The South Asian monsoon provides the drinking water for 1.5 billion people each year. It brings more than two-thirds of India's rainfall and accounts for more than half of the water that Indian farmers use to grow crops.
That means a lot of people want to know how much rain the monsoon will bring. Just a ten percent increase causes major floods, while a 10 percent decrease causes drought.
Konrad Hughen, a senior scientist in Marine Chemistry and Geochemistry at Woods Hole Oceanographic Institution, is one paleoclimatologist trying to unlock the future of the monsoon.
To do this, Hughen is using core samples he took from two boulder corals in the Red Sea that date back to 1760. It turns out, corals act like climate archives by creating visible bands in their skeleton, similar to tree rings.
“Because of their annual banding, that gives us good dating,” Hughen told WCAI. “Because they live for many centuries, they’re like meteorological stations in the ocean recording data continuously. All of those records are out there, just waiting to be read.”
Temperature, salinity, and river runoff can be derived from the coral record. So can the strength of the monsoon winds.
How is that possible? Barium is concentrated in soils and sediments on land, but the ocean is relatively empty of the element. So, river runoff and winds bring a large barium “signal” in the coral skeleton.
The dust blowing off the desert in Sudan into the Red Sea is strongly correlated with the strength of the Asian monsoon.
What do the corals say?
The coral record shows that, over the past 250 years, the monsoon winds have been getting stronger and less variable. The data suggests that there should be more rainfall during the Asian monsoon. But when scientists look at instrumental records across India over the last 50 years, they see that rainfall has been decreasing.
“So, the question becomes why has monsoon rainfall been decreasing when it should be increasing?” Hughen said.
One answer could be an increase in soot and sulfates in the air over India over the past 50 years. This has the tendency to shade the surface and cool the surface. It’s possible that this is counter-acting some of the effects of warming temperatures and stronger winds.
No one is certain of the reason of this mismatch between what scientists expect and what is actually happening with the monsoon rainfall. But Hughen says corals are a good way to keep looking back in the climate record for hints to the future.
“Clearly the value of corals as recorders of past climate change is high,” he said. “And corals should be preserved as much as possible for this reason, as well as many others.”