil fuels). As a result of these anthropogenic emissions, the concentration of methane in the atmosphere has increased by a factor 2.5 since the start of industrialization. Methane is a very potent greenhouse gas, with an estimated contribution to the present day anthropogenic greenhouse effect of about 20%. A reliable understanding of methane emissions is of importance for predicting future global warming and for developing effective mitigation policies. While the main sources of methane have likely been identified, large uncertainties remain regarding their relative importance. In particular the size and inter-annual variability of natural emissions remain poorly quantified. As a result, the underlying causes of large variations in the global growth rate of methane as observed in recent years remain a topic of debate. This phenomenon started with a steady decline in the methane growth rate, modified by large inter-annual variations, between 1985 and 2000. The global average methane concentration remained stable from 2000 to 2007, after which it started rising again. Various explanations have been proposed to explain this growth-rate slowdown, such as a reduction of natural emissions, a reduction of anthropogenic emissions related to the economic collapse of the former Soviet Union, or an increase in the methane sink strength. In this thesis, measurements of methane and its 13C carbon content have been used to derive constraints on the drivers of this methane growth-rate slowdown. In particular, I showed that a commonly supported scenario of temporarily reduced natural wetland emissions balanced by increases in fossil emissions is incompatible with the observed trend in the isotopic signature of methane in the atmosphere. The second part of the thesis explores the use of satellite observations of methane to constrain methane emissions in recent years. I used the inverse-modelling technique, which is a method to derive a statistical best estimate of methane emissions given a set of methane observations and a numerical chemistry-transport model (TM5) to relate these observations to the emissions. Satellites allow a significant extension in global measurement coverage compared with the use of surface observations, in particular in the tropics, which are major contributors to the global emission budget of methane. Satellite observations are, however, difficult to use because of systematic errors that are difficult to identify. I focused on the recently launched GOSAT satellite (2009) and its predecessor SCIAMACHY (launched in 2003), and found that an important fraction of the systematic error budget is caused by the transport model rather than the satellite data. Models are an essential component of the data processing chain. Their development should receive high priority to allow optimal use current and future satellite missions.