A new study indicates the Asian southwest monsoon, which affects the livelihood of millions of people, appears to have increased in intensity during the last four centuries, perhaps as a result of warming in the Northern Hemisphere.
Researcher David M. Anderson of the University of Colorado at Boulder and the National Oceanic and Atmospheric Administration said sediment cores from the Arabian Sea near Oman indicate the wind strength of the monsoon has increased as the Northern Hemisphere has warmed. Anderson and his colleagues hypothesize the southwest monsoon strength will continue to increase during the coming century as greenhouse gases rise and northern latitudes continue to warm.
"Our goal is to understand the natural variability of the climate in hopes of determining whether we are influencing the climate of the planet," said Anderson, a research associate at CU-Boulder's Institute of Arctic and Alpine Research. "Although there are several scenarios as to why the monsoon is increasing, what we are seeing is consistent with the effects of global warming."
A paper by Anderson, the University of Arizona's Jonathan Overpeck and the Indian Institute of Technology's Anil Gupta will appear in the July 26 issue of Science.
Roughly 65 percent of the world's people live in monsoon regions, said Professor Peter Webster of CU-Boulder's Program for Oceanic and Atmospheric Sciences, who was not involved in the study. Because people in monsoon regions are largely dependent on agriculture, too little rain during weak monsoons can cause significant crop failures while too much rain during extremely strong monsoons can cause flooding, crop damage and population displacement.
A monsoon is the seasonal reversal of wind fields that causes dramatic increases in moisture during the warm season, said Anderson, also a NOAA researcher. The two key ingredients to a monsoon are a hot land mass and a cooler ocean. In the case of the Asian southwest monsoon, the key sources are the Tibetan Plateau--which Anderson likened to an "empty frying pan on the stove"--and the Indian Ocean to the south.
The winds blowing along the coast of the Arabian Sea during the monsoon cause upwelling of the water, which dramatically increases the abundance of a tiny marine organism known as G. bulloides. The stronger the monsoon, the more G. bulloides are produced. Over time, a unique fossil record of the species has built up on the sea floor, he said.
The scientists sampled sea sediment cores containing the fossils in 2-millimeter layers from along the coast of Oman and dated them with radiocarbon techniques. The results have provided valuable data on wind speeds during the summer southwest Asian monsoon going back more than 1,000 years.
"We have seen a substantial century-to-century variation on these fossil layers," said Anderson. From 1200 to 1400, for example, the monsoon was strong. The sediments showed the monsoons to be weaker during the 1600s, when a period of cooling in Europe known as the Little Ice Age is believed to have occurred, he said.æ
"But the monsoon wind strength has increased during the past four centuries as the Northern Hemisphere has warmed," Anderson said.
There is an apparent link between decreased Eurasian snow cover and increasing monsoon strength, he said. Alternately, climate-forcing events such as greenhouse gases, volcanic aerosols and solar output may be increasing the strength of the monsoon.
"Either interpretation is consistent with the hypothesis that the southwestern monsoon strength will increase during the coming century as greenhouse gas concentrations continue to rise and northern latitudes continue to warm," the researchers wrote in Science.