Climate change impacts on methane hydrates
How methane ends up in the ocean
People have been burning coal, oil and natural gas for more than a hundred years. Methane hydrates, on the other hand, have only recently come under controversial discussion as a potential future energy source from the ocean. They represent a new and completely untapped reservoir of fossil fuel, because they contain, as their name suggests, immense amounts of methane, which is the main component of natural gas. Methane hydrates belong to a group of substances called clathrates – substances in which one molecule type forms a crystal-like cage structure and encloses another type of molecule. If the cage-forming molecule is water, it is called a hydrate. If the molecule trapped in the water cage is a gas, it is a gas hydrate, in this case methane hydrate.
Methane hydrates can only form under very specific physical, chemical and geological conditions. High water pressures and low temperatures provide the best conditions for methane hydrate formation. If the water is warm, however, the water pressure must be very high in order to press the water molecule into a clathrate cage. In this case, the hydrate only forms at great depths. If the water is very cold, the methane hydrates could conceivably form in shallower water depths, or even at atmospheric pressure. In the open ocean, where the average bottom-water temperatures are around 2 to 4 degrees Celsius, methane hydrates occur starting at depths of around 500 metres.
- 2.16 > Methane hydrate looks like a piece of ice when it is brought up from the sea floor. This lump was retrieved during an expedition to the “hydrate ridge” off the coast of Oregon in the US.
- Surprisingly, there is no methane hydrate in the deepest ocean regions, the areas with the highest pressures, because there is very little methane available here. The reason for this is because methane in the ocean is produced by microbes within the sea floor that break down organic matter that sinks down from the sunlit zone near the surface.
Organic matter is composed, for example, of the remains of dead algae and animals, as well as their excrements. In the deepest areas of the ocean, below around 2000 to 3000 metres, only a very small amount of organic remains reach the bottom because most of them are broken down by other organisms on their way down through the water column. As a rule of thumb, it can be said that only around 1 per cent of the organic material produced at the surface actually ends up in the deep sea. The deeper the sea floor is, the less organic matter settles on the bottom. Methane hydrates therefore primarily occur on the continental slopes, those areas where the continental plates meet the deep-sea regions. Here there is sufficient organic matter accumulating on the bottom and the combination of temperature and pressure is favourable. In very cold regions like the Arctic, methane hydrates even occur on the shallow continental shelf (less than 200 metres of water depth) or on the land in permafrost, the deep-frozen Arctic soil that does not even thaw in the summer.
- 2.17 > Methane hydrate occurs in all of the oceans as well as on land. The green dots show occurrences in the northern permafrost regions. Occurrences identified by geophysical methods are indicated by red. The occurrences shown
by blue dots were verified by direct sampling.
- It is estimated that there could be more potential fossil fuel contained in the methane hydrates than in the classic coal, oil and natural gas reserves. Depending on the mathematical model employed, present calculations of their abundance range between 100 and 530,000 gigatons of carbon. Values between 1000 and 5000 gigatons are most likely. That is around 100 to 500 times as much carbon as is released into the atmosphere annually by the burning of coal, oil and gas. Their possible future excavation would presumably only produce a portion of this as actual usable fuel, because many deposits are inaccessible, or the production would be too expensive or require too much effort. Even so, India, Japan, Korea and other countries are presently engaged in the development of mining techniques in order to be able to use methane hydrates as a source of energy in the future (Chapter 7).
2.18 > In hydrates, the gas (large ball) is enclosed in a cage formed by water molecules. Scientists call this kind of molecular arrangement a clathrate.
Methane hydrates and global warming
Considering that methane hydrates only form under very specific conditions, it is conceivable that global warming, which as a matter of fact includes warming of the oceans, could affect the stability of gas hydrates.
There are indications in the history of the Earth suggesting that climatic changes in the past could have led to the destabilization of methane hydrates and thus to the release of methane. These indications – including measurements of the methane content in ice cores, for instance – are still controversial. Yet be this as it may, the issue is highly topical and is of particular interest to scientists concerned with predicting the possible impacts of a temperature increase on the present deposits of methane hydrate.
Methane is a potent greenhouse gas, around 20 times more effective per molecule than carbon dioxide. An increased release from the ocean into the atmosphere could further intensify the greenhouse effect. Investigations of methane hydrates stability in dependance of temperature fluctuations, as well as of methane behaviour after it is released, are therefore urgently needed.
2.19 > Gas hydrates occur when sufficient methane is produced by organic matter degradation in the sea floor under low temperature and high pressure conditions. These conditions occur predominantly on the continental margins. The warmer the water, the larger the water depths must be to form the hydrate. Deep inside he sea floor, however, the temperature is too high for the formation of methane hydrates because of the Earth’s internal heat.
Many bacteria use methane to provide energy for their metabolism. They take up methane and transform it chemically. In this process the methane releases electrons and is thus oxidized. Some bacteria break the methane down with the help of oxygen. This is called aerobic oxidation. Other bacteria do not need oxygen. This kind of oxidation is called anaerobic.
- Various methods are employed to predict the future development. These include, in particular, mathematic modelling. Computer models first calculate the hypothetical amount of methane hydrates in the sea floor using background data (organic content, pressure, temperature). Then the computer simulates the warming of the seawater, for instance, by 3 or 5 degrees Celsius per 100 years. In this way it is possible to determine how the methane hydrate will behave in different regions. Calculations of methane hydrate deposits can than be coupled with complex mathematical climate and ocean models. With these computer models we get a broad idea of how strongly the methane hydrates would break down under the various scenarios of temperature increase. Today it is assumed that in the worst case, with a steady warming of the ocean of 3 degrees Celsius, around 85 per cent of the methane trapped in the sea floor could be released into the water column.
Other, more sensitive models predict that methane hydrates at great water depths are not threatened by warming. According to these models, only the methane hydrates that are located directly at the boundaries of the stability zones would be primarily affected. At these locations, a temperature increase of only 1 degree Celsius would be sufficient to release large amounts of methane from the hydrates. The methane hydrates in the open ocean at around 500 metres of water depth, and deposits in the shallow regions of the Arctic would mainly be affected.
In the course of the Earth’s warming, it is also expected that sea level will rise due to melting of the polar ice caps and glacial ice. This inevitably results in greater pressure at the sea floor. The increase in pressure, however, would not be sufficient to counteract the effect of increasing temperature to dissolve the methane hydrates. According to the latest calculations, a sea-level rise of ten metres could slow down the methane-hydrate dissolution caused by a warming of one degree Celsius only by a few decades.
A wide variety of mathematical models are used to predict the consequences of global warming. The results of the simulations are likewise very variable. It is therefore difficult to precisely evaluate the consequences of global warming for the gas hydrate deposits, not least of all because of the large differences in the calculations of the size of the present-day gas hydrate deposits. One major goal of the current gas hydrate research is to optimize these models by using ever more precise input parameters. In order to achieve this, further measurements, expeditions, drilling and analyses are essential. >