The 200-Year Drought

New high-resolution analysis enables Scripps scientists to track ancient monsoon shift

In 1877, the failure of the Indian monsoon to deliver rains to that country triggered a famine that killed between five and seven million people.

But history shows that abrupt climate change can make things much, much worse. Some 14,700 years ago, the monsoon was effectively shut off for several hundred years as precipitation bands normally tracked over India pushed south of the equator. Rain fell over latitudes predominantly covered by ocean.

A research team led by Jeff Severinghaus, a professor of geosciences at Scripps Institution of Oceanography, UC San Diego measured the ratio between two types of oxygen preserved in ice cores to produce one of the most detailed records of the event yet. Analysis of the ice core records yielded a timeline of climate change over the past 100,000 years, resolved to time scales of 100 to 500 years. In contrast, previous oxygen histories were only accurate to time scales between 10,000 and 20,000 years.

"We can see all these short time scale changes that we couldn't see before," said Severinghaus.

The journal Science published the analysis in its June 12 edition.

By measuring the ratio between two oxygen isotopes, scientists can infer how much photosynthesis took place on land at a given point in history and thus how much vegetation growth occurred. The record gathered by the research team shows a steep decline in the ratio 14,700 years ago that needed about 200 to rise to levels closer to historical averages.

Besides Severinghaus and colleagues at Scripps, researchers from the Desert Research Institute in Reno, Nev. and Oregon State University took part in the analysis, which was funded by the National Science Foundation.

Severinghaus cautioned that the record of the event does not necessarily serve as an indicator of what current human-caused climate trends could bring. It does however underscore the sensitivity to abrupt climate change of one of the world's most societally important weather patterns.

"These findings underscore just how much and how quickly rainfall patterns can change in some of the most vulnerable societies," he said.

—Robert Monroe

July/August 2009