Our Threatened Oceans by Stefan Rahmstorf

Author:Stefan Rahmstorf [Stefan Rahmstorf, Katherine Richardson]
Language: eng
Format: epub
Tags: Sustainability, Our Threatened Oceans, Ocean
ISBN: 9781906598563
Publisher: Chicago Distribution Center (CDC Presses)

Methane release from the sea bed?

There is one final danger associated with the oceans as a result of climate change that we would like to address in this chapter: the release of methane from the sea bed. The sea bed contains huge carbon deposits in the form of methane hydrate – as an order of magnitude, the quantities involved are comparable to the global coal reserves. Methane hydrate has the appearance of slushy ice but it can be burned. It is a mixture of methane and water, which can form a crystaline structure under very high pressure and at low temperatures, at which point it then becomes solid. The conditions under which methane hydrate is stable are generally given in sea beds below a depth of 500 meters but, in Arctic waters, it can also exist closer to the surface.

Most of this methane has accumulated in the sea bed over a period of millions of years, where it is a waste product created by life in the ocean: it is actually dead biomass, which sediments and is decomposed by bacteria. In some regions, such as the Gulf of Mexico, methane can also seep up from natural gas deposits under the sea, which then combines to hydrate with water in the sediment. The problem here is that, as a consequence of climate change, ocean water is also becoming warmer – and, albeit with a time lag of a few centuries, this also applies to deep water in regions containing methane hydrate deposits. Ultimately, the layer of sediment containing the hydrates will also warm up. As a result, some of the methane will change state and become gaseous. This is because the zone in the sediment with the right conditions for methane hydrate to remain stable will shrink under the effects of warming.

There are now two possible scenarios. The first one is a sudden, episodic release of methane. This could happen if, due to the destabilization of the methane, landslides occur on the sea bed. This scenario is more than just fantasy: today, we still find the tell-tale signs which show instances of this type of landslide throughout the Earth’s history. One famous example is the Storegga Slide off the coast of Norway, an event which took place around 8000 years ago and caused a massive tsunami which hit the Shetland Islands with a height of at least 25 meters and was still 5 meters high when it reached the coast of Britain. Although the amount of methane released in the process – one gigaton – was huge, this was still not enough to have a notable effect on the climate, despite the fact that methane is a very potent greenhouse gas. This example illustrates that, although this type of landslide can cause a devastating flood wave, it does not have a big impact on climate, even in an extreme case such as this.

However, the situation is quite different in the second scenario: a gradual release of methane. To understand this, it

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