Existentialism may be reduced to the philosophy of the late Jean Paul Sartre. It does not mean empiricism or existialism. Existentialism is a personal point of view of experience. That is why the philosopher wrote a sequel to Being and Nothingness named The Critique of Dialectical Reason. That book describes what society or ‘others’ seem to the observer to be doing within one’s existentialist epistemology.
Being and Nothingness is an epistemological work in my opinion, though it usually is regarded as ontology. At any rate, global warming isn’t ontology. One cannot even infer deontological duties from the atmosphere regardless of its temperatures. Being and Nothingness is not a political book or political or even scientific philosophy. Politicians and people in government like to use the term ‘existential threat’ because they have never read Being and Nothingness and have no idea what non-sense they are putting out.