Since it's put on to prevent medical conditions, are there recognized halachic authorities who say that applying sunscreen (a cream NOT lotion) is muttar (permitted) on shabbos?
There are two points which must be addressed:
1) Is cream permitted to be applied on shabbos for medical reasons? See here:
The Gemara (Shabbos 146) writes that it is forbidden miderabanan to spread oil, as it is similar to the act of memarayach, which is forbidden mideoraisa (See Rambam, Shabbos 23:11). R’ Ribiat (The 39 Melochos p919) writes that this applies to hand creams such as Nivea and Vaseline, etc. One may, however, use a liquid hand lotion (which can be poured). R’ Dr. Avraham Avraham writes (Nishmat Avraham 1 OC 328:22b) that both R’ Shlomo Zalman Auerbach (quoted in Shemiras Shabbos Kehilchasa 33:n58) and R’ Eliezer Waldenberg (Tzitz Eliezer 7:30:2) held that while spreading an ointment evenly on one’s skin is forbidden mideoraisa, this is only if it remains on the surface. One may, however, rub cream into the skin if it gets properly absorbed (See Daas Torah 328:26; Minchas Yitzchak 7:20). As one shouldn’t take medicine on Shabbos, this doesn’t apply to medicinal creams, though a bedridden patient (choleh shein bo sakanah) may rub such cream in. The same applies to young children. For creams that are not absorbed, one may press cream (from the back of a spoon, etc.) providing one does not rub the cream in. One is allowed to rub off excess ointments.
2) Is sunscreen cream considered a medical reason? See here:
One cannot justify using sunscreen due to an acute medical need if the need arises from one’s desire to sunbathe. It is permitted to spray liquid sunscreen (Orchot Shabbat 17:20.). This is not considered medicinal because its purpose is not to strengthen the body but to shield one from external injury (see Shulchan Aruch, OC 328:27, which permits covering a healing wound with a bandage).