In the present climate about half the atmospheric greenhouse effect comes from water vapor, about a quarter from clouds, and a fifth from carbon dioxide. According to Vallis. these numbers are necessarily) (i)_________because the effects of the greenhouse gases are not always (ii)_________. If the atmosphere is dry. then adding CO^ makes a big difference to the greenhouse effect, but if there is a large amount of water vapor in the atmosphere already producing a greenhouse effect, then adding CO (iii)_________.
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