Accelerated global warming could lead to a runaway methane global warming effect due to the release of methane currently trapped in unstable methane hydrate deposits in the arctic that could be destabilised by accelerated global warming effects.
Core samples taken from old ocean sediment layers have been used to trace back in time the climate changes that have occurred over the past tens of millions of years. By analysing the incidence of different fossil shell remains of sea creatures occurring in these sediments it is possible to track the changes in the sea water temperatures and levels of atmospheric CO2 occurring at the time the shells were formed and deposited. These shells contain carbon from the CO2 in the atmosphere which was dissolved in the sea water in which the creatures lived just as takes place today.
From these records it appears that there have been short periods of only a few hundred years in the geological past when rapid increases of the Earth's temperature have occurred superimposed on top of the rise and fall of average temperatures over the longer term. For these short periods temperature rises of up to 8 degrees centigrade appear to have occurred on top of existing long term rises of 5 to 7 degrees to give temperatures up to 15 degrees centigrade warmer than today. Temperatures then fell back to the long term trend, the whole rise and fall only lasting a few hundred years.
The most likely cause of this rapid global warming over such a short period is the release of methane into the atmosphere. Methane is 60 times more powerful than CO2 as a greenhouse gas but only remains in the atmosphere for about ten years and so looses it's greenhouse effect quickly compared to CO2 which remains in the atmosphere for 100 years. CO2 would not be available in sufficient quantities to achieve the rapid warming and if CO2 was the cause then the raised temperatures would last a lot longer.
Methane hydrates occur extensively today all over the world. They consist of methane stored within unstable water bound deposits that if disturbed release the methane. They occur in major river deltas such as the Amazon delta and in old delta areas such as the Gulf of Mexico. Major rivers carry millions of tons of silt containing vegetable matter that continues to decay after the silt is deposited in the river delta. This anaerobic decay produces methane which gets trapped in the silt as methane hydrates until the conditions of water temperature and pressure change which can release the methane in vast quantities very quickly.
Another form is a frozen slush/ice methane hydrate where the methane is trapped in an ice/water mixture which releases the methane when it warms up or the pressure on the ice is reduced. Frozen methane hydrates can contain 170 times their own volume of methane. These frozen hydrates occur in the seabed deposits of the Arctic Ocean.
Methane can also be trapped by permafrost layers which over-lay lower unfrozen layers of vegetable material that is decaying and producing methane which remains trapped by the frozen permafrost on top. If the permafrost layer were to melt then the methane in the layers below would escape into the atmosphere. Given the vast areas of permafrost in northern latitudes there is a significant potential for methane to be trapped that would be released if the permafrost melted as a result of global warming.
The theory for these rapid rises and falls of temperature, based on the geological records from 55 million years ago, is that gradual global warming due to some natural cause had resulted in temperatures 5 to 7 degrees centigrade higher than average (i.e. higher than today's temperatures). At this point methane trapped in methane hydrate deposits started to be released into the atmosphere and accelerated the rate of warming. This would result in further warming releasing more methane.
As the atmosphere warmed different types of methane deposits would start to be released and so a cycle of methane release leading to increased warming leading to more methane release from other areas of methane deposits elsewhere in the world would become established as global warming effected different areas of the world.
There is an intriguing photograph of what appears to be a methane plume coming up out of the Arctic ice sheet which indicates that the phenomenon described above can occur. There have also been incidences of oil drilling inadvertently triggering large releases of methane from hydrate deposits. One theory to explain the loss of ships in the so called Bermuda triangle is that they have been engulfed in a sudden methane release which reduces the buoyancy of the sea water so that the ship sinks.
So, does methane pose a threat today? Let us review the situation. We know there are extensive methane hydrate and permafrost deposits all around the world. We have evidence that we are at the beginning of a period of global warming that is probably being made worse by the continuing build up of CO2 in the atmosphere due to fossil fuel burning. Recent computer modelling incorporating the feed back effects of global warming that has already occurred suggests that by about 2050 we may start to loose the beneficial effects of the Amazon rain forest as a carbon sink.
This could lead to temperature rises of 5 to 8 degrees centigrade by 2100. This would be uncharted territory and no one really knows at present how the world's environmental systems would change but we now have the evidence from the geological past. On the basis of this evidence global warming can lead to methane releases which once started would escalate. This would be the worst possible thing to happen because once started there would be no way of stopping a runaway methane global warming event.
We CAN reduce our CO2 emissions from fossil fuels but we COULD NOT reduce methane emissions once they started, huge natural forces would take over and change our world. This would probably result in the melting of the Antarctic icecap which would raise sea levels by 50 metres and would completely change the climates of the world.
So what should we do? We should be careful and not risk starting the sequence events described above. To do this we must reduce total CO2 emissions from now onwards and take measures to protect carbon sinks such as the Amazon rainforest.
If we all carry on burning so much fossil fuel as we do now we will be running the risk of starting an unstoppable runaway methane global warming event within the foreseeable future. Only major absolute reductions in CO2 emissions NOW will avoid this risk.
James Nash is a climate scientist with Greatest Planet (www.greatestplanet.org). Greatest Planet is a non-profit environmental organization specialising in carbon offset investments.
James Nash is solely responsible for the contents of this article.