The left believes that climate change causes terrorism, the reason on the surface is fairly simple. First of all, the left believes climate change is having a tangible impact on the world and that whatever that impact is, it is a bad impact. And it doesn't really matter for our purposes here how climate change comes about, it only matters to the left that there is climate change and that climate change has some sort of negative impact. That negative impact manifests itself, in the mind of a leftist, in a reduction in the overall amount of resources available to the world. Because climate change, supposedly, causes drought, changing weather patterns and etc. this results in fewer of the limited resources available to the people of the world.
Terrorism mostly originates from poorer countries and the reason for this, according to Sanders and the left, is because of the limited resources. But it is not because there are fewer resources that they resort to terrorism, but that western society takes more than its fair share of the world's dwindling limited resources that people in the poorer countries feel the need to resort to terrorism. To someone on the left this sounds like a perfectly rational explanation of terrorism. To someone on the right, not so much.
It can be easy to get mixed up in the idea of limited resources. If you are an American Capitalism (a republican) then you do not see the world in light of limited resources, in fact you do not even believe there are limited resources. Of course there are technically limited resources in that the universe only has so much matter. Thus resources are limited to that amount of matter. Functionally, however, resources, in the mind of an American Capitalist, are unlimited.
It is all about mindset. The mindset of the right is to look at a problem and ask, what can we make/do/develop to solve that problem. The mindset of the left is to look at a problem and ask, how can we make these people have an equal amount of stuff as everyone else. The left is not concerned with producing more stuff and that is why they view the world through the lens of limited resources. The left wants to distribute. The right is concerned with how much stuff people have, they view the world through the lens of wanting people, all people, to increase their quality of life (or whatever metric you want to use to indicate the quality of people's lives).
People on the right cannot believe climate change causes terrorism because the right does not believe in limited resources. It is not climate change that affects the amount of resources we have, but production. It is not climate change that causes poor countries to be poor but a lack of production that causes them to be poor. And whether or not being poor drives people to terrorism is speculative at best because there are plenty of poor countries that do not breed terrorists like the islamic countries of the middle east do.