Monsoon Intensity Linked to Northern Climate

This helps provide a longerhistory of climate shifts that took place in the tropics and some sense of theduration.  The follow up is to complete asimilar mapping to cover the whole of the regions indicated in much finerresolution.  Even better, is to try toavoid conclusions until we have such data.

What is clear is that the variationis comparable to today’s extremes without I assume any reliance on humanactivity although that may turn out to be wrong even back one thousand years.  At most though, we simply do not know.

It is pleasing to see that detailis been slowly provided and a 2500 year span is meaningful whereas a fivehundred year span is only enough to pick up a single trend or two.

Climate Record Suggesting Severe Tropical Droughts asNorthern Temperatures Rise

by Staff Writers

Pittsburgh PA (SPX) May 12, 2011

http://www.terradaily.com/reports/Climate_Record_Suggesting_Severe_Tropical_Droughts_as_Northern_Temperatures_Rise_999.html

A 2,300-year climate record University of Pittsburgh researchersrecovered from an Andes Mountains lake reveals that as temperatures in theNorthern Hemisphere rise, the planet's densely populated tropical regions willmost likely experience severe water shortages as the crucial summer monsoonsbecome drier. The Pitt team found that equatorial regions of South America already are receiving less rainfall than atany point in the past millennium.

The researchers report in the Proceedings of the NationalAcademy of Sciences (PNAS) that anearly 6-foot-long sediment core fromLaguna Pumacocha in Perucontains the most detailed geochemical record of tropical climate fluctuationsyet uncovered. The core shows pronounced dry and wet phases of the SouthAmerican summer monsoons and corresponds with existing geological data ofprecipitation changes in the surrounding regions.

Paired with these sources, the sediment record illustrated thatrainfall during the South American summer monsoon has dropped sharply since1900-exhibiting the greatest shift in precipitation since around 300 BCE-whilethe Northern Hemisphere has experienced warmer temperatures.

"This model suggests that tropical regions are dry to a point wewould not have predicted," Abbott said. "If the monsoons that are socritical to the water supply intropical areas continue to diminish at this pace, it will have devastatingimplications for the water resources of a huge swath of the planet."

The sediment core shows regular fluctuations in rainfall from 300 BCEto 900 CE, with notably heavy precipitation around 550. Beginning in 900,however, a severe drought set in for the next three centuries, with the driestperiod falling between 1000 and 1040.

This period correlates with the well-known demise of regional NativeAmerican populations, Abbott explained, including the Tiwanaku and Wari thatinhabited 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 the1750s 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 awet phase before the rain began waning again in 1900. By July 2007, when thesediment core was collected, there had been a steep, steady increase in dryconditions to a high pointnot surpassed since 1000.

To create a climate record from the sediment core, the team analyzedthe ratio of the oxygen isotope delta-O-18 in each annual layer of lake-bedmud.

This ratio has a negative relationship with rainfall: Levels ofdelta-O-18 are low during the wetter seasons and high when monsoon rain islight. The team found that the rainfall history suggested by the lake corematched that established by delta-O-18 analyses from CascayungaCave in the Peruvian lowlands and theQuelccaya Ice Cap locatedhigh in the Andes.

The Pumacocha core followed the climatological narrative of thesesources between the years 980 and 2006, but provided much more detail, Abbottsaid.

The team then established a connection between rainfall and NorthernHemisphere temperatures by comparing their core to the movement of theIntertropical Convergence Zone (ITCZ),a balmy strip of thunderstorms near the equator where winds from the Northernand Southern Hemispheres meet.

Abbott and his colleagues concluded that warm Northern temperaturessuch as those currently recorded lure the ITCZ-the main source ofmonsoons-north and ultimately reduce the rainfall on which tropical areas rely.

The historical presence of the ITCZ has been gauged by measuring thetitanium concentrations of sea sediment, according to the PNAS report. Highlevels of titanium in the Cariaco Basin north of Venezuela show that the ITCZlingered in the upper climes at the same time the South American monsoon was atits driest, between 900 and 1100.

On the other hand, the wettest period at Pumacocha-between 1400 and1820, which coincided with the Little Ice Age-correlates with the ITCZ'ssojourn to far south of the equator as Northern Hemisphere temperatures cooled.

Study coauthor Mark Abbott, a professor of geology and planetaryscience in Pitt's School of Arts and Sciences whoalso codesigned the project, said that he and his colleagues did not anticipatethe rapid decrease in 20th-century rainfall that they observed. Abbott workedwith lead author and recent Pitt graduate Broxton Bird; Don Rodbell, studycodesigner and a geology professor at Union College in Schenectady, N.Y.;recent Pitt graduate Nathan Stansell; Pitt professor of geology and planetaryscience Mike Rosenmeier; and Mathias Vuille, a professor of atmospheric andenvironmental science at the State University of New York at Albany. Both Birdand Stansell received their PhD degrees in geology from Pitt in 2009.