There is a scientific consensus that the world is warming up because of the greenhouse effect in the atmosphere. It has been caused by a build-up of various gases over the past century, emitted by industrial and agricultural activities. Carbon dioxide (CO2) is the principal greenhouse gas, and efforts are now under way internationally to limit its emissions. Nevertheless, the prognosis regarding future levels of the gas is such that it will be increasingly necessary to bury CO2 wastes deep underground or in the oceans. This burial, technically called sequestration, is described by H. Herzog, B. Eliasson and D. Kaarstad in the February issue of Scientific American.
Certain gases in our atmosphere (principally CO2, nitrous oxide, methane and water vapour) act like a blanket, inhibiting heat loss from the Earth into outer space. The greater the concentration of those gases the more insulating the blanket and the warmer the Earth. CO2 concentration in the atmosphere has risen from 280 parts per million (p.p.m.) in 1850 to 370 p.p.m. today.
Correspondingly, it is estimated the average temperature of the Earth has risen over this time by 0.6 Celsius.
According to Herzog and co-authors, the CO2 concentration in the atmosphere will double the 1850 levels by 2100. This is a conservative estimate and will be exceeded if the most effective measures, including burial, to limit CO2 build-up are not implemented.
The concentration of CO2 in the atmosphere reflects the balance struck by those factors that add or remove the gas from the atmosphere. The main natural emission of CO2 is the "burning" or metabolising of food by animals - we all constantly exhale CO2. It is also released from rocks by natural weathering processes. It is artificially released to the atmosphere whenever carbon-based (organic) material is burned, principally when fossil fuel (coal, peat, oil) is burned.
It is removed naturally from the atmosphere by green plants and microbes, and much CO2 dissolves in the oceans. The natural processes releasing and removing CO2 to and from the atmosphere would, left to themselves, more or less keep atmospheric levels constant. The increasing levels of CO2 in our air are due to human-driven releases.
The first international treaty to stabilise greenhouse gas emission was signed in 1992 in Rio de Janeiro. Since then, demand for fossil fuel has increased globally. More than 85 per cent of global commercial energy needs are currently plied by fossil fuels. Policies of increasing energy efficiency of fossil fuels and use of alternative energy sources (wind, wave, nuclear power) will not grab hold of the problem quickly enough and alternative approaches must also be used, hence the proposals to bury CO2????????????
Burial of CO2 is already practised in Norway. The Sleipper offshore oil and gas field in the North Sea is 240 km off the coast of Norway. One reservoir contains gas diluted with 9 per cent CO2 - customers generally accept no more than 2.5 per cent CO2. Normal practice elsewhere is to remove the excess CO2 and release it to the atmosphere. However, the owners of this gas-field pump the excess CO2 down a well into a 200 metre-thick sandstone layer where it is trapped.
A major incentive for burying CO2 in this instance was the Norwegian offshore carbon dioxide tax at $50 for every ton of gas emitted to the atmosphere. The company buries about 1 million tons of CO2 per year. It invested $80 million in equipment to pump the gas into the earth, but this investment was paid off in just 18 months - a clear example of the beneficial effects of taxing CO2 emissions.
There are various possibilities for the burial of CO2 both on land and at sea. It can be pumped into underground geologic formations such as unmineable coal beds, depleted oil and gas wells, or salty aquifers. It may also be possible to bubble CO2 into the deep ocean at a concentration that doesn't affect the local ecology and at a depth that ensures it stays buried.
Geologists are confident that storage of CO2 in appropriate sites is quite secure. They point to the track record of nature in this regard; for example the McElmo Dome in south-western Colorado has held large quantities of CO2 for centuries. But deep in the ocean doesn't strike one intuitively as a secure location for the gas.
In order to retain CO2 in the ocean for a long time it must be injected into the water at a considerable depth - below the thermocline. The thermocline is a layer of ocean between 100 and 1,000 metres deep in which water temperature decreases steeply with depth. The cooler, denser water below travels only very slowly up through the thermocline and may take many centuries to mix with surface water.
However, when CO2 dissolves in water it forms carbonic acid. Any significant increase in acidity of ocean water would adversely affect marine life, and further testing is necessary in order to prove it would not cause damage.
The most natural way to sequester CO2 from the atmosphere is to plant forests. The green leaves suck the gas from the air and covert it into wood. Planting trees undoubtedly traps unwanted CO2, but is not in itself a panacea. It has been estimated that balancing present emissions by this method would require planting new forests every year equal to the area of India. Large-scale tree planting has other disadvantages such as depressing local bio-diversity and forcing relocation of populations. It has a part to play, but no more than that.
There is no such thing as a free lunch. We have been very inventive in developing ourselves technologically and industrially. One of the prices we now must pay is learning to control a warming world. We will have to be just as clever in devising ways to do this.
William Reville is a senior lecturer in bio-chemistry and director of microscopy at UCC.