This is very interesting cartoon,in which two sharks are celebrating the good news that thanks to global warming， the sea water will drown nearby island in five years where a man is fishing.
问题1 in which 是定语从句吗？in 是从哪里来的，
如何恢复到陈述句
2 thanks to 前省略的主语是什么。是介词短语吗？有何语法点

首先，“This is very interesting cartoon”里差一个不定冠词a，应为This is a very interesting cartoon。nearby island之前加a。
问题1：是修饰cartoon的定语从句，which指代cartoon，在动画片里，故需用介词in 。该句可改写为：This is a very interesting cartoon。In the cartoon two sharks are celebrating the good news that the sea water will drown nearby island in five years where a man is fishing, thanks to global warming.

问题2：thanks to前并未省略任何内容，它是介词短语，常在句子中作状语成分。意思是“多亏、幸亏、由于”。例如：Thanks to your explanation, it's clear to me now.意为：多亏了你的解释，我现在清楚了。
题中该句大意：这是一部很有趣的动画片，里面讲述了两只正在庆祝好消息的鲨鱼的故事。它们的好消息就是：附近的有人正在上面钓鱼的一个岛屿，将在5年后由于全球变暖而被海水淹没。