Over-grazing was responsible for 11.3% of degradation in Oceania,5.5% in Europe and 1.5% in North America, which, when combined, contributed to a larger proportion of land degradation than the other two factors.
这里是which引的非限定性定语从句，所以前面要加逗号。同时由于从定语从句中有前置状语，所以后面也要加逗号隔开，使句子更加清晰。追问
前面那个逗号我明白，后面的意思是which和when连用要用逗号隔开？
追答对，得分开
温馨提示：答案为网友推荐，仅供参考
