 We use oxygen isotopes to work out how the rainfall has changed but also how temperature has changed over the last however many years. And oxygen isotopes work because as the coral sits in its little water column it sits in the surrounding water and as the temperature changes the more water evaporates off the top and this changes the oxygen concentration in the water. So as temperature increases you have more evaporation. This influences the stable oxygen isotopes in the water. Oxygen has two isotopes it has a heavy isotope which is oxygen 18 and a lighter isotope oxygen 16. And so when that evaporation occurs it evaporates off that lower oxygen isotope preferentially so first and it'll leave the heavier isotope in that water column. And same when it rains so when it rains it'll rain the lighter oxygen isotope first and then that heavier oxygen isotope. And this is what we read so as oxygen isotopes drop we have a higher temperature and vice versa and it works the same way for rainfall so if we have a higher rate of rainfall there's more water in that water column and we have our oxygen isotopes change through that.