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Sam Carana: The potential for methane releases in the Arctic to cause runaway global warming

The potential for methane releases in the Arctic to cause runaway global warming

What are the chances of abrupt releases of, say, 1 Gt of methane in the Arctic? What would be the impact of such a release?

by Sam Carana, Arctic News, December 20, 2011, updated January 10, 2012

How much methane is there in the Arctic?

An often-used figure in estimates of the size of permafrost stores is 1672 Gt (or Pg, or billion tonnes) of Carbon. This figure relates to organic carbon and refers to terrestrial permafrost stores. (1)

This figure was recently updated to 1700 Gt of carbon, projected to result in emissions of 30 - 63 Gt of Carbon by 2040, reaching 232 - 380 Gt by 2100 and 549 - 865 Gt by 2300. These figures are carbon dioxide equivalents, combining the effect of carbon released both as carbon dioxide (97.3%) and as methane (2.7%), with almost half the effect likely to be from methane. (2)

In addition to these terrestrial stores, there is methane in the oceans and in sediments below the seafloor. There are methane hydrates and there is methane in the form of free gas. 
Hydrates contain primarily methane and exist within marine sediments particularly in the continental margins and within relic subsea permafrost of the Arctic margins. (3)

Hunter and Haywood estimate that globally between 4700 and 5030 Pg (Gt) of Carbon is locked up within subsea hydrate within the continental margins. This does not include subsea permafrost-hosted hydrates and so those of the shallow Arctic margin (<~300m) were not considered. (3)

Dallimore and Collett (1995) found high methane concentrations in ice-bonded sediments and gas releases suggest that pore-space hydrate may be found at depths as shallow as 119 m. (4) Recent studies indicate that hydrate formation can occur in upper gas-saturated horizons (up to 100-200 m) of permafrost. (5) Furthermore, methane hydrates have been found in Siberia at depths as shallow as 20 m. (6)

Shakhova et al. estimate the accumulated methane potential for the Eastern Siberian Arctic Shelf (ESAS, rectangle on image right) alone as follows:

- organic carbon in permafrost of about 500 Gt;
- about 1000 Gt in hydrate deposits; and
- about 700 Gt in free gas beneath the gas hydrate stability zone. (7)

The East Siberian Arctic Shelf covers about 25% of the Arctic Shelf (3) and additional stores are present in submarine areas elsewhere at high latitudes. Importantly, the hydrate and free gas stores contain virtually 100%  methane, as opposed to the organic carbon which the above study (2) estimates will produce emissions in the ratio of 97.3% carbon dioxide and only 2.7% methane when decomposing.

How stable is this methane?

The sensitivity of gas hydrate stability to changes in local pressure-temperature conditions and their existence beneath relatively shallow marine environments mean that submarine hydrates are vulnerable to changes in bottom water conditions (i.e. changes in sea level and bottom water temperatures). Following dissociation of hydrates, sediments can become unconsolidated, and structural failure of the sediment column has the potential to trigger submarine landslides and further breakdown of hydrate. The potential geohazard presented to coastal regions by tsunami is obvious. (3)

Further shrinking of the Arctic ice-cap results in more open water, which not only absorbs more heat, but which also results in more clouds, increasing the potential for storms that can cause damage to the seafloor in coastal areas such as the East Siberian Arctic Shelf (ESAS, rectangle on image left), where the water is on average only 45 m deep. (8)

Much of the methane released from submarine stores is still broken down by bacteria before reaching the atmosphere. Over time, however, depletion of oxygen and trace elements required for bacteria to break down methane will cause more and more methane to rise to the surface unaffected. (9)

There are only a handful of locations in the Arctic where (flask) samples are taken to monitor the methane. Recently, two of these locations showed ominous levels of methane in the atmosphere (images below). 

The danger is that large abrupt releases will overwhelm the system, not only causing much of the methane to reach the atmosphere unaffected, but also extending the lifetime of the methane in the atmosphere, due to hydroxyl depletion in the atmosphere.

Shakhova et al. consider release of up to 50 Gt of predicted amount of hydrate storage as highly possible for abrupt release at any time. (10)

What would be the impact of methane releases from hydrates in the Arctic? 

If an amount of, say, 1 Gt of methane from hydrates in the Arctic would abruptly enter the atmosphere, what would be the impact? 

Methane's global warming potential (GWP) depends on many variables, such as methane's lifetime, which changes with the size of emissions and the location of emissions (hydroxyl depletion already is a big problem in the Arctic atmosphere), the wind, the time of year (when it's winter, there can be little or no sunshine in the Arctic, so there's less greenhouse effect), etc. One of the variables is the indirect effect of large emissions and what's often overlooked is that large emissions will trigger further emissions of methane, thus further extending the lifetime of both the new and the earlier-emitted methane, which can make the methane persist locally for decades. 

The IPCC (2007) gives methane a lifetime of 12 years, and a GWP of 25 over 100 years and 72 over 20 years. (11) 

The image by Dessus (2008) below illustrates how methane's GWP depends on the horizon over which its impact is calculated. (12) 

Drew Shindell (2009) points out that the IPCC figures do not include direct+indirect radiative effects of aerosol responses to methane releases that increase methane's GWP to 105 over 20 years when included. (13)

Using the IPCC figures, applying a GWP of 72 times carbon dioxide would give 1 Gt of methane a greenhouse effect equivalent to 72 Pg of carbon dioxide over 20 years. Applying a GWP of 105 times carbon dioxide would give 1 Gt of methane a greenhouse effect equivalent to 105 Pg of carbon dioxide over 20 years.  

By comparison, atmospheric carbon dioxide levels rose from 288 ppmv in 1850 to 369.5 ppmv in 2000, for an increase of 81.5 ppmv, or 174 Pg C. (14)

Note that this 174 Pg C was released over a period of 150 years, allowing sinks time to absorb part of the burden. Note also that, as emissions continue to rise, some sinks may turn into net emitters, if they haven't already done so.

The image on the left shows the impact of 1 Gt of methane, compared with annual fluxes of carbon dioxide based on the NOAA carbon tracker. (15) 

Fossil fuel and fires have been adding an annual flux of just under 10 Pg C since 2000 and a good part of this is still being absorbed by land and ocean sinks. 

In other words, the total burden of all carbon dioxide emitted by people since the start of the industrial revolution has been partly mitigated by sinks, since it was released over a long period of time.

Furthermore, the carbon dioxide was emitted (and partly absorbed) all over the globe, whereas methane from such abrupt releases in the Arctic would - at least initially - be concentrated in a relatively small area, and likely cause oxygen depletion in the water and hydroxyl depletion in the atmosphere, extending methane's lifetime, while triggering further releases from hydrates in the Arctic.

This makes it appropriate to expect a high initial impact from an abrupt 1 Gt methane release, i.e. at a GWP of well over 100 times the greenhouse effect of carbon dioxide, which will last for decades. 

Even more terrifying is the prospect that this would trigger further methane releases. Given that there already is ~5 Gt in the atmosphere, the impact of this initial 1 Gt combined with further releases of, say, 4 Gt of methane would result in a burden of 10 Gt of methane. When applying a GWP of 105 times carbon dioxide, this would result in a greenhouse effect equivalent to 1050 Pg of carbon dioxide over 20 years.

In conclusion, a release of 1 Gt of methane in the Arctic would be catastrophic and the methane wouldn't go away quickly either, since this would be likely to keep triggering further releases. While some models project rapid decay of the methane, those models often use global decay values and long periods, which is not applicable in case of such abrupt releases in the Arctic.  

Instead, the methane is likely to stay active in the Arctic for decades at a very high warming potential, due to depletion of hydroxyl and oxygen, while the resulting summer warming (when the sun doesn't set) is likely to keep triggering further releases in the Arctic. 

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