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the Mid-Atlantic Ridge (Original)


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Carolyn Silver
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« Reply #30 on: July 28, 2008, 10:15:04 pm »

Abrupt Climate Change During Glacial Times
The Younger Dryas


Some of the best-documented events are dramatic, rapid rearrangements of the entire climate system as the earth shifted from glacial (ice age) to interglacial (warm) periods. These events include the prominent Younger Dryas event, as well as the numerous Dansgaard/Oeschger events.

The Younger Dryas was an over 1,000 year long cold period between the last ice age and modern conditions. The Earth's climate abruptly warmed at the end of the last glacial period approximately 14,500 years ago. It then cooled back to glacial conditions over the next 3,000 years. After 1,000 years of conditions comparable to the last glacial climate, the Earth's climate suddenly warmed, with much of the change happening in less than a decade.


Figure 18. Ice core reconstruction of temperature and snow accumulation from Alley 2000.

The Younger Dryas is best known from two sources. Originally, it was described from pollen data, denoting a period when the cold-loving dryas flowers were much more common across much of Europe. It was not until the 1989-1994 U.S. and European projects GISP2 and GRIP drilled their long ice cores in Greenland that scientists could understand the rapidity with which climate changed during the Younger Dryas ( Alley 2000, Cuffey and Clow 1997). As you can see from the GISP2 data (Figure 18), temperatures rapidly rose around 10° C in a very short time around 11,500 B.P. Detailed analysis of the ice cores revealed that most of the increase occurred in less than a decade.

Hulu Cave Record

Figure 19. Comparison of oxygen isotope records in a Greenland ice core (red) and a stalagmite from Hulu Cave, China (blue). The Younger Dryas event is well known as an abrupt cool event in the North Atlantic region (more negative values indicate colder conditions in Greenland). The significance of the Hulu Cave record is that a concident change occured half-way around the world in summer rainfall. More negative values for Hulu Cave are interpreted pimarily as an indicator of more summer monsoon rainfall relative to winter rainfall. Thus the east Asian summer monsoon was weaker during the Younger Dryas when the North Atlantic was cooler. The time 16,000 to 10,000 years before present spans the transition from the glacial to interglacial state.


Unlike some abrupt change events, records of the Younger Dryas can be found from around the globe. The recent stalagmite record from Hulu cave (figure 19) shows that the changes in oxygen isotopes found in Greenland ice are matched in cave deposits in eastern China (Wang et al. 2001).

Records of the Younger Dryas are prominent across most of the northern hemisphere, and some manifestations of the event may spread worldwide. The Younger Dryas has now been even more precisely dated using sediments from the tropical Atlantic off Venezuela ( Hughen et al. 1996, Hughen et al. 2000, Haug et al 2001, Lea et al. 2003).

http://www.ncdc.noaa.gov/paleo/abrupt/data_glacial.html
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