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When coal is used for generating electricity, CO2 is emitted into the atmosphere, which ultimately has an adverse effect on the climate process. It is however possible to capture the greenhouse gas before it is released into the atmosphere and then store it in underground rock formations. This could be a beneficial practice until our energy needs can be met by renewable energies.

In Ketzin, near Berlin, is Europe's only mainland site for testing the geological storage of CO2. It is the test site for the European project CO2SINK where monitoring techniques are being developed. 100 tonnes of CO2 are housed in two massive tanks before being pumped into a sandstone layer 630 metres underground. However, choosing the right location for storage is critical; required is a rock formation where sandstone is enclosed by an impermeable layer (a claystone layer, for example).

Carbon storage is currently considered by scientists as an acceptable short and medium term approach for reducing CO2 emissions. The Italian researcher Sergio Persoglia supports the concept of returning the CO2 to its place of origin, as it was extracted in the form of coal.

Sergio Persoglia is the coordinator of CO2GeoNet, a European network of experts on the geological storage of CO2. Researchers from CO2GeoNet are studying natural processes involving CO2 gas - an example of which is the leaking of CO2 from an extinct volcano. They are looking at the underground migration of gas and the potential dangers of a gas escape from a storage site. One side-effect of high levels of CO2 in soil is that plants cannot grow due to lack of oxygen.

At a geological research site, in a residential area near Rome, the barren earth is evidence for the very high levels of CO2 in the soil. This is the result of underground pressure caused by volcanic activity that pushes the gas through channels in the rock, known as fractures or faults. Here would be a poor choice of storage site for CO2. A storage site must therefore be free from pressure changes and faults in the rock.

The site at Ketzin has been in successful operation since June 2008. During this period of testing the techniques for observing and measuring the flow of the gas can be refined. These include measurements of electric field and temperature and will ensure that a tiny volume of escaped gas will not go unnoticed.

Appropriate storage sites can be found in most European countries. It is nonetheless estimated that the worldwide capacity would last for just 80 to 100 years. So for the time being excessive CO2 in the atmosphere could be avoided, but this is only a temporary solution. The only long term solution lies in the use of renewable energies.