Clearing the Air (2009)

![Figure][1] 

PHOTO CREDIT: JUPITERIMAGES

In 1895, the Great Swedish chemist Svante Arrhenius used basic physical concepts, already well understood at the time, to describe how variations in trace gases in the atmosphere—particularly CO2—should influence the heat budget of Earth. By establishing the connection between higher concentrations of atmospheric CO2 and higher atmospheric temperatures, Arrhenius laid the foundation of modern climate-change research, which is dominated by the understanding that the steadily increasing atmospheric CO2 concentration, caused mostly by fossil-fuel burning, is the main reason why the world's climate is warming so much and so rapidly. If too much CO2 in the air is the cause of our climate dilemma, then it seems obvious that we should slow down, stop, or even reverse its current rise to mitigate the risk of unwanted climate changes. Carbon capture and sequestration, or storage, (CCS) is rapidly becoming a key element in our nascent efforts to minimize the amount of CO2 we emit and perhaps even to regulate the amount of CO2 in the atmosphere.

This special issue aims to elucidate some of the main approaches to CCS and how they might be accomplished. In the Review, Haszeldine (p. [1647][2]) surveys efforts around the world to capture and store CO2 emitted by power plants, discussing both the technological challenges of carbon capture, transport, and storage and the political hurdles that remain to be overcome. Four Perspectives then highlight specific approaches to both CO2 capture and CO2 sequestration. Rochelle (p. [1652][3]) discusses how CO2 can be removed from the flue gas of power plants by the well-established technique of amine scrubbing, an especially important activity given the current reliance on coal. Keith (p. [1654][4]) presents ideas about removing CO2 directly from ambient air, one of the few approaches that offer the possibility of reducing the concentration of CO2 in the atmosphere on a time scale of decades. Orr (p. [1656][5]) discusses the current strategy of choice for sequestering captured CO2 permanently: the storage of CO2 in onshore geologic formations. Finally, Schrag (p. [1658][6]) argues for another promising but rarely discussed way to sequester CO2: through storage in offshore sediments.

In News, Dennis Normile (p. [1642][7]) offers a guide to the carbon cycle, a quick introduction to carbon sources and sinks, and a look back at how atmospheric CO2 has varied over the past 500 million years or so. Robert Service (p. [1644][8]) provides a map of major CCS projects around the world, showing mainly projects already in progress or under construction. Finally, Josh Fenn (p. [1646][9]) profiles two CCS projects in China which, though at early stages, are potentially important in a country with such a surging demand for fossil fuels.

The prospects of CCS are uncertain, but its promise is great. If this special issue has any single most important message, it is that there are abundant reasons to hope that CCS can be implemented effectively.

 [1]: pending:yes
 [2]: /lookup/volpage/325/1647?iss=5948
 [3]: /lookup/volpage/325/1652?iss=5948
 [4]: /lookup/doi/10.1126/science.1175680
 [5]: /lookup/volpage/325/1656?iss=5948
 [6]: /lookup/doi/10.1126/science.1175750
 [7]: /lookup/doi/10.1126/science.325_1642
 [8]: /lookup/doi/10.1126/science.325_1644
 [9]: /lookup/doi/10.1126/science.325_1646