The early Mission buildings here were built out of adobe bricks. Local mud, straw, and water were mixed and slapped into forms. After drying in the sun, they were placed into walls and cemented together with more adobe mud. According to Wikipedia, Adobe buildings were very long lasting in dry climates.
The weather mavens all tell us that we are about to become a very wet climate. The El Ninos are coming, they say, and this year they will be the worst ever. Many historic buildings in California are threatened. They will wash away back to mud and straw….something no one wants to see.
Other sites around the world are also threatened. Chan Chan, a World Heritage Site in Peru, has made arrangements with “Freedom National Culture Institute of Peru with contributions from the World Heritage Foundation - WHR, ICCROM, and GCI. The plan was approved by the Peruvian Government.”
As far as my tentative research has explored, there is no comprehensive plan for the adobe Missions of Baja and Alta California. While we all are being urged to buy flood insurance, having our water rates raised, and told to stock up on foods and water, the string of historic California missions is threatened with melting.