One school of thought believes that the planet is warmingwithout humans doing anything to make it happen.

The reflexive pronoun ‘itself’ is missing in the given sentence. Reflexive pronouns reflect the action back onto the subject. A reflexive pronoun should be used with a transitive verb and it can never be used as a subject in a sentence. Hence ‘itself’ should be used after ‘warming’ in order to make the sentence grammatically correct.