Emergent patterns in nature and society

Wet and dry monsoon in South America

Researchers have found evidence of the two-mode South American monsoon by studying lake sediments from Laguna Pumacocha in Peru. They discovered that the monsoon can have dry and wet regimes. In addition, their data suggest that the monsoon is shifting towards its dry mode given the fact that precipitation has sharply drop during the last century. I quote from EurekAlert:

A 2,300-year climate record University of Pittsburgh researchers recovered from an Andes Mountains lake reveals that as temperatures in the Northern Hemisphere rise, the planet’s densely populated tropical regions will most likely experience severe water shortages as the crucial summer monsoons become drier. The Pitt team found that equatorial regions of South America already are receiving less rainfall than at any point in the past millennium.

[…]

the sediment record illustrated that rainfall during the South American summer monsoon has dropped sharply since 1900—exhibiting the greatest shift in precipitation since around 300 BCE—while the Northern Hemisphere has experienced warmer temperatures.

“This model suggests that tropical regions are dry to a point we would not have predicted,” Abbott said. “If the monsoons that are so critical to the water supply in tropical areas continue to diminish at this pace, it will have devastating implications for the water resources of a huge swath of the planet.”

The sediment core shows regular fluctuations in rainfall from 300 BCE to 900 CE, with notably heavy precipitation around 550. Beginning in 900, however, a severe drought set in for the next three centuries, with the driest period falling between 1000 and 1040. This period correlates with the well-known demise of regional Native American populations, Abbott explained, including the Tiwanaku and Wari that inhabited present-day Boliva, Chile, and Peru.

After 1300, monsoons increasingly drenched the South American tropics. The wettest period of the past 2,300 years lasted from roughly 1500 to the 1750s during the time span known as the Little Ice Age, a period of cooler global temperatures. Around 1820, a dry cycle crept in briefly, but quickly gave way to a wet phase before the rain began waning again in 1900. By July 2007, when the sediment core was collected, there had been a steep, steady increase in dry conditions to a high point not surpassed since 1000.

If you want to follow up the paper, it was recently published in PNAS: A 2,300-year-long annually resolved record of the South American Summer monsoon from the Peruvian Andes by Bird and colleagues.

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