More than half the world's population depends on the Asian monsoon to bring much needed moisture for agriculture and basic human needs.
But the yearly rains can also bring peril. Surging flood waters from last summer's monsoon killed more than 800 people in India, Nepal, and Bangladesh, displacing millions of others.
Scientists have observed that the Asian monsoon has been gaining strength during the past few centuries, possibly due to rising global temperatures. The Intergovernmental Panel on Climate Change concluded in 1994 that global warming could intensify the monsoon and increase monsoon variability.
But recent research showing a link between the climate of the North Atlantic Ocean and Asian monsoon suggests that abrupt climate changes could actually sap the strength of future monsoons, according to a University of Arizona geoscientist.
"If the North Atlantic should cool without warning, as some scientists suggest it might, one of the results could be a weakened monsoon and less water for all the people that depend on it," says Jonathan Overpeck, a professor of geosciences and director of the UA Institute for the Study of Planet Earth.
Overpeck, along with Anil K. Gupta from the Indian Institute of Technology in Kharagpur and David M. Anderson from the National Oceanic and Atmospheric Administration's Paleoclimatology Program in Boulder, Colo., used sediments from the floor of the Arabian sea near Oman to reconstruct monsoon strength in the region for the past 10,000 years. Their work appears in the January 23 issue of Nature.
The researchers used fossils of the plankton G. bulloides to estimate wind intensity. During a monsoon, the seasonal reversal of winds brings moisture from the ocean onto land. The winds also blow surface waters off shore, Overpeck says, causing an upwelling of colder, nutrient-rich water where the microscopic marine animals can thrive.
By counting the levels G. bulloides present in different layers of the sediment and using radiocarbon dating, the scientists were able to approximate monsoon strength from 10,500 years ago on up to the present. The resulting record showed a natural variation in the monsoon from one century to the next.
"We have new evidence that the strength of Asian monsoon varies substantially on century to millennial time scales," Overpeck says. "We need to understand this if we're going to ensure human and ecological sustainability in the Tibet, China, India, and the rest of Southeast Asia.
Earlier studies by Overpeck and other scientists on the last ice age-- 80,000 to 10,000 years ago--suggested a possible link between monsoon variations and changes in North Atlantic climate.
To see if this same link persisted in the current interglacial period, the researchers compared the Arabian sediment record to iceberg debris that had settled into the floor of the North Atlantic ocean over the same time period. The tiny iron-stained grains found in the sediment provide a record of temperatures in the region spanning from Greenland south to the British Isles.
The records revealed seven intervals of weakened monsoon coinciding with cold spells in the North Atlantic region. The most intense monsoons occurred at times when the North Atlantic was warmest.
"It is satisfying because we now find this same hypothesized link operating through many oscillations in the last 10,000 years," Overpeck says. "But this also leads us to scary implications."
Ocean circulation patterns in the North Atlantic play a key role in global climate, working like a conveyor belt to bring warmth from the tropics to northern latitudes in Europe, Overpeck says.
"Heat and salt drive the circulation of the world's oceans," he explains. "If you somehow disrupt the ability of this conveyor to work, you disrupt the ability to transport heat to the north."
Climate reconstructions show that large influxes of freshwater from melting ice disrupted the conveyor in the past, resulting in rapid cooling of the North Atlantic. One such event, 8,200 years ago, correlates to a weak monsoon period evidenced in the researchers' record.
The Intergovernmental Panel on Climate Change predicts global temperatures will rise 3 to 10 degrees Fahrenheit by the end of the century. This could cause large-scale melting of the Greenland ice sheet--resulting in a surge of freshwater that could possibly slow down North Atlantic circulation. Even without such a dramatic event, Overpeck notes, global warming brings other changes, such as increases in rainfall, that could disrupt circulation patterns.
"The North Atlantic is freshening right now in ways we don't fully understand," Overpeck says. "In the past, this possibility concerned Europeans the most, but now those dependent on monsoon rains also have a good reason to worry."
While researchers aren't sure on the exact causes of the link between the North Atlantic and the Asian monsoon, earlier research showed the amount of snow on the Tibetan plateau may play a critical role, Overpeck says. As the land warms in the spring, the air rises above the land causing a pressure gradient that drives the monsoon.
"More snow on the plateau in spring or early summer uses up all the sun's heating because it has to be melted and evaporated before the land can warm," Overpeck says. "So the more snow you have in winter, the weaker the monsoon the following summer."
The authors speculate that when the North Atlantic is cold, areas downwind like the Tibetan plateau stay cold longer, allowing more snow to persist and setting up a weakened monsoon.
"The monsoon snow-cover link may lead to a stronger or more variable monsoon in the coming century as the northern hemisphere continues to warm faster than the tropics," says Anderson, who is also a research scientist with the Institute of Arctic and Alpine Research at the University of Colorado, Boulder. "This may last until a time when the conveyor abruptly weakens, leading to an abrupt weakening in the monsoon.
Other studies show that changes in the amount of sunlight correlate to variations in both the North Atlantic climate and the Asian monsoon. The researchers aren¥t certain if the sun affects each system directly or if solar radiation influences the North Atlantic circulation, which in turn impacts the monsoon.
"More research is needed to identify the role of solar variability, and the remote influence of the north Atlantic climate," Anderson says.
In an earlier study, the authors found evidence from sediments in the same region showing an increase in monsoon strength in the past 400 years. Their work was published in the July 26, 2002 issue of Science.
"The monsoon has strengthened significantly and in an unprecedented way over the last couple centuries," Overpeck says. "It appears that at least some of this must be the result of human-caused global warming.
While stronger monsoons could bring relief to Asia's dry lowlands, scientists aren't certain where increased rains would fall. Data seems to point to Tibet, Overpeck says. The headwaters for all the major rivers in Asia come from the Tibetan Plateau, he notes, indicating increased potential for flooding if the monsoon continues to strengthen.
"Either way you look at it, global warming could cause real trouble for the monsoon system of Asia," Overpeck says. "This could generate substantial human suffering and political instability."
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