Environmental challenges related to methane hydrate decomposition from climate change scenario and anthropic activities: State of the art, potential consequences and monitoring solutions

Natural gas hydrate deposits (NGHD) have been investigated for decades and represent one of the major methane reservoirs on Earth. They are encountered in sediment of both the continental margins and the permafrost region; areas considered to host amongst the most climate-sensitive ecosystems on Earth. With worldwide temperature increases affecting continental margins and the permafrost, it is important to raise concern about the fate of the NGHD in the coming centuries. Thus, this review presents an overview of the potential consequences of hydrate decomposition on its surrounding areas. It compiles and discusses hydrate-derived methane fluxes measured or inferred from in situ data at several sites by considering both dissociation and dissolution. Depending on the magnitude and the duration of hydrate decomposition, the amounts of methane released can affect to varying degrees the seafloor and the microbial communities that sustain the methane cycle and regulate its transfer from the sediment to the water column; and that aspect is addressed in this review. Here, we also considered the transfer of methane from NGHDs and more broadly from marine emissions to the atmosphere, as it is assumed that such transfer will likely increase in the future. Finally, multi-scale monitoring in space and time is a key element to evaluate the impacts of natural and anthropic perturbations on NGHDs. We thus propose potential engineering solutions for the monitoring of NGHD, mainly based on the long-term deployment of sensor systems.