The capture and storage of CO2 deep underground may prove an environmentally risky solution to global warming, a US government experiment revealed.

US researchers who injected carbon dioxide in a depleted oil field in Texas found it caused the minerals underground to dissolve, raising fresh doubts about carbon capture and storage technology as a viable solution to global warming.

Yousif Kharaka, the geochemist who led the experiment, said the 1,600 tonnes of liquid CO2 injected underground changed the acidity of the minerals, causing them to dissolve. This, she said, has environmental implications as the liquid CO2 could then leak into ground water or find its way back into the atmosphere and aggravate the greenhouse effect.

The results of the study, performed in October 2004, were published in the July 2006 edition of journal Geology. Experimental carbon capture and storage (CCS) projects are currently being implemented around the globe. The technology is believed to hold the promise of a future where fossil fuels such as oil and coal can become clean of CO2 emissions, the most important gas held responsible for global warming.

In Europe, the largest project involves the injection of liquid CO2 to force more oil out of a field in the North Sea. The project, which is supported by the governments of Norway and Great Britain, is due to be phased in by 2010 by companies Shell and Statoil.

The EU Commission is due to present a policy paper on carbon capture and storage at the end of 2006 that will address the use of the technology to reduce emissions from all fossil fuels, especially in the coal sector.