Asian monsoons have been intensifying over the last 400 years, and they’re slated to get worse, a team of earth scientists says. Stronger monsoon rains could cause severe flooding and erosion that would affect up to half the world’s population.
The South Asian monsoon carries much-needed rain to billions of people in India, China, Bangladesh, and other countries. The monsoon season begins in summer when northeast trade winds reverse direction and carry water-saturated air inland.
“The South Asian monsoon . . . is key to agriculture and water resources,” comments Gerald A. Meehl at the National Center for Atmospheric Research in Boulder, Colo. “The past and future behavior of the monsoon is therefore of critical interest.”
Researchers have chronicled changes in monsoon intensity over tens of thousands of years, but few have examined variations on the smaller time scales relevant to human civilization. Now, researchers in the United States and India have used the fossil record to piece together variations in monsoon strength over the past millennium.
For evidence on monsoons, David M. Anderson of the University of Colorado in Boulder and his colleagues looked to a seeming unrelated subject: the microscopic, hard-shelled foraminifer Globigerina bulloides in sediments of the Arabian Sea.
As “a happy side effect” of the Asian monsoon, winds blow along the coasts of Saudi Arabia and Oman, says Anderson. These winds churn up deep waters and transport minerals to the otherwise nutrient-poor surface waters. In years when monsoon winds are strong, shallow-living G. bulloides undergoes a population boom and abundant shells end up in sediments below.
The researchers took 100-millimeter-deep sediment cores and separated each into 2-mm layers that they carbon dated and examined for G. bulloides. The team accumulated a 1,000-year record on the fossils’ abundance–and, therefore, monsoon intensity.
The results show that following a low in monsoon wind intensity around the year 1600, there has been a steady increase. The abundances of G. bulloides remains suggest a more marked increase in monsoon winds during the past 100 years, which the researchers attribute to global warming. The findings are detailed in the July 26 Science.
In Asia, global warming may create a greater summertime disparity between land and ocean temperatures, says Anderson. This, in turn, would increase monsoon intensity, he says.
Most climate-change studies measure surface temperature. “This study provides additional evidence of anthropogenic climate change,” comments Meehl, who is not a member of the research team.
Increased monsoon intensity might mean fewer crop failures, says Meehl, but it could create more flooding and erosion that would damage the livelihood of millions, as recent flooding in Bangladesh did.