How do we know what coral reefs looked like hundreds of years ago? Often times, we are simply left wondering. Scientists can get an idea from naturalist recordings, but there are many unknowns and they are relying on interpreting personal observations. The landscape of coral reefs has certainly changed since 1846, when Joseph Jukes recorded his impressions of the Great Barrier Reef, and there is no way of knowing how many species he saw that might have gone extinct, never to be recorded.
This phenomena, where the current state of an ecosystem or species is assumed to be the same as that in the past, is referred to as a "shifting baseline." Scientists are studying the past, through radioactive dating, historical and fossil records, to get a better idea of what these places looked like hundreds, or even thousands of years ago. Watch this video from CoralWatch to learn more.