The research has shown that four gases were not around in the atmosphere at all until the 1960s, which suggests they are man-made. The scientists discovered the gases by analyzing polar snow pack. Air from this snow is a natural archive of what was in the atmosphere up t0 100 years ago. The researchers also looked at modern air samples, collected at remote Cape Grim in Tasmania.

　　They estimate that about 74,000 tonnes of these gases have been released into the atmosphere. Two of the gases are accumulating at significant rates. However, they don't know where the new gases are being released from and this should be investigated. Possible sources include chemicals

　　for insecticide（杀虫剂） production and solvents（溶液）

　　for cleaning electronic components. The three CFCs are being destroyed very slowly in the atmosphere - so even if emissions（散发） were to stop immediately, they will still be around for many decades to come.

　　Of the four species identified, CFC-113a seems the most worrying as there is a very small but growing emission source somewhere, maybe from agricultural insecticides. We should find it and take it out of production.

　　63. What do we know about the newly discovered gases?

　　A. Some are surely produced by the development of agriculture.

　　B. The CFCs will have a long impact once they are released.

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