The Pontifical Academy of Sciences accepts for its members anyone who is an expert in his or her respective field, regardless of his or her religious beliefs or lack thereof:
The new members of the Academy are elected by the body of Academicians and chosen from men and women of every race and religion based on the high scientific value of their activities and their high moral profile. They are then officially appointed by the Roman Pontiff.
In the case of Hans Schellnhuber, he is "one of the leading climate scientists worldwide" and "has published more than 250 scientific papers and authored, co-authored or edited 50 books or book chapters."