Today's climate is mild.

"Today's climate is mild" is a sick sentence. "Mild weather" is a sick sentence with improper subject-predicate collocation. The word "mild" is used to refer to people's attitude, not the weather, and should be changed to "warm". There are several types of improper collocation, such as improper collocation of verb and object, improper collocation of subject and predicate, improper collocation of part, improper part of speech and so on. The sentence "mild weather" belongs to the type of improper subject-predicate collocation.