Thursday, February 28, 2008

Monsoon intensity driven by Earth's orbit, says a study that uses a new measuring technique

A little bit down in this story from Terra Daily (via Agence France-Presse) is a mention of new tool for paleoclimatology – measuring oxygen isotope ratios in stalagmites: The monsoon rains that drench tropical and subtropical Asia from June through September vary in duration and intensity in keeping with tiny wobbles in Earth's orbit as it circles the Sun, according to a study released Wednesday. These cycles wax and wane every 23,000 years, said the study, based on the breakthrough use of stalagmites from a cave in central-eastern China to measure changes in climate patterns over the last quarter million years.

"The implications are that the present Asian summer monsoon is relatively weak in comparison to a few thousand years ago and that is will stay at this level for centuries more," lead researcher Hai Cheng of Nanjing Normal University in China's Jiangsu Province told AFP. The findings, however, do not take into account the relatively recent impact of greenhouse gas-driven global warming, which climate scientists predict could significant alter monsoon patterns.

Three irregularities in the movement of Earth -- its orbit, the angle at which it is tilted, and the axis of rotation -- all combine to create a periodic variation in the amount of incoming solar radiation, explained Cheng. It is this so-called precessional cycle that is largely responsible for longterm changes in monsoon duration and strength, the researchers found.

Monsoons occur with the seasonal reversals of wind directions caused by temperature differences between the land and sea. While found elsewhere in the world, they are most pronounced in Asia in part due to the impact of the massive Tibetan Plateau.

Economies in tropical and sub-tropical Asia, especially around the Indian subcontinent, depend on monsoon rainfall to grow crops on land that is largely unirrigated. But heavy monsoons can also bring massive flooding, causing severe economic damage and loss of life.

As significant as the findings, arguably, are the methods used to collect them. Cheng and his colleagues measured the oxygen isotope ratios locked in the stalagmites built up from the floor of the Sanbao Cave to determine changes in climate over millennia, said the study, published in the British journal Nature. Compared with other commonly used proxies of paleoclimatology such as tree-rings and ice cores, speleothems -- as these mineral deposits are called -- provide a record over a much longer timescale.

This technique "will likely replace the Greenland ice records as the chronological benchmark for correlating and calibrating climate variability," said Cheng. It also allows for a new level of precision, achieved by measuring the growth of the isotope thorium-230 from the slow radioactive decay of uranium, found in trace amounts in the deposits.

"What emerges is a record of monsoon variation unprecedented in its detail and chronology stretching back 224,000 years," said Jonathan Overpeck and Julia Cole, both geologists at the University of Arizona, in a commentary, also in Nature. The word "monsoon" is thought to have originated from the Arabic word "mausim," which means season.

Cave vulcanos on the floor of Gruta da Torrinha, Chapada Diamantina, Bahia, Brasil, by Renata Shibuta Marques, Wikimedia Commons

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