This year, heat waves and droughts threaten health and crops in the United States, while heavy rains and floods savage Asia. Easterly trade winds normally blow ocean water in the opposite direction, but when these tropical winds slacken, the piled-up water sloshes back towards South America. The displaced warmth it carries with it changes the circulation of the tropical Pacific as well as air flow over a large part of Earth. The last change shows up clearly in fossil corals that lived 124,000 years ago near the present location of Sulawesi Island, Indonesia. When conditions are dry, oxygen 16 evaporates more quickly from the water, so the heavier oxygen 18 dominates in the coral skeleton.
Corals build their skeletons in annual rings, like tree rings.
These rings can be dated, and their oxygen ratios reveal how rainfall varied from year to year. Despite the time gap, which includes an ice age about 60,000 years long, the records appear strikingly similar. You can't tell one from the other until 1976. During normal winters, cold water wells up from the depths of the eastern Pacific, and this upwelling leaves its mark on the corals. This record reveals a sudden shift in 1976.