Crutzen has suggested that the Antropocene began in the late 18th century, when, ice cores show, carbon dioxide levels began what has since proved to be an interrupted rise.

1 本句主句谓语has suggested意为“暗示、表明”，而不是“建议”，所以其后that从句用陈述语气谓语。

2 when从句不是时间状语从句，而是非限制性定语从句，修饰the late 18th century. when=during which time.

3 when从句的内容是研究者后来研究冰层推测出来的，而不是当时测量的，所以说话人以插入语的形式，插入ice cores show, 对此作补充说明。

4 what has since proved to be an interrupted rise为名词性关系分句（名词性从句）作began的宾语。what为融合型关系代词=the thing which.