如下列句子：

Rather, they contend, it was almost certainly the result of climatic shifts, prompted in part by cumulative effects of the el nino/la nina phenomenon on Pacific Ocean temperatures, culminating in a harsh winter in which huge numbers of salmon eggs were frozen.

——culminating in a harsh winter，如何确定culminating的逻辑主语是climatic shifts 还是cumulative effects呢？虽然从句意来说是后者，那能从语法上来判断吗，谢谢，（想知道为什么就不能是climatic shifts呢）