It may be that changes to reefs that are causing concern are simply natural variations. Two divers descend on a very large Porites coral on the Great Barrier Reef. There are biological and geological systems which leave a record of both their responses to their environment and the time at which those responses occurred. For example, annual rings are formed in trees growing in temperate and sub-polar regions. The width of an individual ring records how well the tree grew in that year.
Wet years are good years for trees growing in arid regions.
Warm years are good years for trees growing high on mountains. Recovery of environmental information from trees is now so sophisticated and advanced that it has become the science of dendroclimatology. None of these gives good information about tropical ocean regions. A source of proxy information for such regions would fill a large gap in the existing proxy records. Annual density bands in coral skeletons have promised to be the tropical marine proxy record since their discovery 20 years ago.