The interaction of climate change and methane hydrates

Abstract

Gas hydrate, a frozen, naturally‐occurring, and highly‐concentrated form of methane, sequesters significant carbon in the global system and is stable only over a range of low‐temperature and moderate‐pressure conditions. Gas hydrate is widespread in the sediments of marine continental margins and permafrost areas, locations where ocean and atmospheric warming may perturb the hydrate stability field and lead to release of the sequestered methane into the overlying sediments and soils. Methane and methane‐derived carbon that escape from sediments and soils and reach the atmosphere could exacerbate greenhouse warming. The synergy between warming climate and gas hydrate dissociation feeds a popular perception that global warming could drive catastrophic methane releases from the contemporary gas hydrate reservoir. Appropriate evaluation of the two sides of the climate‐methane hydrate synergy requires assessing direct and indirect observational data related to gas hydrate dissociation phenomena and numerical models that track the interaction of gas hydrates/methane with the ocean and/or atmosphere. Methane hydrate is likely undergoing dissociation now on global upper continental slopes and on continental shelves that ring the Arctic Ocean. Many factors—the depth of the gas hydrates in sediments, strong sediment and water column sinks, and the inability of bubbles emitted at the seafloor to deliver methane to the sea‐air interface in most cases—mitigate the impact of gas hydrate dissociation on atmospheric greenhouse gas concentrations though. There is no conclusive proof that hydrate‐derived methane is reaching the atmosphere now, but more observational data and improved numerical models will better characterize the climate‐hydrate synergy in the future.

Gas hydrate in nature

“…methane hydrate breakdown is truly contributing methane to the atmosphere now or is likely to do so in the next century or more (Ruppel and Kessler, 2017).…”

“…Globally, most gas hydrate deposits are so deeply buried that the gas hydrate is likely to be stable for centuries or more under most model scenarios for future warming (Ruppel and Kessler, 2017).…”

The U.S. Geological Survey’s Gas Hydrates Project

“…In the oceans, most of the methane emitted at the sea floor from degrading gas hydrates or other sources dissolves in the overlying waters, and some of the methane is converted to carbon dioxide by bacteria. Very little of the methane reaches the atmosphere if it is emitted at water depths of more than several hundred feet (Ruppel and Kessler, 2017). There are two settings in which gas hydrate is most susceptible to breaking down as oceans warm: one is on upper continental slopes at water depths of 300-800 meters (approximately 1,000-2,600 feet) and the other is on continental shelves that ring the Arctic Ocean and are underlain by permafrost (approximate water depth as much as 100 meters or 330 feet).…”

“…For comparison, consumption of natural gas in the United States in 2016 exceeded 27.49 TCF according to the U.S. Energy Information Administration. Studies indicate that gas hydrates may prevent at least 10-15 percent of the global carbon inventory from circulating in the ocean-atmosphere system (Ruppel and Kessler, 2017).…”

Submarine vortices derived from natural gas hydrate conversion: a mechanism for ocean mixing

“…The existence of the hydrate physical-chemical system provides a potential natural mechanism for the delivery of cooler, less saline, water from the seafloor into otherwise generally quiescent deep water masses. NGH present within the EMB seafloor sediments that formed during the previous glacial episode would have dissociated owing to warming of abyssal Mediterranean waters as part of the interglacial equilibration of temperature in the atmosphereocean system and the underlying sediment (e.g., Ruppel & Kessler, 2016). Water from converted hydrate is very low-salinity, and even after local mixing with pore water would be considerably less dense and more buoyant than seawater or marine sediment pore water.…”