| We all know about the role of carbon dioxide in causing global warming. |
| Obviously, society needs to reduce the release of carbon dioxide, otherwise known as CO2. |
| This gas comes from the burning of fossil fuels, such as coal and oil, |
| and it is virtually impossible for society to prevent, or even limit such activity. |
| Our need for energy and power is just too great. |
| Instead, a more practical idea is to collect the carbon dioxide from the burning process, |
| for example, directly from the chimneys of power stations, |
| and somehow prevent this gas from being released into the environment. |
| To do that, you need to store it somehow, and that has to be essentially forever. |
| It is perhaps for this reason that many believe that, |
| rather than storing the carbon dioxide as a gas, |
| it is better to react it with metal oxides, such as magnesium or calcium, |
| which results in the formation of a hard carbonate material. |
| The gas is, in effect, turned into a stable and unreactive solid, |
| which can simply be dumped anywhere. |
| This process actually occurs naturally, although very very slowly, |
| and is one cause of the surface limestone in the world. |
| But this slow reaction speed is the problem. |
| Even when enhancing this process through high temperature and pressure, |
| or pre-treatment of the mineral, it is still far too slow to be economical. |
| One other technique which has been suggested is to pump the gas to the bottom of the ocean, |
| where it would react with compounds in the seawater, forming carbonic acid. |
| However, this alternative has now been ruled out. |
| The CO2 may be removed from the atmosphere, |
| but the high oceanic acidity which would result raises its own set of problems, |
| mostly with all the delicate life and the intricate food chains in the seawater, |
| some on which we ourselves depend, |
| and that's something which no one wants to experiment with. |
| Perhaps because of the lack of alternatives, |
| the most commonly discussed solution to the problem of disposing of carbon dioxide is to pump the gas underground - |
| a technique known as 'geosequestration'. |
| In this system, the CO2, for example, |
| could be pumped into underground pockets within depleted oil wells, or disused coal tunnels. |
| This carries, however, three serious disadvantages, |
| namely: the risk of leaks, the considerable costs involved, |
| and finally, the unproven effectiveness. |
| Let us look at those three disadvantages in detail. |
| Firstly, there is the risk of leaks. |
| Although the gas would be deep and sealed over by masses of rock and earth, |
| the huge pressures in these spaces would turn the gas into a liquid state |
| capable of moving through rock fissures or faults. |
| This could allow the gas to eventually be released to the surface. |
| Since CO2 is heavier than air, |
| and thus pushes oxygen aside, |
| such leaks could result in the suffocation of thousands, or tens of thousands of people - |
| certainly not a consequence to be taken lightly. |
| Natural CO2 leakage from volcanic build-up has already witnessed such deadly events. |
| The other problem of geosequestration is the cost. |
| The time and effort spent on materials and construction, |
| primarily the pipework through which the gas would travel, does not come cheaply. |
| So, if this system were to be implemented in, say, coal-fired power plants, |
| the extra cost would have to be paid by the electricity user, |
| whose bills would almost double as a consequence. |
| Few people are prepared to pay this much simply to make a small dent on the effects of global warming. |
| And this leads to the final problem. |
| The most basic question is whether geosequestration actually reduces global warming. |
| The problem here is that the energy needed to create |
| and drive the sequestration process would require approximately |
| a quarter of a coal-fired electricity plant's output. |
| In other words, the plant would have to burn one quarter more of its coal |
| just to account for the sequestration of the carbon dioxide, |
| and with coal producing other noxious pollutants, |
| such as sulphur, ash, and heavy metals, |
| the environment is hardly benefited at all. |
| Nevertheless, there are many active experimental efforts underway, |
| primarily in oil production sites. |
| These are small but intensively monitored and analysed. |
| All we can say now is that the jury is still out on whether underground carbon storage will one day be feasible. |