U. PITTSBURGH (US) — A sediment core from a lake in the Andes Mountains suggests that as temperatures rise, summer monsoons will lessen, causing densely populated tropical regions to experience severe water shortages.
Equatorial regions of South America are already receiving less rainfall than at any point in the past millennium, according to a new study published in the Proceedings of the National Academy of Sciences.
The nearly 6-foot-long sediment core from Laguna Pumacocha in Peru, containing the most detailed geochemical record of tropical climate fluctuations yet uncovered shows pronounced dry and wet phases of the South American summer monsoons and corresponds with existing geological data of precipitation changes in the surrounding regions.
Paired with these sources, the sediment record illustrates 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,” says Mark Abbott, professor of geology and planetary science at University of Pittsburgh.
“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, correlating with the demise of regional Native American populations, 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.
To create a climate record from the sediment core, the team analyzed the ratio of the oxygen isotope delta-O-18 in each annual layer of lake-bed mud. This ratio has a negative relationship with rainfall: Levels of delta-O-18 are low during the wetter seasons and high when monsoon rain is light. The team found that the rainfall history suggested by the lake core matched that established by delta-O-18 analyses from Cascayunga Cave in the Peruvian lowlands and the Quelccaya Ice Cap located high in the Andes.
The Pumacocha core followed the climatological narrative of these sources between the years 980 and 2006, but provided much more detail, Abbott says.
A connection was then established between rainfall and Northern Hemisphere temperatures by comparing their core to the movement of the Intertropical Convergence Zone (ITCZ), a balmy strip of thunderstorms near the equator where winds from the Northern and Southern Hemispheres meet.
Warm Northern temperatures such as those currently recorded lure the ITCZ—the main source of monsoons—north and ultimately reduce the rainfall on which tropical areas rely.
The historical presence of the ITCZ has been gauged by measuring the titanium concentrations of sea sediment. High levels of titanium in the Cariaco Basin north of Venezuela show that the ITCZ lingered in the upper climes at the same time the South American monsoon was at its driest, between 900 and 1100.
On the other hand, the wettest period at Pumacocha—between 1400 and 1820, which coincided with the Little Ice Age—correlates with the ITCZ’s sojourn to far south of the equator as Northern Hemisphere temperatures cooled.
Researchers from Union College and the State University of New York at Albany contributed to the study.
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