"Dessen" is basically the answer to "Wessen?", just like "Das." is the answer to "Was?", to put it simple. If you have a Genitiv-case, for example "Das ist das Buch des Lehrers.", you ask "Wessen Buch ist das?" and the answer would be "Des Lehrers." The question to your sentence "Dessen war sich die Politikerin nicht bewusst." would be "Wessen war sich die Politikerin nicht bewusst?" - answer: "Dessen." Here, "dessen" is referring back to the first sentence you wrote "Die Ökologin ist pessimistisch." If you say, for example, "She is sad. I didn't even know that." then "that" is referring back to "She is sad". It's basically the same here.