WUWT reader “Jos” advises in Tips and Notes of this essay in Nature’s Knowledge Project. Excerpts follow:
Methane Hydrates and Contemporary Climate Change
Concern about the long-term stability of global gas hydrate deposits is rooted in the potential impact that a large CH4 release might have on global climate. CH4 is ~20 times more potent than CO2 as a GHG, but it oxidizes to CO2 after about a decade in the atmosphere. In recent models, the longer-lived CO2 oxidation product (Archer et al. 2009), not the CH4 itself (e.g., Harvey & Huang 1992), is credited with causing most of the excess atmospheric warming that would follow large-scale dissociation of methane hydrates.
The present-day concentration of CH4 in the atmosphere is ~200 times lower than that of CO2, but CH4 concentrations have risen by ~150% since pre-industrial times, compared to only ~40% for CO2 (IPCC 2001). Rising atmospheric CH4 concentrations lead to more rapid depletion of the hydroxyl radicals needed for oxidation, longer CH4 residence times, and thus increased CH4-induced warming (Lelieveld et al. 1998). Present-day CH4 emissions are dominated by wetlands, ruminants, fossil fuel production, and rice cultivation (IPCC 2001), sources that fluctuate with season, human behavior, and other factors. The IPCC (2001, 2007) relies on estimates and models (Fung et al. 1991, Lelieveld et al. 1998, Wang et al. 2004) to attribute ~3.75×10-4 Gt C (IPCC 2001), or ~1% of annual global CH4 emissions, to dissociating gas hydrates. This estimate has never been validated, nor do any observational data unequivocally link specific CH4 emissions or regionally integrated CH4 fluxes to dissociating gas hydrates. Against the backdrop of a well-mixed atmosphere, strong annual fluctuations, and a significant increase in CH4 concentrations over the 20th century, detecting a CH4 signal that is directly attributable to dissociating gas hydrates may always remain challenging.
Gas Hydrates and Past Warming Events
The geologic record is punctuated by warming events that may provide clues about future interactions between methane hydrates and climate change. The Paleocene-Eocene Thermal Maximum (PETM) at ~54.95 Ma is the most intensely studied of these hyperthermals (e.g., Dickens et al. 1995, Schmidt & Shindell 2003, Renssen et al. 2004, Zachos et al. 2001, 2005, Röhl et al. 2007). The large, negative carbon isotopic excursion (CIE) recorded in both marine and terrestrial sediments during the PETM has been interpreted as reflecting widespread release of isotopically-light (microbial) carbon from dissociating marine methane hydrates (e.g., Dickens et al. 1995, Zachos et al. 2005). This explanation is also invoked to explain CIEs in other deep-time warm periods (Hesselbo et al. 2000, Jiang et al. 2003). Negative CIEs also undergird the clathrate gun hypothesis (CGH), which postulates that repeated warming of intermediate ocean waters during the Late Quaternary (since 400 ka) triggered periodic dissociation events (Kennett et al. 2003).Ice core data (Sowers 2006) and geologic studies (Maslin et al. 2003) have challenged the CGH, while Bock et al. (2010) and Petrenko et al. (2009) question the role of gas hydrate dissociation in contributing to increased atmospheric CH4 during more recent warming events as well. Northern hemisphere wetlands, which may experience increased production of isotopically-light CH4 in response to local warming, appear to be the key culprit in enhancing atmospheric CH4 concentrations during several Pleistocene (~2.6 Ma to 10 ka) and Holocene (since 10 ka) warming events.
Catastrophic, widespread dissociation of methane gas hydrates will not be triggered by continued climate warming at contemporary rates (0.2ºC per decade; IPCC 2007) over timescales of a few hundred years. Most of Earth’s gas hydrates occur at low saturations and in sediments at such great depths below the seafloor or onshore permafrost that they will barely be affected by warming over even 10^3 yr. Even when CH4 is liberated from gas hydrates, oxidative and physical processes may greatly reduce the amount that reaches the atmosphere as CH4. The CO2 produced by oxidation of CH4 released from dissociating gas hydrates will likely have a greater impact on the Earth system (e.g., on ocean chemistry and atmospheric CO2 concentrations; Archer et al. 2009) than will the CH4 that remains after passing through various sinks.
Ruppel, C. D. (2011) Methane Hydrates and Contemporary Climate Change. Nature Education Knowledge 3(10):29