I'm not sure that the argument presented here justifies being called an "extinction-level event". Displacement of hundreds of millions of people, sure, but how

I'm not sure that the argument presented here justifies being called an "extinction-level event". Displacement of hundreds of millions of people, sure, but how are melting glaciers and rising sea levels alone going to drive us extinct? Wouldn't desertification be more of a concern, especially in tropical regions of Asia, America, and Africa?