A Stanford geophysicist, Mark Zoback, has warned that "underground storage of carbon dioxide may trigger earthquakes which could allow the gas to seep back into the atmosphere, rendering the emissions mitigation approach ineffective."
The problem is that burning coal produces so much carbon dioxide that the volume is too great to be stored safely underground.
For carbon sequestration to make a significant contribution, massive volumes of the gas would need to be injected into the ground at thousands of sites around the world. According to Professor Zoback, injecting such massive amounts of carbon dioxide would inevitably trigger faults and cause small earthquakes. These would create pathways for the carbon dioxide to escape and render the whole process pointless.