这还只是美国。气象学家建立的模型发现，如果喷发是在夏季，释放出来的气雾可能会扩散到全球。在短期内，由于有毒云团阻挡阳光，全球平均气温可能会大幅下降，并在数年内无法恢复正常。降雨量将急剧下降。这可能足以引发热带雨林灭绝。从中西部开始，农业可能会崩溃。一组研究人员在 ...

The Yellowstone wolf project has been a valuable experiment to help biologists decide whether to reintroduce wolves to other parts of the country as well. 28.What is the text mainly about?