Geoscience Reference
In-Depth Information
Gas hydrates
Currently, below the world's oceans and permafrost is stored a large amount of carbon in
the form of gas hydrates. These are a mixture of water and methane, which is solid at low
temperatures and high pressures. These gas hydrates are cages of water molecules, which
hold individual molecules of methane and other gases within the cage. The methane comes
from decaying organic matter found deep in ocean sediments and in soils beneath perma-
frost. These gas hydrate reservoirs could be unstable, as an increase in temperature or de-
crease in pressure would cause them to destabilize and to release the trapped methane.
Climate change is warming up of both the oceans and the permafrost, threatening the sta-
bility of gas hydrates. Methane is a strong GHG, 21 times more powerful than carbon diox-
ide. If enough were released, it would raise global temperatures, which could led to the re-
lease even more gas hydrates—producing a runaway effect. Scientists really have no idea
how much methane is stored in the gas hydrates beneath our feet: estimates are between
1,000 and 10,000 gigatonnes, which is a huge range (compared with ~800 gigatonnes of
carbon currently in the atmosphere). Without a more precise estimate, it is very difficult to
assess the risk posed by gas hydrates.
The reason why scientists are so worried about this issue is because there is evidence that a
super greenhouse effect occurred 55 million years ago, during what is called the Palaeo-
cene-Eocene Thermal Maximum (PETM). During this hot-house event, scientists think
that up to 1,500 gigatonnes of gas hydrates may have been released. This huge injection of
methane into the atmosphere accelerated the natural greenhouse effect, producing an extra
5°C of warming. There is still, however, considerable debate over the PETM. For example,
was it gas hydrate or volcanic carbon dioxide release that caused the warming? The current
consensus is that the ocean reserves of gas hydrate are likely to remain stable this century.
This is because as ocean temperatures change, the change has to be transmitted through the
gas hydrate layer, causing some of it to melt, but if this process is slow enough, the gas re-
leased might migrate up in the ocean sediment column and re-freeze at a higher level.
However, if carbon emissions are not curbed then by the next century we could see this
process speed up leading to the release of some of the methane stored in the deep ocean. It
is also clear that there will be some permafrost gas hydrate destabilization as climate
change is having the greatest impact on high latitude temperatures, and permafrost melting
has been reported in almost all areas. However, we still do not have an indication of how
much methane is stored beneath the world's permafrost regions. So at the moment our best
estimate suggests a global warming of 3°C could release between 35 and 940 gigatonnes of
carbon, which could add between 0.02°C to 0.5°C to global temperatures.
Search WWH ::




Custom Search