Every year new generations of corals build their homes on the stony remnants of their ancestors. Pulling calcium—or strontium—from the surrounding seawater, they form calcium carbonate shells that give coral reefs their unique shapes and structures. But, much like tree rings, these coral reefs also lay down layer after layer of environmental records, a catalogue of the sea's temperature and other information. By studying a record of such coral cores stretching back roughly 6,500 years, scientists at the Australian National University have pieced together how climate change may affect the weather in their region.

The ratio of strontium to calcium in a given layer of coral reef—as well as the amount of a heavier isotope of oxygen in the carbonate itself—reflect the temperature in this historical record, but the isotopic information also reveals rainfall. "When you measure the two of them together, you can use the strontium-calcium ratio to take away the temperature and that gives you a clear rainfall signal," explains paleoclimatologist Nerilie Abrams of the British Antarctic Survey. Looking at this rainfall signal, Abrams and her colleagues were able to determine historical climate patterns in the Indian Ocean region.

These coral proxies—drilled out of still-living reefs as well as those that had been uplifted by earthquakes in the Mentawai Islands of western Indonesia—closely matched more recent historical measures, noting strong droughts in 1961, 1994 and 1997. Each of these monitored droughts are linked to oscillations in the so-called Indian Ocean Dipole—the El Nio of that region, in which the normally warm eastern ocean becomes quite cold and the western ocean, near Africa, warms.

Looking further back in the coral record, the researchers found that this pattern held up over thousands of years. "Every time we had one of these coolings, there has been a very strong drought in western Indonesia to accompany it," Abrams notes. This was particularly true at more distant dates as the Indian monsoons were stronger, thanks to more sunlight hitting Earth due to orbital variations.

At present, such Indian Ocean Dipole events are typically cut off by the end of the monsoon season, as the monsoon winds die down so too does the cooling near the coast of Sumatra. But in the past, such drought-spawning events actually occurred during the monsoon season; "the drought peaks at the time when normally this area is receiving its maximum month of rainfall," Abrams says.

This could mean that the stronger monsoons expected as a result of global warming will lead to stronger droughts in Indonesia and Australia. And, of course, the Indian Ocean Dipole's interaction with the actual El Nio in the Pacific, which starts off the coast of Indonesia, could further exacerbate this effect. "Longer droughts will have many additional social and economic consequences, for example on food supply, health and hydropower," climatologist Jonathan Overpeck writes in a commentary on the research published in Nature January 18. "Indonesia is also a pivotal biodiversity hot spot and drought intensification could bring significant challenges to conservation management."

It remains too soon to tell exactly how this climate system will work under changed conditions and other environmental factors—such as whether the cooling effect of the soot generated by industry and burning forests outweighs the warming effect of greenhouse gases—which may play large roles. But the corals provide clues from the planet's past. "There is a lot more we can learn from the corals," Abrams adds. "It's a matter of analyzing them."