Gas Hydrates and Climate WarmingWhy a Methane Catastrophe Is Unlikely
[Modified from USGS Science Pick.]
News stories and Web postings have raised concerns that climate warming will release large volumes of methane from gas hydrates, kicking off a chain reaction of warming and methane releases. But recent research indicates that most of the world’s gas hydrate deposits should remain stable for the next few thousand years. Of the gas hydrates likely to become unstable, few are likely to release methane that could reach the atmosphere and intensify climate warming.
Gas Hydrates Primer
Gas hydrates are an ice-like combination of natural gas and water that can form in deep-water ocean sediments near the continents and within or beneath continuous permafrost. Specific temperatures and pressures and an ample supply of natural gas are required for gas hydrates to form and remain stable.
An estimated 99 percent of gas hydrates are in ocean sediment and the remaining 1 percent in permafrost areas (see map). Methane hydrate or “methane ice,” which is the most common type of gas hydrate, represents a highly concentrated form of methane: one cubic foot of methane hydrate traps about 164 cubic feet of methane gas.
The amount of methane trapped in the Earth’s gas hydrate deposits is uncertain, but even the most conservative estimates conclude that about 1,000 times more methane is trapped in hydrates than is consumed annually worldwide to meet energy needs. The most active area of gas-hydrate research focuses on gas hydrates’ potential as an alternative source of natural gas (for example, see http://web.mit.edu/mitei/research/studies/documents/natural-gas-2011/Supplementary_Paper_SP_2_4_Hydrates.pdf [842 KB PDF]); the U.S. Geological Survey (USGS) Gas Hydrates Project has several programs addressing this topic (see http://energy.usgs.gov/OilGas/UnconventionalOilGas/GasHydrates.aspx).
Gas Hydrates and Climate Change
Gas hydrate researchers are examining the link between climate change and the stability of methane-hydrate deposits. Warming climate could cause gas hydrates to break down (dissociate), releasing the methane that they now trap.
Methane is a potent greenhouse gas. For a given volume, methane causes 15 to 20 times more greenhouse-gas warming than carbon dioxide, and so the release of large volumes of methane to the atmosphere could, in theory, exacerbate climate warming and cause more gas hydrates to destabilize.
Some research suggests that such large-scale, climate-driven dissociation events have occurred in the past. For example, extreme warming during the Paleocene-Eocene Thermal Maximum about 55 million years ago may have been related to a large-scale release of methane from global methane hydrates. Some scientists have also advanced the clathrate-gun hypothesis to explain observations that may be consistent with repeated, catastrophic dissociation of gas hydrates and triggering of submarine landslides during the late Quaternary (400,000 to 10,000 years ago).
Methane As a Greenhouse Gas
The atmospheric concentration of methane, like that of carbon dioxide, has increased since the onset of the Industrial Revolution. Methane in the atmosphere comes from many sources, including wetlands, rice cultivation, termites, cows and other ruminants, forest fires, and fossil-fuel production. Some researchers have estimated that as much as 2 percent of atmospheric methane may originate with dissociation of global gas hydrates. Currently, scientists do not have a tool to say with certainty how much, if any, atmospheric methane comes from hydrates.
Although methane is a potent greenhouse gas, it does not remain in the atmosphere for long; within about 10 years, it reacts with other compounds in the atmosphere to form carbon dioxide and water. Thus, methane that is released to the atmosphere ultimately adds to the amount of carbon dioxide, the main greenhouse gas.
Climate-Driven Gas Hydrate Dissociation
For the most part, warming at rates documented by the Intergovernmental Panel on Climate Change for the 20th century should not lead to catastrophic breakdown of methane hydrates or major leakage of methane to the ocean-atmosphere system from gas hydrates that dissociate. Although most methane hydrates would have to experience sustained warming over thousands of years before dissociation was triggered, gas hydrates in some places are dissociating now in response to short- and long-term climatic processes.
The following discussion refers to the numbered type locales or sectors shown in the diagram of gas-hydrate deposits below.
Sector 1, Thick Onshore Permafrost: Gas hydrates that occur within or beneath thick terrestrial permafrost will remain largely stable even if climate warming lasts hundreds of years. Over thousands of years, warming could cause gas hydrates at the top of the stability zone, about 625 feet (190 meters) below the Earth’s surface, to begin to dissociate.
Sector 2, Shallow Arctic Shelf: The shallow-water continental shelves that circle parts of the Arctic Ocean were formed when sea-level rise during the past 10,000 years inundated permafrost that was at the coastline. Subsea permafrost is thawing beneath these continental shelves, and associated methane hydrates are likely dissociating now. (For example, see related Sound Waves article "Degradation of Subsea Permafrost and Associated Gas Hydrates Offshore of Alaska in Response to Climate Change.") If methane from these gas hydrates reaches the seafloor, much of it will likely be emitted to the atmosphere. Less than 1 percent of the world’s gas hydrates probably occur in this setting, but this estimate could be revised as scientists learn more.
Sector 3, Upper Edge of Stability: Gas hydrates on upper continental slopes, beneath 1,000 to 1,600 feet (300 to 500 meters) of water, lie at the shallowest water depth for which methane hydrates are stable. The upper continental slopes, which ring all of the world’s continents, could host gas hydrate in zones that are roughly 30 feet (10 meters) thick. Warming ocean waters could completely dissociate these gas hydrates in less than 100 years. Methane emitted at these water depths will probably dissolve or be oxidized in the water column and is unlikely to reach the atmosphere. About 3.5 percent of the Earth’s gas hydrates occur in this climate-sensitive setting.
Sector 4, Deepwater: Most of the Earth’s gas hydrates, about 95 percent, occur in water depths greater than 3,000 feet (1,000 meters). They are likely to remain stable even with a sustained increase in bottom temperatures over thousands of years. Most of the gas hydrates in these settings occur deep within the sediments. If the gas hydrates do dissociate, the released methane should remain trapped in the sediments, migrate upward to form new gas hydrates, or be consumed by oxidation in near-seafloor sediments. Most methane released at the seafloor would likely dissolve or be oxidized in the water column. A recent article, “Methane Hydrates and Contemporary Climate Change,” provides more detail.
USGS Research Activities
The USGS Gas Hydrates Project is studying Arctic methane hydrates, methane emissions, and their relation to short- and long-term climate change. Since 2009, the USGS Gas Hydrates Project has been conducting field research to determine whether gas hydrates are currently dissociating in response to climate warming and, if so, how much methane emitted from these gas hydrates might reach the atmosphere. Study areas include the U.S. Beaufort Sea (for example, see http://soundwaves.usgs.gov/2010/11/) and Alaska’s North Slope (see http://soundwaves.usgs.gov/2009/10/). The USGS has also organized workshops to identify priorities in climate-hydrates research (see Fire in the Ice, http://www.netl.doe.gov/technologies/oil-gas/FutureSupply/MethaneHydrates/newsletter/newsletter.htm, v. 11, no. 1, p. 18) and to plan ocean-drilling projects related to these issues (see http://iodp-usssp.org/workshop/catching-climate-change/).
in this issue: