单选题 .  ①As of late 1980s, neither theorists nor large-scale computer climate models could accurately predict whether cloud systems would help or hurt a warming globe. ②Some studies suggested that a four percent increase in stratocumulus clouds over the ocean could compensate for a doubling in atmospheric carbon dioxide, preventing a potentially disastrous planetwide temperature increase. ③On the other hand, an increase in cirrus clouds could increase global warming.
    ①That clouds represented the weakest element in climate models was illustrated by a study of fourteen such models. ②Comparing climate forecasts for a world with double the current amount of carbon dioxide, researchers found that the models agreed quite well if clouds were not included. ③But when clouds were incorporated, a wide range of forecasts was produced. ④With such discrepancies plaguing the models, scientists could not easily predict how quickly the world's climate would change, nor could they tell which regions would face dustier droughts or deadlier monsoons.16.  The author of the passage is primarily concerned with ______