Defusing the Arctic Methane Time Bomb

The Arctic methane time bomb keeps ticking… From Scientific American…

Climatewire

More Arctic Methane Bubbles into Atmosphere

A new study suggests more than twice as much of the potent greenhouse gas is bubbling out of the rapidly warming Arctic Ocean, speeding climate change

By Stephanie Paige Ogburn and ClimateWire

Arctic Ocean: A new study reports that methane releases from one part of the Arctic Ocean are more than twice what scientists previously thought.

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SciAm

If the Arctic Methane Time Bomb is really twice as bad as “scientists previously thought,” one of two things must be happening:

1. The Arctic methane time bomb is about to go off and turn Earth into Venus.
2. “Scientists” preconceptions about the climatic hazards of Arctic methane are very wrong.

Arctic methane is currently trapped in permafrost and in methane hydrate deposits. Some methane from these traps escapes to the atmosphere every year, particularly during warm summer months. However, there is absolutely no indication that this represents some sort of Arctic methane time bomb, ticking its way to some sort of carbon Apocalypse.

Permafrost

Permafrost is ground that is frozen below the active layer (~30-100 mm) for multi-year periods. Some Arctic permafrost has been frozen for at least several thousand years. The active layer may thaw seasonally; however the permafrost substrate remains frozen year-round. The frozen nature of the soil below the active layer prevents it from adequately draining. This results in a very boggy active layer with abundant decaying plant matter. As such, permafrost is generally very methane-rich.

A rapid and extensive thawing of Arctic permafrost could theoretically release a lot of methane into the atmosphere. There’s just very little reason to think that this is even a remote possibility now or in the foreseeable future.

News in Brief: Warming may not release Arctic carbon

Element could stay locked in soil, 20-year study suggests

By Erin Wayman

Web edition: May 15, 2013

Print edition: June 15, 2013; Vol.183 #12 (p. 13)

Researchers used greenhouses to artificially warm tundra (shown, in autumn) for 20 years. They found no net change in the amount of carbon stored in the soil.

Sadie Iverson

The Arctic’s stockpile of carbon may be more secure than scientists thought. In a 20-year experiment that warmed patches of chilly ground, tundra soil kept its stored carbon, researchers report.

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Science News

In the Alaska experiment, they warmed the permafrost by 2°C over a 20-yr period (10 times the actual rate of warming since the 1800s) and there wasn’t the slightest hint of an accelerated methane release.

There is no evidence of widespread thawing of Arctic permafrost since Marine Isotope Stage 11 (MIS-11), approximately 450,000 years ago. None of the subsequent interglacial stages indicate widespread permafrost thawing, above 60°N, not even MIS-5 (Eemian/Sangamonian), which was about 2°C warmer than present day, possibly as much as 5°C warmer in the Arctic.

The last interglacial stage (MIS-5, Sangamonian/Eemian) was considerably warmer than the current interglacial and sea level was 3-6 meters higher than modern times. It was particularly warmer in the Arctic. Oxygen isotope ratios from the NGRIP ice core indicate that the Arctic was approximately 5°C warmer at the peak of MIS-5 (~135,000 years ago).

It also appears that it was significantly warmer in the Arctic during the Holocene Climatic Optimum (~7,000 years ago) than modern times. The Arctic was routinely ice-free during summer for most of the Holocene up until about 1,000 years ago. McKay et al., 2008 demonstrated that the modern Arctic sea ice cover is anomalously high and the Arctic summer sea surface temperature is anomalously low relative to the rest of the Holocene…

Modern sea-ice cover in the study area, expressed here as the number of months/year with >50% coverage, averages 10.6 ±1.2 months/year… Present day SST and SSS in August are 1.1 ± 2.4 8C and 28.5 ±1.3, respectively… In the Holocene record of core HLY0501-05, sea-ice cover has ranged between 5.5 and 9 months/year, summer SSS has varied between 22 and 30, and summer SST has ranged from 3 to 7.5 8C (Fig. 7).

McKay et al., 2008

Vaks et al., 2013 found no evidence of widespread permafrost thawing above 60°N since MIS-11, not even during MIS-5…

The absence of any observed speleothem growth since MIS 11 in the northerly Lenskaya Ledyanaya cave (despite dating outer edges of 7 speleothems), suggests the permanent presence of permafrost at this latitude since the end of MIS-11. Speleothem growth in this cave occurred in early MIS-11, ruling out the possibility that the unusual length of MIS-11 caused the permafrost thawing.

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The degradation of permafrost at 60°N during MIS-11 allows an assessment of the warming required globally to cause such extensive change in the permafrost boundary.

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There is clear evidence that the Arctic was at least 5°C warmer during MIS-11 than it is today…

Several so-called “superinterglacials” have been identified in the Quaternary sediment record from LakeEl’gygytgyn (Melles et al.,2012). Among these “superinterglacials”, marine isotope stage (MIS) 11c and 31 appear to be the most outstanding in terms of their temperature, vegetation cover, in-lake productivity, and in the case of MIS11c also duration (Melles et al.,2012). Quantitative climate reconstructions for MIS11c and 31 at Lake El’gygytgyn imply that temperatures and annual precipitation values were up to ca. 5°C and ca. 300mm higher if compared to the Holocene (Melles et al.,2012)

Vogel et al., 2013

The lack of evidence of permafrost melt during MIS-5 tends to indicate that MIS-11 may have been more than 5°C warmer. So, the notion that we are on the verge of a permafrost meltdown is patently absurd.

Methane Hydrate Deposits

Methane hydrates (or gas hydrate) are composed of molecules of methane encased in a lattice of ice crystals. These accumulations are fairly common in marine sediments.

Gas hydrate is an ice like substance formed when methane or some other gases combine with water at appropriate pressure and temperature conditions. Gas hydrates sequester large amounts of methane and are widespread in marine sediments and sediments of permafrost areas.

USGS

99% of methane hydrate deposits are thought to be in deepwater environments. The only way that climate change could destabilize these deposits would be through a sudden drop in sea level. The thermocline of the deepwater deposits changes very little (not at all at depth) even with 20 °C of surface warming over a 1,000-yr period.

Methane Hydrates and Contemporary Climate Change

By: Carolyn D. Ruppel (U.S. Geological Survey, Woods Hole, MA) © 2011 Nature Education

Citation: Ruppel, C. D. (2011) Methane Hydrates and Contemporary Climate Change. Nature Education Knowledge 3(10):29

Methane Hydrate Primer

Methane hydrate is an ice-like substance formed when CH4 and water combine at low temperature (up to ~25ºC) and moderate pressure (greater than 3-5 MPa, which corresponds to combined water and sediment depths of 300 to 500 m). Globally, an estimated 99% of gas hydrates occurs in the sediments of marine continental margins at saturations as high as 20% to 80% in some lithologies; the remaining 1% is mostly associated with sediments in and beneath areas of high-latitude, continuous permafrost (McIver 1981, Collett et al. 2009). Nominally, methane hydrate concentrates CH4 by ~164 times on a volumetric basis compared to gas at standard pressure and temperature. Warming a small volume of gas hydrate could thus liberate large volumes of gas.

A challenge for assessing the impact of contemporary climate change on methane hydrates is continued uncertainty about the size of the global gas hydrate inventory and the portion of the inventory that is susceptible to climate warming. This paper addresses the latter issue, while the former remains under active debate.

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Fate of Contemporary Methane Hydrates During Warming Climate

The susceptibility of gas hydrates to warming climate depends on the duration of the warming event, their depth beneath the seafloor or tundra surface, and the amount of warming required to heat sediments to the point of dissociating gas hydrates. A rudimentary estimate of the depth to which sediments are affected by an instantaneous, sustained temperature change DT in the overlying air or ocean waters can be made using the diffusive length scale 1 = √kt , which describes the depth (m) that 0.5 DT will propagate in elapsed time t (s). k denotes thermal diffusivity, which ranges from ~0.6 to 1×10-6 m2/s for unconsolidated sediments. Over 10, 100, and 1000 yr, the calculation yields maximum of 18 m, 56 m, and 178 m, respectively, regardless of the magnitude of DT. In real situations, DT is usually small and may have short- (e.g., seasonal) or long-term fluctuations that swamp the signal associated with climate warming trends. Even over 103 yr, only gas hydrates close to the seafloor and initially within a few degrees of the thermodynamic stability boundary might experience dissociation in response to reasonable rates of warming. As discussed below, less than 5% of the gas hydrate inventory may meet these criteria.

Even when gas hydrate dissociates, several factors mitigate the impact of the liberated CH4 on the sediment-ocean-atmosphere system. In marine sediments, the released CH4 may dissolve in local pore waters, remain trapped as gas, or rise toward the seafloor as bubbles. Up to 90% or more of the CH4 that reaches the sulfate reduction zone (SRZ) in the near-seafloor sediments may be consumed by anaerobic CH4 oxidation (Hinrichs & Boetius 2002, Treude et al. 2003, Reeburgh 2007, Knittel & Boetius 2009). At the highest flux sites (seeps), the SRZ may vanish, allowing CH4 to be injected directly into the water column or, in some cases, partially consumed by aerobic microbes (Niemann et al. 2006).

Methane emitted at the seafloor only rarely survives the trip through the water column to reach the atmosphere.

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Global Warming and Gas Hydrate Type Locales

Methane hydrates occur in five geographic settings (or sectors) that must be individually evaluated to determine their susceptibility to warming climate (Figure 1). The percentages assigned to each sector below assume that 99% of global gas hydrate is within the deepwater marine realm (McIver 1981, Collett et al. 2009). Future refinements of the global ratio of marine to permafrost-associated gas hydrates will require adjustment of the assigned percentages. Owing to the orders of magnitude uncertainty in the estimated volume of CH4 trapped in global gas hydrate deposits, the percentages below have not been converted to Gt C.

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Conclusions

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 103 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.

Contemporary and future gas hydrate degradation will occur primarily on the circum-Arctic Ocean continental shelves (Sector 2; Macdonald 1990, Lachenbruch et al. 1994, Maslin 2010), where subsea permafrost thawing and methane hydrate dissociation have been triggered by warming and inundation since Late Pleistocene time, and at the feather edge of the GHSZ on upper continental slopes (Sector 3), where the zone’s full thickness can dissociate rapidly due to modest warming of intermediate waters. More CH4 may be sequestered in upper continental slope gas hydrates than in those associated with subsea permafrost; however, CH4 that reaches the seafloor from dissociating Arctic Ocean shelf gas hydrates is much more likely to enter the atmosphere rapidly and as CH4, not CO2. Proof is still lacking that gas hydrate dissociation currently contributes to seepage from upper continental slopes or to elevated seawater CH4 concentrations on circum-Arctic Ocean shelves. An even greater challenge for the future is determining the contribution of global gas hydrate dissociation to contemporary and future atmospheric CH4 concentrations.

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Nature Knowledge

The infamous photos, often posted by alarmists, of methane bubbling up from the Arctic sea floor and lake beds account for less than 1% of global methane hydrate deposits. These deposits are unstable in any temperature regime at depths of less than 200 m. They were already bubbling long before Al Gore invented CAGW.

Arctic Methane Time Bomb Defused

A substantial permafrost thaw above 60° N would require the Arctic to warm by more than 5°C relative to current conditions

A substantial destabilization of methane hydrate deposits is highly unlikely even with 20°C of warming relative to current conditions.

Arctic methane time bomb defused… QED.

References