Estimating methane sources and sinks in the Arctic using atmospheric data assimilation

• Main Author: Wittig, Sophie
• Other Authors: Laboratoire des Sciences du Climat et de l'Environnement Gif-sur-Yvette (LSCE), Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Institut national des sciences de l'Univers (INSU - CNRS)-Université Paris-Saclay-Centre National de la Recherche Scientifique (CNRS), Université Paris-Saclay, Isabelle Pison, Antoine Berchet
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: HAL CCSD 2023
• Subjects: Methane; Arctic; Inversion; Emission; Méthane; Arctique; Inversions; Émissions; [SDU.OCEAN]Sciences of the Universe [physics]/Ocean; Atmosphere; Arctique*; Climate change; Ice; permafrost; Sea ice
• Online Access: https://theses.hal.science/tel-03997135; https://theses.hal.science/tel-03997135/document; https://theses.hal.science/tel-03997135/file/117416_WITTIG_2023_archivage.pdf

The Arctic is an especially critical region with regard to global climate change. With temperatures in high northern latitudes increasing around three times faster than the global average, changes in environmental characteristics such as declining sea ice extend or thawing permafrost soils are proceeding rapidly. As a consequence, positive climatic feedbacks are triggered in this region, whereby the Arctic warming is accelerated even further. One important impact is the high risk of increased methane (CH4) emissions. CH4 is a potent greenhouse gas whose global average concentration in the atmosphere has increased by about 160 % since pre-industrial times, a trend that is also reflected in the Arctic. Currently, various CH4 sources, both natural and anthropogenic, are contributing to CH4 emissions from the Arctic region. Anthropogenic emissions are predominantly caused by the extraction and distribution of fossil fuels by the Arctic nations. The dominant natural source of CH4 are high northern latitude wetlands; however, other freshwater systems, wildfire events, geological fluxes and oceanic emissions of different origins also contribute to the Arctic CH4 budget. Furthermore, the thawing and destabilization of terrestrial and sub-sea permafrost will potentially increase CH4 emissions in the near future, for instance by the exposure of degradable soil organic matter. However, assessing the amount of CH4 emissions in the Arctic and their contribution to the global budget still remains challenging. This is on the one hand due to the difficulties in carrying out accurate measurements in such remote areas. Besides, high variations in the spatial distribution of methane sources and a poor understanding of the effects of ongoing changes in carbon decomposition, vegetation and hydrology also complicate the assessment. Therefore, the aim of this work is to reduce uncertainties on current bottom-up estimates of CH4 emissions as well as soil oxidation by implementing an inverse modeling approach in order to better quantify ...