Impossible to answer? It's actually quite easy to answer.
A rational person would not believe in the Global Warming religion because Global Warming theology is in direct contradiction to logic (as it is based upon a buzzword ('global warming') that warmizombies and climate lemmings will not and cannot unambiguously define), science (as it rejects numerous laws of science, typically the laws of thermodynamics and the stefan boltzmann law, without first falsifying those laws), and mathematics (as it rejects the rules of statistical mathematics, including but definitely not limited to: unambiguously defining boundaries, declaring a desired margin of error at the outset, removing all known biases, using raw data, declaring and justifying a variance value, and calculating the margin of error).
So back to the question in the OP: Why should any rational adult believe as warmizombies and climate lemmings believe?
Answer: They shouldn't, as the Global Warming religious dogma irrationally rejects logic, science, and mathematics. Rational adults do not willingly reject such things.