The answer is "ominous".
Al Gore made considerable use of metaphors during his Nobel Prize acceptance speech in 2007. One of the most memorable phrases of the speech was: "As a result, the earth has a fever. And the fever is rising." This phrase clearly makes reference to the claim that the Earth is subject to ever-growing temperatures.
In order to give formality to the speech, he makes use of the word "ominous", which is an alternative way to warn that an important event which may have severe consequences is about to occur. In this case, it is an irreversible state of the planet that would put the human species into danger.
