Hay una enorme cantidad de metano (CH₄) en la tierra congelada en un tipo de hielo llamado hidrato de metano. Hidratos pueden formar con casi cualquier gas y consiste en una “Jaula” de las moléculas de agua que rodea el gas. (El término "clatrato" describe en general más sólidos que consisten en gases que se encuentran atrapados dentro de cualquier tipo de “jaula”, mientras que el hidrato es el término específico para cuando la jaula está hecha de moléculas de agua). Hay hidratos de CO₂ en Marte, mientras que en la Tierra la mayor parte de los hidratos están llenos de metano. La mayoría de ellos se encuentran en los sedimentos del océano, pero algunos se asocian con suelos de permafrost. Los hidratos de metano, intuitivamente parece ser la más precaria de las cosas. El hidrato de metano se derrite si se pone demasiado caliente, y flota en el agua. El metano es un potente gas invernadero, y se degrada en CO₂, otro gas de efecto invernadero que se acumula en la atmósfera al igual que los combustibles fósiles como el CO₂ lo hace. Y hay mucho de ella, posiblemente más que los tradicionales depósitos de combustibles fósiles. Concebible, los cambios climáticos pueden afectar a estos depósitos. Entonces ¿Qué sabemos de la película-catástrofe potencial de los hidratos de metano? Hidrátos del océano. La mayoría de los hidrato de metano se encuentra en los sedimentos del océano. De éste, la mayoría es lo que puede llamarse depósitos de tipo estratigráfica. Carbono orgánico de plancton que está enterrado durante millones de años. Cientos de metros bajo el fondo del mar, las bacterias producen metano de entre el plancton muerto. Si el metano se produce con la suficiente rapidez, algunos de ellos van a congelar en hidratos de metano. Este tipo de depósito tiene miles de gigatoneladas de carbono como el metano [Buffett y Archer, 2004; Milkov, 2004]. Por comparación, el tipo más abundante de los combustibles fósiles tradicionales es el carbón, que suele ser acreditado con cerca de 5000 Gton C [Rogner, 1997].   A veces el metano se mueve en torno a la tierra, y se colecta en algunos lugares, formando lo que se llama depósitos estructurales de hidratos. El Golfo de Mexico, por ejemplo, es básicamente un campo de combustible lleno de fugas [MacDonald et al., 2005]. Una implicación de gas en movimiento y en torno a la puesta en común de este tipo es que el hidrato de concentración puede ser mayor, hasta el punto de lo que ellos llaman depósitos masivos, trozos de casi hidrato puro. La segunda conclusión es que el hidrato se puede encontrar mucho más cerca del lecho marino, e incluso en el fondo del mar. El hidrato se derrite si se pone demasiado caliente. El océano está lo suficientemente frío en una escala de profundidad por decir 500 metros hacia abajo (200 metros en el Ártico). Más allá de el fondo del mar, la temperatura aumenta con la profundidad, a lo largo del gradiente de temperatura geotérmica. En cierta profundidad se convierte demasiado caliente para el hidrato, por lo que el hidrato se derrite es enterrado más profundo que esta profundidad. A menudo existe una capa de burbujas debajo de la zona de estabilidad de hidrato. Las burbujas sísmicas reflejan las ondas sonoras, y se muestran claramente en las prospecciones sísmicas en todo el mundo [Buffett, 2000]. Las colinas y los valles de capas de burbujas siguen las colinas y los valles del lecho marino, por lo que esta capa se le llama reflector de simulación de fondo (BSR). Ahora calentemos el agua en la parte superior de la columna de sedimentos. En última instancia, el nuevo perfil de temperatura tendrá casi la misma pendiente como antes, el geotérmico. El hidrato en la zona de estabilidad se hará más delgada, con un aumento en la temperatura de la columna de sedimentos. Lo importante a señalar es que, se hace más delgado desde la parte de abajo, y no desde arriba. El hidrato en la base de la zona de estabilidad original se encuentra en derretimiento. Si la estabilidad de la zona aún existe, será menor en la columna de sedimentos que en las burbujas de metano recién liberadas, por lo que podrían actuar como una trampa fría para evitar que el gas metano liberado escape. Sin embargo, estudios sísmicos muestran a menudo " zonas liberadas" donde la BSR esta perdida, y todas las capas de la estructura de la columna de sedimentos por encima de los desaparecidos BSR es suavizado. Estos se consideran áreas donde el gas se ha roto a través de la estructura del sedimento para escapar al océano [Wood et al., 2002]. Una teoría es que la migración ascendente de fluido lleva consigo el calor, evitando la congelación del metano, ya que viaja a través de la estabilidad nominal de la zona. Los sedimentos superficiales de los océanos del mundo tienen agujeros llamados pockmarks [Hill et al., 2004], interpretado como lo que estas explosiones de gas se verían desde la superficie. Y existe la posibilidad de deslizamientos de tierra. Cuando se unde el hidrato y produce burbujas, hay un aumento en el volumen. La idea es que las burbujas podrían levantar los granos fuera de sí, desestabilizando la columna de sedimentos. El mayor deslizamiento de tierra submarino conocido es frente a las costas de Noruega, llamado Storegga [Bryn et al., 2005; Mienert et al., 2005]. El lado excavado en la parte superior media de 250 meters of sediment over a swath hundreds of kilometers wide, stretching half-way from Norway to Greenland. There have been comparable slides on the Norwegian margin every approximately 100 kyr, synchronous with the glacial cycles [Solheim et al., 2005]. The last one occurred 2-3 kyr years after the stability zone thinned due to increasing water temperature [Mienert et al., 2005], about 8150 years ago. The slide started at a few hundred meters water depth, just off the continental slope, where Mienert calculates the maximum change in HSZ. The Storegga slide area today contains methane clathrate deposits as indicated by a seismic BSR corresponding to the base of the HSZ at 200-300 meters, and pockmarks indicating gas expulsion from the sediment. However, there is another also apparently plausible hypothesis for Storegga, which doesn't involve hydrates at all. This is the rapid accumulation of glacial sediment shed by the Fennoscandian ice sheet [Bryn et al., 2005]. Rapid sediment loading traps pore water in the sediment column faster than it can be expelled by the increasing sediment load. At some point, the sediment column floats in its own porewater. This mechanism has the capacity to explain why the Norwegian continental margin, of all places in the world, should have landslides synchronous with climate change. The Storegga slide generated a tsunami in what is now the United Kingdom, but it does not appear to have had any climate connections. It was not a catastrophic amount of methane loss. The volume of sediment moved was about 2500 km³. Assuming 1% hydrate by pore water volume were released on average from the slide volume, you get a methane release of about 0.8 Gton of C. Even if all of the hydrate made it to the atmosphere, it would have had a smaller climate impact than a volcanic eruption (I calculated the methane impact on the radiative budget here).   Actually, the truth be told, the Storegga slide occurred spookily close in time to the 8.2k climate event, but there doesn't appear to be any connection. The 8.2k event was a century-long cool interval, most probably in response to fresh-water release from Glacial Lake Aggasiz to the North Atlantic and was coincident with a ~75 ppbv drop in methane, not a rise. Methane can leave the sediment in three possible forms: dissolved, bubbles, and hydrate. Dissolved methane is chemically unstable in the oxic water column of the ocean, but it has a lifetime of decades (shorter in high-flux environments) [Valentine et al., 2001], so if the methane is released shallow enough in the ocean, it has a good chance of escaping to the atmosphere. Bubbles of methane are typically only able to rise a few hundred meters before they dissolve. Hydrate floats in water just like regular ice floats in water, carrying methane to the atmosphere much more efficiently than bubbles [Brewer et al., 2002]. For most parts of the ocean, melting of hydrates is a slow process. It takes decades to centuries to warm up the water 1000 meters down in the ocean, and centuries more to diffuse that heat down into the sediment where the base of the stability zone is. The Arctic Ocean may be a special case, because of the shallower stability zone due to the colder water column, and because warming is expected to be more intense in high latitudes. Permafrost. You've maybe read about permafrost in the paper a lot lately. Permafrost soils are defined as those which remain frozen year-round (actually, the technical definition is a soil which has been frozen for the last two years). There is sometimes a zone near the sediment surface that thaws in the summer. In the permafrost literature, this zone is called the active zone, and it has been observed to be getting larger with time [Sazonova et al., 2004]. Melting of surface soils is one reason why the high latitude Arctic is expected to be a part of the land surface that responds most dramatically to climate change [Bala et al., 2005].   The other reason is that temperature changes are more dramatic in high latitudes than the global average, especially high northern latitudes. There has been a stream of anecdotal reports of the effects of melting permafrosts on the Arctic landscape, tilted buildings and "drunken forests" near Fairbanks, for example [Pearce, 2005; Stockstad, 2004]. Much of the Alaskan oil pipeline is anchored in permafrost soils. Hydrates are sometimes associated with permafrost deposits, but not too close to the soil surface, because of the requirement for high pressure. The other factor that determines whether you find hydrate is the permeability of the soils. Sometimes freezing, flowing groundwater creates a sealed ice layer in the soil, which can elevate the pressure in the pore space below. Hydrate in a one permafrost core [Dallimore and Collett, 1995] was reported below sealed ice layers. Lakes have been reported to suddenly drain away as some subsurface sealed ice layer is apparently breached. The grand-daddy of subsurface sealed ice layers is a very large structure in Siberia called the ice complex [Hubberten and Romanovskii, 2001].   The most important means of eroding the ice complex is laterally, by a melt-erosion process called thermokarst erosion [Gavrilov et al., 2003]. The ice layer is exposed to the warming waters of the ocean. As the ice melts, the land collapses, exposing more ice. The northern coast of Siberia has been eroding for thousands of years, but rates are accelerating. Entire islands have disappeared in historical time [Romankevich, 1984]. Concentrations of dissolved methane on the Siberian shelf reached 25 times higher than atmospheric saturation, indicating escape of methane from coastal erosion into the atmosphere [Shakhova et al., 2005]. Total amounts of methane hydrate in permafrost soils are very poorly known, with estimates ranging from 7.5 to 400 Gton C (estimates compiled by [Gornitz and Fung, 1994]). The Future. The juiciest disaster-movie scenario would be a release of enough methane to significantly change the atmospheric concentration, on a time scale that is fast compared with the lifetime of methane. This would generate a spike in methane concentration. For a scale of how much would be a large methane release, the amount of methane that would be required to equal the radiative forcing of doubled CO2 would be about ten times the present methane concentration. That would be disaster movie. Or, the difference between the worst case IPCC scenario and the best conceivable 'alternative scenario' by 2050 is only about 1 W/m2 mean radiative energy imbalance. A radiative forcing on that order from methane would probably make it impossible to remain below a 'dangerous' level of 2 deg above pre-industrial. I calculate here that it would take about 6 ppm of methane to get 1 W/m2 over present-day. A methane concentration of 6 ppm would be a disaster in the real world. The atmosphere currently contains about 3.5 Gton C as methane. An instantaneous release of 10 Gton C would kick us up past 6 ppm. This is probably an order of magnitude larger than any of the catastrophes that anyone has proposed. Landslides release maybe a gigaton and pockmark explosions considerably less. Permafrost hydrates are melting, but no one thinks they are going to explode all at once. There is an event documented in sediments from 55 million years ago called the Paleocene Eocene Thermal Maximum, during which (allegedly) several thousand Gton C of methane was released to the atmosphere and ocean, driving 5° C warming of the intermediate depth ocean. It is not easy to constrain how quickly things happen so long ago, but the best guess is that the methane was released over perhaps a thousand years, i.e. not catastrophically [Zachos et al., 2001; Schmidt and Shindell, 2003]. The other possibility for our future is an increase in the year-in, year-out chronic rate of methane emission to the atmosphere. The ongoing release of methane is what supplies, and determines the concentration of, the ongoing concentration of methane in the atmosphere. Double the source, and you’d double the concentration, more or less. (A little more, actually, because the methane lifetime increases.) The methane is oxidized to CO₂, another greenhouse gas that accumulates for hundreds of thousands of years, same as fossil fuel CO₂ does. Models of chronic methane release often show that the accumulating CO₂ contributes as much to warming as does the transient methane concentration. Anthropogenic methane sources, such as rice paddies, the fossil fuel industry, and livestock, have already more than doubled the methane concentration in the atmosphere from pre-industrial levels. Currently methane levels appear stable, but the reasons for this relatively recent phenomena are not yet clear. The amount of permafrost hydrate methane is not known very well, but it would not take too much methane, say 60 Gton C released over 100 years, to double atmospheric methane yet again. Peat deposits may be a comparable methane source to melting permafrost hydrate. When peat that has been frozen for thousands of years thaws, it still contains viable populations of methanotrophic bacteria [Rivkina et al., 2004] that begin to convert the peat into CO₂ and CH₄. It’s not too difficult to imagine 60 Gton C over 100 years from peat, either. Changes in methane production in existing wetlands and swamps due to changes in rainfall and temperature could also be important. 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