The world population is rising so quickly that the world has

ecome 是表示变化的连系动词,意思是“变得”,后接的crowded 是过去分词作形容词用.世界人口正在急剧增长,世界已经变得过分拥挤了.说话的时间立足点是现在,根据这个意思就必须使用现在完成时态.所以有了这个句子：The world population is rising so quickly that the world has become too crowded.
我们也可以说：世界人口正在急剧增长,世界正在变得越来越拥挤了.说话的时间立足点还是现在,根据这个意思我们就必须使用现在进行时态：The world population is rising so quickly that the world is becoming more and more crowded / is becoming increasingly crowded.